Wiz830

Hello everybody, I ported my tiny screen editor to Hi-Tech C on Z80-MBC2. See http://www.andreadrian.de/tiny_screen_editor/index.html And I wrote floating point conversion for Z80 in C and assembler. See http://www.andreadrian.de/fp_conversion2/index.html Have fun with computing as in the 1980s, but without the bugs.

Andre, are you aware of Tony Nicholson's project to incorporate various bug fixes into Hi-Tech C? It can be found at https://github.com/agn453/HI-TECH-Z80-C

Hello, first, let me say this is a great project !

I have a question about the design. I see an RS flip-flop connected to both IORQ and WAIT_RST, the idea being to automatically trigger WAIT at the beginning of the I/O cycle.

I'm a bit puzzled by how resetting it works, though. For WAIT_RST to have any effect, IORQ should first be high again. To do that, you pull BUSRQ low, get control of the bus (which would put IORQ in HiZ state, thus high thanks to the pullup resistor), reset WAIT and finally release bus control (BUSRQ back to high).

But how does that even work? Can the Z80 take BUSRQ into account even when in a wait state? The chronograms from the Z80 user's guide left me with the idea that it wouldn't give up the bus until the end of the I/O cycle.

Can you enlighten my confused mind on this part of the design?

Hi, if I remember well I checked the behaviour using a logic analyser as the official diagrams don't show up this particular case clearly... (it was a my own assumption at first...).

The trick is to use the BUSRQ signal to force the CPU in HiZ when it exits from an I/O operation (before the next machine cycle), giving the time to the Atmega to do the needed operations on the data bus.

BTW: I used the "undefined state" of the RS FF to save parts on exit (theoretically undefined as U1D, who drives the WAIT signal, is under the Atmega control in any case...).

Thanks for the answer.

I still don't understand, though. BUSRQ will force the CPU into HiZ mode only at the end of the I/O operation, exactly as you said. But that end cannot be reached while WAIT is low. And it won't go high again in the specific case of an ongoing I/O request, since then the RS output will be 'undefined'.

Or is there something that somehow guarantees that the 'undefined' output of the RS FF is actually seen by the Z80 as 'undefined'?

(I'd have thought that you'd need to OR the IORQ signal with not-WAIT_RST, so that when the reset signal goes low, the S input of the RS FF goes high regardless of IORQ state, and thus the WAIT line is properly released, allowing the CPU to get out of the I/O cycle and end up in HiZ.)

Maybe it should be clarified what "undefined" means in our case.
Look at the schematic the RS FF (U1D and U1C).
When the Atmega needs to end the I/O operation and resume the Z80 from the Wait state, it puts WAIT_RES LOW.
As IORQ is LOW too, the RS FF enters in the "undefined region", where it stops to work as a FF ("undefined" means that here).
In any case U1D is a NAND gate, so forcing the input pin 13 (WAIT_RES) LOW will make the output HIGH.
But the output (pin 11) is the WAIT signal, and when HIGH it brings the Z80 out of the wait state.
When the Z80 runs normally the IORQ signal is deactivated by the CPU and goes HIGH, and at this point the RS FF exits from the "undefined region" and it is safe deactivate the WAIT_RES too (HIGH). Now the RS FF is at the reset state.
At the next I/O operation the RS FF will be triggered again (in the set state) by the CPU using the IORQ signal and the "dance" will start again...

Indeed. I was totally blind for gods know whatever reason to the fact that, indeed, the NAND gate output would definitely be high with WAIT_RES low !

Thanks a lot for your explanation and your patience, this was really helpful !

I've just built a Z80-MBC2 from a kit. All parts were provided with the kit. Having built it, it boots fine to the IOS menu using reset and the user button, however it does not boot to BASIC, Forth or CPM. What happens is that the BIN files load and the message about running on the Z80 is the last thing I get before the HALT LED comes on. I am going to check things like Z80 power and clock but if anyone has any thoughts I'd be glad to hear them. The Z80 tests fine in a Spectrum BTW, but I do also need to check the CPU is a genuine CMOS version, as if it isn't then I guess that could be the cause. EDIT the CPU is a Sharp LH0080A, which according to the datasheet is only a 4Mhz part.

Hi, the LH0080A is a NMOS Z80 CPU version rated at 4MHz. Here you need a CMOS version rated (at least) at 8MHz (as explained in the project description or manual).

