(2014-04-17) A review of an inexpensive microcontroller starter set.
At $40, this evaluation kit (also available at your localRadio-Shack store) is $10 less expensive thanthe bs2-ic Modulebuilt into it, even though it includes a $4USB A to Mini B Cableand a little solderless breadboard, (and ten 22-gauge jumper wires, for good measure).
Other Similar Parallax Products :
The downside is that the "HomeWork Board" can't be used to program anactual BS2 for permanent installation in a less bulky standalone project.
Like an olderBS1 module (and/or the low-costBS1 evaluation board)a stand-alone BS2 would only need a 6 V to 12 V DC power supply. The other units require 6 V to 7.5 V DC.
Opening the box and getting started :
The 555-28188 comes with four stick-on rubber feet to be placed next to the corner holes(which are meant for stand-off screw posts, if you decide to put the board in a protectiveenclosure later).
The instructions are silk-screened on the board's flipside. They read:
You should only connect the board to your computer (using the USB cable provided) after installing the Parallaxsoftware on the computer and putting in a fresh 9V battery (not included).
The installation puts a "Basic Stamp Editor v2.5.3" (or possibly some other version number)on the run menu but not on your desktop (copy the icon from the start menu toyour desktop and/or taskbar if you so desire). The first time you run that software, it asks you if you want to associate therelated file types to it (.bs1 .bs2 .bse .bsx .bsp .bpe .bpx). Allowing it to do so will simplify your life (those file types are not of any use outside the Basic Stamp world, anyway). At startup, you're shown one of 36 "tips of the day". The first of these doesn't apply to USB boards but it mentions incidentallythat there's an online manual. I quote:
Use a standard straight-through serial cable to program the BASIC Stamp. A null-modem will not work. If you're careful, you can make your own cable...just follow the wiring diagram in theBASIC Stamp Manual v2.0.
Following the instructions on the backside of the board, we click the Help menu and choose the first item in it ("BASIC Stamp Help..."). Then, we click "Getting Started" on the left-hand column.
The first task is to identify your board. The board reviewed here corresponds to the fourth picture listed (the last one in the v2.5.3 revision of the software). Just clickthe "next" button corresponding to that pictureand perform the setup tasks described, starting with affixing therubber feet (in case you've resisted the temptation to do so earlier).
For the initial "Run/Identify test", you should wait a few secondsafter the USB cable is plugged-in for the first time ever (this gives your computer a little bit of timeto install the proper drivers and flash the message "Device is now ready to use" when that's done). It's useful to know that the F6 key can be used to perform the "Run/Identify test"whenever the BS communication sofware is up and running.
Since the unit has no power switch, you must disconnect the battery to turn it off. However, it's virtually impossible not to discover that one of the contacts comes off more easily than the other,so that you can just leave one side plugged-in and rotate the battery to switch theunit on and off (albeit unreliably so). I've seen at least one other person do that publicly, at the end of avideo-proof that people do have fun with this thing (seeSorting M&Ms by color).
A more reliable low-cost solution, without defacing the board, is toconnect the battery vertically to only one contact (I prefer the outside one, the negative ground) and close the circuit with a battery snap connector cut in half,with some switch soldered between its two leads. Either that or use an unbutchered snap obtain a pair of wireswhich can connected (with am on/off switch) to any suitable DC power source (there's a nice low-drop LM2940L voltageregulator on board, so voltage requirements for an outside power supplyshouldn't be too strict).
I decided to base my own solution on a 9V power supply with an "M" plug (5.5 mm OD, 2.1 mm ID, positive center) specifically marketed forArduino microcontroller boards.
By itself, the board draws about 9.1 mA (measured) at rest when poweredwith a 9 V battery. The TL082 which we shall soon add as part of ourfirst "real" interface (converting digital PWM into a clean analog signal) will draw an additional 2.5 mA at rest...
Wiring some Standard Interfaces
Before I even touched this unit or looked at the online documentation,I had a few fun applications in mind for this initial review. One of them was generating DTMF tones. I was therefore shocked and amused to discoverthat this idea is so common that one of the 42 commands in the PBASIC language(DTMFOUT)is dedicated to that and makes the software part trivial. The command generates the tone digitally in PWM mode (pulse-width modulation) since there's no digital-to-analog converter on board. This can be directed to any any pin but I'll dedicate P1 to that usage (for a trivial reason which shall be given soon) and puta goodactive filter on it (Parallax recommends a 2000 Hz cutoff frequency but I'll use 3300 Hz toallow a larger range of audio tones, beyond TouchTone®).
It's always a good idea to give several nonconflicting functions to the same I/O pin,especially for decision intended to be permanent (we don't want to waste pins, as we only have 16 to play with). So, I'll also attach a bicolor diode to P1 (RadioShack #276-0012) which will be yellow (= red+green) when there's audio activity, red in a 0 state, green in a 1 stateand unlit in the high-impedance state. The bicolor diode consists of a red LED and a green LED mounted in parallelwith opposite polarities in a single package with two pins (marked like the green LED would be if it was by itself). One way to achieve that functionality would be toconnect the bicolor LED between P1 and a point connected to both supply railsby the proper current limiting resistors (equal values of 1k or so work fine because the twodiodes have similar voltage drops of 2.0 V and 2.1 V). It works fine by presents one design flaw: A current of several milliampereswill drain the battery even when the unit goes into sleep mode (where only microamperes are darwn). A better solution is to connect the bicolor LED between P1 and somethingwhose polarity is opposite to that of P1 when P1 isn't in the high-impedancestate. So, we'll make sure that the first stage of our active filteris simply an inverting amplifier in the DC regime (at 0 Hz) so we can connect our bicolor LED (with a current-limiting resistor) between P1 and the output of that stage. At first, we may be satisfiedwith just one stage and a single capacitor.
(2014-04-22) Converting a PWM digital signal into an analog signal.
It's certainly not necessary to have access to an oscilloscope to use the Stamp. However, a few oscillosope screenshots will help illustrates the concepts we have to teach now.
Pulse width modulation (PWM) denotes any encoding of an analog levelby a digital two-level signal whose duty cycle is proportional to the amplitude of the analog level. The method used in BasicStamps to convert an 8-bit input level is to add the corresponding 8-bit value (between 0 and 255) to an 8-bit accumulator. The value of the carry bit generated by thisoperation is what the binary output (0 or 1) will be for the duration of the current PWMinterval (a unit of time which, for the unit reviewed here, is equal to 3.8 microsecondsfor all PWM-related commands except one, as discussed below).
The beauty of this scheme is its simplicity and the fact that it remains valid if the desiredanalog level changes over time (just adjust the content of the accumulatorto reflect the level change). This is how the Stamp generates siwaves and dual tones. Let's use an example to design, once and for all, a proper audio output for our microcontroller.
We shall be aiming for the same cutoff frequency as traditional telephone service,namely 3400 Hz. A simple first-order lowpass filter is used for the preliminary demonstration. A 3400 Hz cutoff would be approximately achieved witha 4.7 k resistor and a 10 nF capacitor. (or 47 k and 1 nF). Counting the 220 built-in resistor on the output pinand using the measured values of the components on hand for this demo, our first test wasactually conducted with a corner frequency of 3175 Hz.
We dedicate a pin to properly trigger a Rigol DS1102E oscilloscope: The signal displayed above on channel 1 (yellow) is meant for the scope's "external trigger" input (usinga pair of clips attached to a BNC cable).
Note that the width of the pulse can be precisely measured to be 184.8 us using a higher timebase resolution (that's a more precise value than the "180.0 us" in theabove screenshot at low resolution). That's an interesting measurement of the time it takesto execute a very simple PBASIC step, which translates into 5400 steps per second (the BS2 isadvertised to execute 4000 "typical" PBASIC instructions per second).
The rising and falling edges of the trigger signal need not be adjacent. Place them separately anywhere you like in your program to examine two different parts of themain loop in more complicated cases, without the need to tax the scope's delayed timebase. The two triggering options can then be selected on the scope without reprogramming the microcontrollerunder test (although it takes only a few seconds to do so).
We now show details of the 2000 Hz "sinewave" burstfrom the above test, along with the driving PWM digital signal (yellow trace).
Transitions can only appear at regular intervals of 3.8 us This PWM cycle, used in the algorithm described above,corresponds to 76 cycles of the BS2 clock, at theadvertised rate of 20 MHz.
In this screenshot, the scope shows the correct duration (4 units = 15.2 us) of the first whole interval on the screen andunderestimates the second one by 5% (it can find the correct value of 3.800 us using shorter time-scales).
This 3.8 us constant interval, corresponding to a PWM samplingfrequency of about 263.16 kHz, is the same for all PWM waves produced by the BS2 firmware (single frequency, double frequencies or touchtone). Unexplicably, it's slightly longer (4.4 us) for thePWMcommand itself, which merely produces the PWM equivalent of a static level.
To make it easy on yourself,one cost-effective way to go beyond the above first-order filter is touse a commercialDSL filter, which is normallydesigned to have the aforementioned corner frequency (around 3400 Hz). Such filters are typically third-order or better...
(2014-05-18) The trick is to attenuate the PWM square wave before filtering it.
So far, we have decoded the PWM digital output from the microccontroller into a goodanalog signal. The BasicStamp firmware could easily have encoded differentanalog levels in its PWM tones. Unfortunately, it did not. So, wehave to find some hardware solution to the problem of allowing software to controlthe audio volume.