Side note 1: it has been reported that a NMOS Z80 "could run" here, but that can't be guaranteed. In your case check if the clock is set at 4MHz as last try...

Side note 2: the only "certified working" kits are those one sold by McJohn here: https://shop.mcjohn.it/en/8-diy-kit (or better you buy components by yourself...), as suggested in the project description or manual.

Regards.

My research has concluded much the same: It's the wrong CPU. The seller on eBay says he tested the CPU and it was fine, even at 8Mhz, but it's only a 4Mhz part, but it doesn't work for me at any speed and it is clearly not a CMOS part.

OK, so I have got to the bottom of things, it turns out the RAM IC was dead. Whilst I have installed a CMOS 8Mhz rated CPU, but the fix was replacing the RAM.

Hallo, on my Z80-MBC v3 Board only a RTC DS1307 works, the DS3231 will not be recognized. But it works on other system.

Found a solution. Solder the address pins and it works!

Am I missing anything or is it not possible to access the i2c port by software?

I2C is used for the optional GPIO expander (and for the RTC too). If you want use it for something different and access it from SW you have to modify the IOS FW accordingly.

Ok. Then I will do that. Will post it here if I have something :)

Thanks for the great project 

I see on facebook that one user add LCD display to the board where can I find some information and code ?

Try to ask on FB.

Does anyone know if the ATMega32 can be reprogrammed (sketch not bootloader) while still in circuit using the Z80-MBC2 serial port? I want to update my firmware to the latest version. Thanks.

In your case you have to use the ICSP connector with an external programmer as the USBasp (as if you had a brand new Atmega32), and you can do it leaving the Atmega32 and the other chips on the board (be sure that the SD module is taken out from the board). There is no chance to program the Atmega with the serial port without  a bootloader.

OK am is there any BOM with all of the components like the resistor values and their number on the board? I can't find it anywhere.

Edit: I must not have looked very hard! 

I just wanted to take a moment to say that the Z80-MBC2 is an amazing project! It allowed me to explore an era of home computing I had missed out on since my first was the TI-99/4A in 1981. It reminds me of a modern incarnation of vintage machines like the SWTPC 6800 and others.

It was a pure joy to build and get up and running. Now it's time to build the uTerm and design a case for everything to go into!

So this classic geek says THANK YOU, Just4Fun!

Here is my full build log, perhaps it will help someone. :)

https://theclassicgeek.blogspot.com/2022/09/the-z80-mbc2-retro-homebrew-computer.html

Hi this is a verry nice project. Good work. I have an error when trying to compile the arduino sketch. 

Arduino : 1.8.19 (Windows 10), Carte : "Arduino Mega or Mega 2560, ATmega2560 (Mega 2560)"

D:\Users\g4b3\Downloads\F11RKKSJNEVGGU4\S220718-R190918_IOS-Z80-MBC2\S220718-R190918_IOS-Z80-MBC2.ino: In function 'void setup()':

S220718-R190918_IOS-Z80-MBC2:690:3: error: 'TCCR2' was not declared in this scope

   TCCR2 |= (1 << CS20);                           // Set Timer2 clock to "no prescaling"

   ^~~~~

D:\Users\g4b3\Downloads\F11RKKSJNEVGGU4\S220718-R190918_IOS-Z80-MBC2\S220718-R190918_IOS-Z80-MBC2.ino:690:3: note: suggested alternative: 'TCCR2A'

   TCCR2 |= (1 << CS20);                           // Set Timer2 clock to "no prescaling"

   ^~~~~

   TCCR2A

S220718-R190918_IOS-Z80-MBC2:694:19: error: 'COM20' was not declared in this scope

   TCCR2 |= (1 <<  COM20);                         // Set "toggle OC2 on compare match"

                   ^~~~~

D:\Users\g4b3\Downloads\F11RKKSJNEVGGU4\S220718-R190918_IOS-Z80-MBC2\S220718-R190918_IOS-Z80-MBC2.ino:694:19: note: suggested alternative: 'COM2A0'

   TCCR2 |= (1 <<  COM20);                         // Set "toggle OC2 on compare match"

                   ^~~~~

                   COM2A0

S220718-R190918_IOS-Z80-MBC2:695:19: error: 'COM21' was not declared in this scope

   TCCR2 &= ~(1 << COM21);