The analog counterpart of what we are after is called a voltage-controlledvariable-gain amplifier. As a purely analog project, that would be afairly intricate endeavor, well beyond our current scope. However, we can simplify things greatly by noticing that a PWM signalcan simply be used to chop up (turn on and off) some constantDC level different from the whole supply voltage. Filtering that square wave will result in a signal proportional to the DC level used.
The first step is to produce a steady analog voltage from a PWM signal onsome other pin. To do that we use the second high-impedance opampin the IC (TL082 or TL072) whose first half we've already used as an active filter. Let's dedicate one pin (which we'll call LEVEL) to the audio gain control. We connect LEVEL to the non-inverting input of the operational amplifier wired asa voltage follower (i.e., the output is connected to the inverting input). Let's now connect a 22 nF capacitor, say, to that pin (and nothing else, not even a probe tip).
The charge on the capacitor has essentially nowhere to go when the microcontroller pin isin its high-impedance state. So, we have a constant voltage at the output of theoperational amplifier which will take a very long time to decay... Much more than what we need to maintain the output volume of every separate tone, for a few seconds, at most.
for all pathes to ground in this casual test. That would mean that the volume of a continuous tone would decrease ata negligible rate of 0.0024 dB/s (3 dB after 20 minutes).
Note that we should not use the OUTPUT command for the LEVELpin. Doing so would force the state of the pin low or high for a substantialamount of time (at least 184.4 us, remember?) and we don't need that;the PWM command makes the pin an output pin only for the duration of its execution,which is exactly what we want.
(2014-06-22) With a BS2 (orequivalent) PBASIC offers no built-in LCD support.
As this is really a continuation of the previous educational introduction, usingthe inexpensiveHomeWork Board equivalent to a Basic Stamp II (BS2) we'll do more work to really get to the bottom of things.
Our reward for not cutting corners will be a couple of final answers to software and hardware issues which are very often badly butchered...
For units beyond the BS2, Parallax provides 3 specific PBASIC commands whichfacilitate 4-bit parallel interfacing withan LCD (liquid crystal display) driven by the industry-standardHitachi HD44780character-LCD controller (namely,LCDCMD,LCDINandLCDOUT). We'll discover that the scheme has one imperfection, in theory if not in practice (a fourth specialized command should have been provided for the early steps of initialization,accomplished in 8-bit mode with an irrelevant low-nibble).
The HD44780-compatible LCDs are commonly available in several formats:
We'll make our wiring compatible with one of the configurations supported by Parallax in theaforementioned specialized LCD primitives. Then, we'll discuss PBASIC software to support that configurationand turn the whole thing into the educational experience it was meant to be.
7 pins are normally used to interface with the LCD. Some people use only 6 pins for one-way communications with an LCD (write-only) by tying to ground the R/W pin (number 5 on the standard LCD connector). I don't recommend that shortcut at all. With the sole exception of the enable pin (number 6 on the LCD connector) all pins are used only temporarily by the LCD and can be shared by other devices.
LCD units without backlighting have connectors with only 14 pins instead of . Pin 15 is the anode (+) of thebacklight and pin 16 is the cathode (-). Connect 16 to ground (Vss) and connect 16 to the positive 5V power rail through a current-limiting resistor.
The potentiometer on pin 3 of the connector (often dubbed "contrast") is crucial. It establishes the Vee voltage of the LCD, which should be ajusted accordingto room temperature for best results. If Vee is too low, off-pixels are visible. By increasing Vee, an optimal voltage is reachedwhere the off-pixels disappear, but barely so (increasing the voltage beyond thiswould make the on-pixels fade away). At this writing for example, room temperature is 28°C (83°F is typical of Los Angeles in June) and the optimal Vee for my test unit is 780 mV (better described as -4.2 V with respect to the LCD anode). Because the optimal "Vee" depends on temperature, it's somewhat unpredictableand the user should be able to adjust it with a trimmer. If the unit is meant to operate in various environments, an automatic compensationfor temperature and/or supply voltage could be considered (especially when using amicrocontroller with a spare analog voltage control). Sometimes, the best voltage can be negative withrespect to the cathode as well (especially for 3.3 V circuits) and a negative rail becomes indispensable for best results.
Different LCD Formats :
On a multi-line LCD character display, the positions of the characters in a given line are alwaysnumbered consecutively from left to right. What varies from one LCD to the next are the numbers assigned to the first character of each line.
The displays that feature 4 lines of 20 characters behave as if they consistedof 2 lines of 40 characters. whose second halves appear as the third and fourth lines ofthe actual screen, which is filled by autoincrementing from 0 to 79 (and back to 0) as follows:
Sequential numbering of the 80 positions in an LCD of 4 lines of 20 characters
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
However, this is not the way direct-access indices are organized. Instead, the beginnings of the two lines are respectively at addresses 0 and 64. The two sets of forbidden entry points at the "end" of either line (40-63 and 104-127)behave exactly like the valid beginning of the next line (64 and 0, respectively). Both forbidden sets are skipped when the LCD is accessed sequentially, by autoincrementing.
4 by 20 LCD random-access addresses (40-63 same as 64 ,104-127 same as 0 )
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
At left is the standard 5x8 matrix font for the HD44780. It's sometimes called a 5x7font because the bottom row is blank in all standard characters to leave room for an underscore cursor. However, this need not be the case for custom characters, especially in applicationswhere the underscore cursor isn't used.
The HD44780 provides only 64 bytes of CGRAM (character generator RAM) for custom characters. In this 5x8 format, that's enough for 8 custom characters (each 5-bit row uses a byte) encoded from 0 to 7 at the highlighted positions in the table.
In the rare 5x11 format (not covered here) those 64 bytes would only allow 5 custom characters,numbered from 0 to 4, with 9 leftover bytes.
Like DDRAM (data display RAM) the 64 locations of CGRAM are accessible by address and successivewrites will cause an autoincrement to allow access to the next location. Also like DDRAM, this autoincrement wraps around to the beginning of the available space. If you must know, so do the codes of the custom characters. (In practice, this means that the characters you defined for codes 0,1,2...will also be available at 8,9,10...)
A dot indicates an irrelevant bit ("don't care"). Bit-letters are explained below. In the last column are execution times (microseconds) for a 250 kHz clock rate.
Name
RS
RW
Data Bus
Instruction (action to perform)
s
clear
0
0
0000 0001
Clear every display byte, reset cursor.
1640
home
0
0
0000 001.
Reset display shift, reset cursor.
1640
entry
0
0
0000 01as
Autoincrement (a=1). Allow shift (s=1).
40
show
0
0
0000 1dub
Display (d), underscore (u), blinker (b).
40
move
0
0
0001 cm..
Cursor (c=0) or display. Left (m=0) or right.
40
set
0
0
001w nf..
4-bit (w=0). Single line (n=0). 5x8 dots (f=0).
40
cgram
0
0
01,address
6-bit address in character-generator space.
40
ddram
0
0
1, address
Set 7-bit address in data-display space.
40
busy
0
1
BF,address
Fetch current address ("busy flag" in MSBit).
0
write
1
0
data
Write 8-bit data to current address.
40
read
1
1
data
Read 8-bit data from current address.
40
a (autoincrement flag) pertains to DDRAM access only. a=1 is the normal mode whereby successiveaccesses correspond to indices in increasing order. To go backwards, use autodecrement mode (a=0).
The HD44780 data bus has pull-up resistors :
Let's nvestigate the high-speed protocol which avoids unduedelays for a fast microntroller (this doesn't apply to the BS2 running PBASIC, with asingle exception which is easily disposed of). In the process of this investigation, I discovered thatthe LCD's enable pin (pin 6 on the LCD connector, often called"E" and dubbed LCD in the source code provided here) doesn't put the LCD data bus in a high-impedance state, as expected. Instead, the data lines sport 30k pull-up resistors,which the designer should keep in mind when sharing this bus with other devices.
Once the LCD has been properly initialized, we may attempt to time the longest LCDinstruction to see if any extra delays are needed on the BS2. The following BS2 program fragment is meant to visualize on the oscilloscope whattime is leftover in the worst case...
DO ' Testing BF = Db7 with an oscilloscope :char = LCDclr ' %00000001 (Clear LCD) is the SLOWEST commandGOSUB LCDctrl ' Send command (and leave RS low)DIRD = %0000 ' Use 4-bit bus D as input (pins 12,13,14,15) HIGH rw ' This rising edge of RW will trigger the scope HIGH lcd ' Enable LCD PAUSE 1 ' Monitor Db7 = BF with a 10k pull-down resistorLOW lcd ' Test complete, disable LCDLOW rw ' Back to write modeLOOP ' Repeat (more than 200 times per second)
The highlighted part of that program corresponds to the oscilogram below (the yellow trace is R/W). The 10 k external pull-down resistor on BFreveals the presence of the aforementioned 30k internal pull-up resistor.
The 30k value itself can be deduced from the screenshot at left,where a 25% level is observed on Db7 = BF (blue trace) as the pull-up resistor fights the external 10k pull-down resistor (temporarily installed for this test only).
Likewise, the failure of the BF signal from the LCD to reach the 100% logic level(which doesn't exist without the pull-down resistor)can be attributed to an output impedance of 1.5k or so.
If reading the LCD is permanently disabled by grounding R/W,the LCD can't be used at full speed in machine language. PBASIC, on the other hand, is so slow that the issue only arisesin the "clear-screen" command (used in theabove oscilloscope test). The "write-only" use in PBASICof an HD44780-driven LCD is thus totally safe if one simply waits anadditional 2 milliseconds after clearing the screen. That's all there is to it...