                   ^~~~~

D:\Users\g4b3\Downloads\F11RKKSJNEVGGU4\S220718-R190918_IOS-Z80-MBC2\S220718-R190918_IOS-Z80-MBC2.ino:695:19: note: suggested alternative: 'COM2A1'

   TCCR2 &= ~(1 << COM21);

                   ^~~~~

                   COM2A1

S220718-R190918_IOS-Z80-MBC2:696:3: error: 'OCR2' was not declared in this scope

   OCR2 = clockMode;                               // Set the compare value to toggle OC2 (0 = low or 1 = high)

   ^~~~

D:\Users\g4b3\Downloads\F11RKKSJNEVGGU4\S220718-R190918_IOS-Z80-MBC2\S220718-R190918_IOS-Z80-MBC2.ino:696:3: note: suggested alternative: 'OCR2A'

   OCR2 = clockMode;                               // Set the compare value to toggle OC2 (0 = low or 1 = high)

   ^~~~

   OCR2A

exit status 1

'TCCR2' was not declared in this scope

Ce rapport pourrait être plus détaillé avec
l'option "Afficher les résultats détaillés de la compilation"
activée dans Fichier -> Préférences.

I am not an arduino  user. So if somebody can help please. My goal is to try using the board with a french Minitel and try to do a sort of BBS on it.

Thank you.

Hi, In order to compile the source in Arduino you need to install the correct core using the board manager. The core required is https://github.com/MCUdude/MightyCore the page has details on how to install the core. Once installed you need to select ATmega32 with an external 16MHz crystal. You need to select the same port as you connect on your terminal so you can't do both.

It looks like you have the wrong MCU set. It should be Atemga32A not Mega2560. It's good to use the facebook page, probably a faster responce.

make sure you have selected ATmega32, I guess you have installed MyghtyCore under arduino, these errors come when it does not recognize the MCU, you put atmega2560 card? you can not compile it on this card.

I am impressed by your SBC.  I have seen some of your YouTube videos demonstrating the construction and operation of the MBC2 and MBC3.

I would like to see some videos about the evolution of the designed.  For example, why did you chose the static ram that you did?  What compromises went into the choice?  Why did you choose this particular AVR?  Are there things that you put in the Flash memory of the chip that you could have put on the SD card or vise versa?

I noticed that in the Rev3 board you connected A8 of the Z80 to A12 of the RAM.  I assume that this was done to simplify PCB layout.  When did you make this decision, and where there any other considerations?  You bank the memory. How did you decide on the bank size (64k, 32k, 16k, etc)?  Z80 address A15 goes to the AVR. What advantages and difficulties does that cause?

All communications between the Z80 and AVR appears to be via I/O ports 0 and 1. What went into that design decision? What have been the advantages and disadvantages to that choice?

You went from an SR flip flop and a WAIT to driving an interrupt on the AVR. How has this been better?

I would love to see a multipart series of videos with schematics and code segments to explain everything that went in the design and its evolution.

Thank you for sharing your design with the rest of the world.

Is there a Z80-MBC3?

https://github.com/eprive/Z80-MBC3

Do you have a Chromebook? The Z80-MBC2 will most likely work with it. If the Chromebook doesn't immediately detect the board, try re-plugging it a few more times.

The Z80-MBC2 is working fine with my i7 ASUS Chromebox 3 under chromeOS 103. It's worth noting the board is detected only under Crostini Linux, not from the Android container or chromeOS itself.

Greetings all

I recently finished building a Z80-MBC2 and am having trouble with it.  When I apply power and it reset the IOS light blinks at about a 3Hz rate but there's no other sign of activity on the system except a short pulse when I press the reset button.  The ATmega32 seems to have been flashed successfully via an avrtiny.  Fuse bits are set properly.  Swapped out the atmega32 and Z80 chips, no joy.  Any pointers to what to check,or pointer to a discussion group or some such where I can discuss this more fully?

Thanks in advance

-Bob

I just finished building one yesterday and have exactly this issue.  I have double checked solder joints and begun looking for troubleshooting tips.  I also carefully compared my build with the detailed pictures in this project.  I am most suspicious of my serial connection since the board looks alive, like Bob's, and I successfully flashed the 32A with this a USBTinyISP programmer.  I have tried 1152008n1, 96008n1 with both IOS and IOS-lite.  Even tried multiple flashes of the 32A.  Next up for me is re-reading the info on this page and other's replies below and making time to investigate clocks and the like with an oscilloscope.  I'll update when I learn more about what I might have done wrong.