With a microcontroller programmed in assembly language however, it's more satisfyingto drive the LCD in the proper high-speed manner (reading BF before writing to the LCD).
When the microcontroller is in sleep mode,output pins are still driven but will go out for about 18 msevery 2.304 s. During those regular outages, spurious strobes could occur on the LCD's "enable" pinif we didn't install a pull-down resistor on that pin (which is the only one permanently dedicated to the LCD unit, besides the backlight pin).
"Jitter" reveals the exact frequency of the LCD local clock:
Jitter is the phenomenon whereby events which could naively be expectedto happen repeatedly at a predictable time are actually observed to be randomly early or late,within a definte interval of measurable width. (In some contexts, that's also called phase noise.)
In the above, one can observe an obvious jitter on the falling edge of the BF signal (blue trace). Its width can easily be measured to be about 17.6 us (with the oscilloscope at 2us per division observe the jitter for several seconds,placing one cursor on its lower limit and another on the upper one). If we assume that the jitter is due to thelack of synchronization of the BS2 clock and the microcontroller clock,then the width of the jitter is equal to a whole number of cycles of the LCD clock (presumably, the time it takes the HD44780 to execute each cycle of a waiting loop). This means the clock of the tested LCD unit is a multiple of 56.8 kHz or so. As the datasheet of the HD44780 specify a clock frequency between 190 kHz and 250 kHz, we may expect the clock frequencyof that particular unit to be about 227 kHz and deduce that the HD44780 can't pinpoint the time of externalevents to an accuracy better than 4 clock cycles, which seems reasonable...
(2014-04-19) Giving birth to a standalone HW1 by cutting its USB umbilical cord.
Our goal now is to turn the HomeWork Board (HWB)into a controlling device which can be operated without its USB umbilical cord. We're already halfway into this with theLCD provided above. In this section, we'll complete the process by endowing the unit with a 16-key keypad. After this, the resulting standalone device deserves an enclosure and a name: HW1.
I'll install an off-the-shelf 16-key keyboard (the usual telephone keys plus anABCD column corresponding to the four extra standard touchtones). The supporting software will recognize 136 possible input combinations from this: 16 single keys and 120 combinations of two keys pressed simultaneously. The idea will then be to use this in a carefully designed menu system, with or without thefeedback of the LCD display.
(2014-05-23) Bouncing ultrasound (40 kHz) off an object to measure its distance.
The HC-SR04 is a 4-pin device. Besides the power rails (labeled Vcc and GND) there's an input pin labeled TRIG and an outpout pin labeled ECHO. The device consists of a microcontroller driving two identical acoustic transducersworking at a nominal frequency of 40 kHz (ultrasound). It sends an 8-cycle sound burst and measures the time it takes to receive the echo.
The unit is activated by a positive pulse on the TRIG pinand responds with a positive pulse on the ECHO pin after a fixed amount oftime (on my unit, the leading edge of ECHO comes 466 us after the falling edge of TRIG). The width of that pulse is the time it took to receive an echo (i.e., twice the distance between the sensor and the object, divided by the speed of sound).
This demo lights up an LED when the measured distance is 204 mm or less:
' {$STAMP BS2}' {$PBASIC 2.5}trig PIN 1echo PIN 2led PIN 3time VAR WordLOW trig ' Equivalent to OUTPUT trig : trig=0INPUT echoLOW ledDOPULSOUT trig,25 ' 50 us on a BS2 (10 us on a BS2sx)PULSIN echo,1,time ' Receive pulse widthIF time<= 300 THEN ' 204 mm at 340 m/s led=1 : ELSE : led=0 : ENDIFLOOP
Clearly, the unit could have been designed to combine the output and input functions on a single pin. Such single-pin devices are indeed available but they happen to be much more expensivethan the unit reviewed here...
(2014-04-22) Example: Real-time clock (RTC) using the I2C bus.
Our introduction to the I2C protocol will be based on a unit which containstwo independent I2C nodes; a real-time clock and a mid-sized EEPROM (4096 bytes of non-volatile memory).
The tiny board at left retailsbelow $6(with shipping). It's built around a DS3231 with anintegrated temperature-compensated crystal oscillator (TCXO).
On the "32K" pin is the output of the 32768 Hz oscillator (300 nsrise time, 32% duty, 15 ns fall). The programmable SQW output pin can delivera 1 Hz signal (300 ns rise, 20 ns fall, 50%).
Straight out of the box, without using its digital trimming capabilities,the DS3231 has a 2 ppm accuracy (between 0°C and 40°C) which translates into 1.2 s per week, or 1 minute per year. To measure the accuracy of the clock in just a few minutes,you may use the stable 1 Hz signal (often termed PPS for "pulse per second") from a GPS receiver. Use that signal to trigger a digital oscilloscope and observe the 32768 Hz output with the timebase set at 2 us per division (so you can alwayssee several wavefronts on the screen). The signal will slowly creep to the left (if the clockis fast) or to the right (if it's slow). Measure the time T be the time it takes for the trace to drift on the screen by a distancecorresponding to a given interval (one microsecond, say). The relative accuracy of the clock is simply t/T. For example, with the factory settings ("aging register" set to zero) I found that the trace was drifting to the left on the screen by 2.5 us in 36.32 seconds at a temperature of 31¼°C (according to the chip, but probably more like 29°8) so that clock is too fast by the following relative amount:
(2.5 us) / (36 s) = 0.07 ppm = 2 seconds per year
A few minutes later, at the same temperature, it took only 26 seconds for the same drift (that's 1 ppm). Then the drift seemed to stop when thetemperature (as measured by the chip itself) dropped only a quarter of a degree... The data sheet says that the oscillator can be trimmed in steps of roughly 0.1 ppm (the user-supplied correction is stored in register $10 termed"aging register" and is updated every 64 seconds or upon request). If found the influence of the aging register, if any, to be far less than that in theconditions described above. It looks as though the chip is significantly more precise than what's described in thedatasheet, but less adjustable... My first impression about the unit test is that its untrimmed performance isat the best level that digital trimming would permit, according to the datasheet. The influence of digital trimming, if any, is certainly far below the0.1 ppm per step quoted in the datasheet. My observations are thus consistent with the fact that the unit I am looking atwas trimmed at the factory to the best precision the hardware would allow (roughly 0.1 ppm) with user-adjustable trimming now turned off.
The venerable DS1307 doesn't begin to approach that level of performance of the DS3231. (In typical applications, the DS1307 relies on an uncompensated external 32768 Hz crystal.)
Besides the real-time clock itself, the above DS3231 board (like its DS1307-based predecessors) includes an independent32 kbit EEPROM(4096 bytes of non-volatile data) which could be another good didactic I2C example,albeit a duller one... Before I got may hands on the part,my intention was to use that EEPROM to store aging data on it (the variation of optimal digital trimming over the years). Because of the above considerations, this is now moot.
One nice feature of the DS1307 is that is has static RAM (SRAM) maintained by the clock batteryon the portion of the 8-bit address space not used by the actual RTC registers. The DS3231 doesn't have that, but there is an otherwise identical IC (the DS3232) which does.
Unlike the DS1307, the DS3231/DS3232 features a century toggle (located atthe most significant position of the month-number byte). This is toggled whenever the century changes, to deal with potential bugsat the beginnings of the years 2100, 2200, etc. (Remember the Y2Kmillennium bug?)
To be able to properly determine leap years, a chip like that would needto keep track of the centurymodulo 4. That's only two bits, but the DS3231 doesn't have them. Instead, the DS3231 uses the Julian rule (which call for every year divisible by 4 to be a leap year) and ignores the Gregorian modification to that rule used in our modern calendar (where years divisible by 100 aren't leap years unless they are also divisible by 400). The DS3231 will therefore fail on the first day of March in 2100, 2200, 2300, 2500...
The first bad day for the DS3231 comes only 59 days after the DS1307 will failfor lack of a "century toggle".
The I2C (Inter IC) Bus
SHIFTIN sda, scl, MSBPRE, [frame\9]
This is part of a clock program where we only need to fetch a single bit(the parity of the least significant digit in the count of seconds) to check as fast as possible whether an update is needed,so any trick goes... Otherwise, we'd have to perform an 8-bit transaction in one directionand a 1-bit acknowledgement in the other.
The slave changes the state of the data line (SDA) in blue, just aftereach clock pulse.
Not visible at this time-scale is a reactionlag of about 350 ns from the RTC chip and arise time (10-90%) of 165 nswith a 2.15 k pull-up resistor on SDA (designed for 400 kHz "Fm mode" use).
As examplified above,the BS2 can be used to communicate over an I2C bus as a master (not as a slave, which is a much more demanding task). The basic pace of the above example cannot be adjusted on the BS2 platform (take it or leave it). The basic BS2 clock rate for IC2 communications is 16.6 kHzThe width of each clock pulse is 14.24 us and the intervalbetween them is 46 us. Almost all I2C devices are rated for at least 100 kHz and will very easily accomodate the above pace.
Note that the BS2 uses push-pull on both bus lines in the relevant primitive,instead of the open-collector architecture I2C is based on. Models beyond the BS2 feature PBASIC primitives specifically designed for I2C (namelyI2CIN andI2COUT) but we are not reviewing those here.
This isn't much of a problem in practice, as long as the BS2 master never pushes a hard "1" on a bus line (by contrast with a soft "1" produced by thehigh-impedance "input" state) at times when the I2C protocol allows a slave to pull is low.
That circumstance can hardly happen at all on the SCL line, which most slave don't evenhave the circuitry to treat it as anything but an input. A few slaves may be able to "hold the clock" low to tell a fast master to wait during high-speedtransfers. This type of protocal is beyond the scope of this article.
When the master (the BS2) is speaking on the SDA line, all slaves are listening and it makesvery little difference whether the BS2 is using open-collector or push-pull logic (the only difference on an oscilloscop is that rising times are faster, with a possible overshoot,when push-pull is used).
Don't even think of allowing the BS2 to share an I2C bus with other masters or withslaves that can "hold the clock".
On our side of things, it's essential never to let the BS2 attempt to writeanything but a zero at a time when it's supposed to be listening on the SDA line (but doesn't care about the information). Or else a condition may result which istechnically a bad short circuit (no physical damage will result only becauseof the BS2's built-in protection resistors on I/O pins). In practice, this word of caution only applies to the writing counterpart of the 9-bitread discussed above. The ninth bit in an I2C write is supposed to be writtenby the slave (and it's normally low). If the BS2 chooses to overwritethat bit, it better be a zero. In other words, don't EVER use the following instruction unless you are absolutelysure that "frame" is even!
SHIFTOUT sda, scl, MSBFIRST, [frame\9]
Below is a close-up view of the acknowledgment by a DS3231 chip of an I2Crequest by a Basic Stamp 2.
Because its pin is protected by a substantial resistance, the BS2can pull the SDA bus line only so low (696 mV) against a 2.15 k pull-up resistor. After the falling edge of the positive pulse on SDA, an intermediary level (476 mV here, but it's sensitive to fidgeting with the breadboard) is quickly reached which is maintained for asurprisingly long time (165 us)
The I2C standard can achieve synchronous serial communications between many ICsover a distance of a few meters, using only a single pair of shared lines (besides the two power rails).
Every transfer over an I2C bus begins with a START signal (more about that later) followed by a control byte containing a 7-bit code followed by a one-bit mode (0=write, 1=read). The 7-bit code can be one of 112 short slave addresses or it can be one of the 16 commands tabulated next, which ordinary (7-bit) I2C slave devices may safely ignore.
7-bit Code
R/W
Meaning
000 0000
0
General call
000 0000
1
START byte
000 0001
-
CBUS format
000 0010
-
Other format (reserved)
000 0011
-
Reserved for future use
000 0011
-
Reserved for future use
000 0100
0
Hs mode master code test (0)
000 01xy
z
Hs mode master code xyz (1 to 7)
000 0011
-
Reserved for future use
111 10xy
R/W
10-bit address (xy bits & next byte)
111 1100
-
Device ID
111 1101
-
Reserved
111 1110
-
Reserved
111 1111
-
Reserved
Two I2C devices with the same address shouldn't coexist on the same bus.
The maximum capacitance allowed on each bus line (400 pF) restrictsthe physical characteristics of the bus and the number of connected devices.
The two I2C bus lines are called clock and data and denoted by two standard 3-letter abbreviations:
SCL : Serial clock line.
SDA : Serial data line.
Both lines are open collector with pull-up resistors connected to thepositive power rail (that name assumes the lines are driven byNPN transistorswith grounded emitters, but other equivalent technologies can be usedand the designation open drain is also common). The logical state of either line is thus high by default and becomes "0" only whenit's actively pulled to ground by a conducting transistor.
This holds for the normal I2C bus (with a maximum clock rate of 100 kHz) and all upgrades thereof, except the ultra-fast mode introduced in 2012 (supporting clock rates up to 5 MHz) which uses push-pull logic on two bus lines withdifferent names (USCL and USDA).
I2C buses are rated according to the highest clock rate they can handle
Mode
Clock (max)
Year
Structure
Slow
10 kHz
1982
Open collector
Normal
100 kHz
1982
Open collector
Fm
Fast
400 kHz
1992
Open collector
Fm+
Fast+
1 MHz
2007
Open collector
Hs
High-speed
3.4 MHz
1998
Open collector
UFm
Ultra fast
5 MHz
2012
Push-pull
Atmel call their version of I2C "Two-Wire Interface" (TWI). They currently do not support 10-bit addressing or high speeds.
The I2C bus is very similar to theSMBus introduced by Intel in 1995 (the other accepted abbreviation for "System Management Bus" is SMB;please avoid "SMB bus" for grammatical reasons).
One significant difference is that SMBusallows dynamic allocation of slave addresses (for "plug and play"operation of removable devices) which is rare in the I2C world. SMB devices aren't allowed to operate at very lowfrequencies (which makes them unsuitable for educational I2C demonstrations wherewhere the bus is operated manually): They have a minimum operating frequency of 10 kHz and a timeout of 35 ms. SMB devices must be operated below 100 kHz. Many implementations no not follow the official SMB recommendation of pull-up resistorsof 14k or more (in 5V systems) which forces sluggish operations. There are also differences between allowed voltages and current levels but, for themost part, both standards are compatible below 100 kHz.
Bus Masters :
Every transaction over the I2C bus is between two nodes dubbed master and slave. Those rôles pertain to a single transaction;several nodes may be capable of acting as masters of the bus. When several masters can compete for control of the bus, every one of them must be a qualified multimaster willing and able to follow strict arbitration procedures. Usually, a BS2 only act as a singlemaster on an I2C bus where all other nodes are slaves. It takes heroic efforts to turn it into a proper multimaster.
A master obtains control of the bus by creating a start condition (namely, causing a high-to-low transition on SDA when SCL is high). It's solely responsible for generating the clock signal (SCL) and formulating requests (issuing additional START signals as needed) until it gives up control of the bus by creating a stop condition. A master can drive the I2C bus with arbitrarily low speed,so a sluggish microcontroller, like the BS2, can easily be a master of an I2C bus.
A well-behaved multimaster would at least need to monitor the I2C buscontinuously to know when it's busy (between a START and a STOP). This part is easily handled in hardware (it would be foolish to attempt it in software,even usinginterrupts) by creating a BUSY indicator available to all potential masters sharing the bus. The START procedure in a multimaster environment is:
Make sure the bus isn't BUSY (poll hardware indicator).
Pull SDA low to create a START signal.
... / ...
Slaves :
An I2C slave device must be able of recognize its own address quicklyto respond to a master's request. A microcontroller can hardly function as a slave unlessit can handle hardwareinterrupts, which the BS2 can't do.
In normal synchronous data transfer, the logical state of the data bus linecan only change when the clock line is low (a high clock thus indicatesstable valid data which can be safely read by the receiver). A data transition when the clock is high indicates either a start bit (when the data line goes from high to low) or a stop bit (for a low to high data transition).
After the start bit, the master sends an 8-bit piece of data (always starting with the most significant bit) containing the 7-bit address of the slave it wishes to communicate with,followed by a R/W bit set to "0" ifit wants to write to the slave or "1" if it wants to read from it. The slave so addressed should send an acknowledge bit (ACK) by pulling SDA low during the entire high time of the ninth clock pulse on SCL.
Once communication is established in this way, a normal transfer of datatakes place in the direction previously indicated by the master,which keeps clocking the bus (not faster than the rate used forthe above initial handshake). The slave is responsible for issuing an ACK bit after each byte transferred. Failure to do so is a NACK condition, which tells the master it should terminatethat multi-byte transaction (with a stop bit) and liberate the bus for thenext transaction.
A slave can deny access to a master at any part of a write transaction (from themaster's perspective) or at the beginning of a read request simply bydoing nothing, instead of pulling SDA low before the master issues itsninth clock pulse. Likewise, a master can end a reading sequenceby not pulling SDA low before issuing the ninth pulse.
However, no part of the I2C protocol allows a slave to request terminationof a read sequence once it has started. It can either do nothing (which will look to the master as if theslave is sending an endless sequence of $FF) or "wrap around"its own address space as if its first register followed its last. For some obscure reason, the latter solution is more popular than the former.
Properly resetting a singlemaster I2C bus and its connected interfaces :
The procedure described below should be made part of the initializationroutine of any microcontroller with a reset button or any microcontrollerwhich can be powered down independently of some devices connectedto its I2C bus.
The problem is that the microcontroller in chargecould have been reset at any point in the middleof an I2C transaction, so a slave could be pulling SDA low forever, waiting for a clock pulse which never comes.
Making the following procedure part of the microcontroller initializationwill remedy that situation and put any interrupted interfaceback to its normal state after every microcontroller reset. This indispensable piece of I2C folklore is now mentioned,more or less precisely, in the datasheets from several I2C manufacturers (including Maxim and Atmel).
The trick is to toggle SCL (up to 9 times) until SDAis brought to a high-level while SCL is high. At this point, we can simply pull SDA low to generate a start condition. I like to complete the initialization by letting SDA go high again, which creates a stop condition and releases the I2C bus for normal use (in pristine condition, with both lines pulled up).
(2014-04-22) Reacting to external events as they happen.
The Basic Stamp doesn't support hardware interruptswhich would allow the fastest possible reaction to an external event atno cost in extra time. Instead, a mecanism is provided which can check for some predeterminedevent after the execution of every PBASIC instruction. It's the next best thing.
(2014-04-21) Parallax P8X32A QuickStart Board for Propeller MCU.