What os, if windows 11 then you need serial port drivers for the 2102 chip. You can see if the serial program is working ok by unplugging the mbc2 from the adapter put a dupont connector from RX to TX. Run your terminal software, make sure the keys you press are echoed back to the screen. You should see TX/RX led flash. If nothing then 99% driver problem. 

Hello guys! 

A year ago I managed to get this up and runing. 

Now I want to update the IOS, but I am unable to find a way to flash the ATMEGA32 from the included ISP header using Arduino as ISP. I am connecting both ISP headers from the arduino to the Z80-MBC2, and when I try to flash, it resets the MBC2, then waits, and throws: avrdude: stk500_getsync() attempt 6 of 10: not in sync: resp=0xe0

I know I could probably take out the ATMEGA and put it in a breadboard, throw in a crystal, and flash it, but I think there must be a combination of keypresses to put it into "bootloader" mode?

Thanks for any hint :)

Solved: I used a BusPirate to program with avrdude with no problem... it looks like Arduino as ISP does not quite work! THanks :)

Hi all.  I've succesfully built the MBC2 on breadboard and can get the OS Lite (no SD Card) working!  When I add the SD Card, and flash the full OS, the system starts, but hangs on the boot menu and reports SD Error 2 NOT_READY on MOUNT operation.  
I've doubled checked the wiring.  I can successfully use the SD Card module with PetitFS running on an Arduino Uno.  I can even flash PetitFS and run the test on to a brand new ATMega128.  So I know the SD Card reader is working.  It just won't work with the MBC2.
I have also checked the SCK, CS output on a logic analyser and from what I can see the clock is cycling and there are pulse on the CS, but no data on MISO or MOSI.  I'm stuck!    I'm using a 16GB micro SD card so not sure if that is too big??   Any help appreciated. Cheers

After lots of testing and cursing, I determined that the USB breadboard power supply unit (YuRobot) was only supply 3.3V even though it was jumped for five.  Turns out this unit needs to be feed 6.5-12V in order to output 5V!  I had powered directly from USB 5V which for some reason it steps down to 3.3V - not enough to power the SD Card, but apparently enough to power the Z80, RAM, and ATMega128.   I attached 5V and everything worked immediately!  Great project!

I know your post is over 3 years old. I had the same problem with the SD card and solved it as follows: There is a file called "pffArduino.h". In this file you search for SD_CS_PIN. The value is “10”. This value must be replaced with “4”. Then recompile and transfer to the ATMEGA32. The card reader will then work.
 I found this in an old post from Frank N. 21 Dec 2018. https://www.thebackshed.com/forum/ViewTopic_mobile.php?TID=10957&P=2

Hi all. I've successfully completed the Z8-MBC2 and it works fine. I've just completed the uTerm board and programmed the STM32 successfully following the instructions on the site. I have a problem now in the monitor says "no input detected".  I tried the test points TP1 - TP4. TP1 (RGB) I get 2.6v and nothing on TP3/4. Not sure what values I should have, can't find any info for these. Using a new VGA cable and a monitor that was working fine.  Not sure what to do next.  Any help would be appreciated. Thanks.

Check if the keyboard's led work (NUM LOCK, CAPS LOCK...). If not there is a problem with yours STM32.

J4F, many thanks for your prompt reply. Tried the keyboard, no lights! Resoldered the SMT32, doesn't look pretty but  SUCCESS!!!  Once again many thanks, regards Alex.

I have trouble in communicating with the board! Using putty (9600 bauds, 8 data bits, 1 stop bit, no parity, no flow control) I receive (after pressing the reset button) a flow of about 550 chars, but they are gibberish! LEDs are blinking "normally"' and the behavior is repeatable, so I presume the program is correctly loaded - how could I go further?

IOS serial wants 115200bps 8N1, IOS LITE 9600bps 8N1.

Hi j4f, great project, nice job. I assembled the kit bought by Mc John. It worked immediately as soon as mounted, but when I set the frequency of 8 MHz the bootstrap stops at "IOS Z80 is running from now". If I set the working frequency to 4 MHz then it works normally. Thanks for any suggestions.

Hi, this is a very unusual issue... Have you checked the board assembly if it is correctly assembled (values, solder joints, etc...)?  Check D8, D9 and R19 and R13 if they are the right values and correctly assembled. Then the only thing I can suggest is to try to change the Z80 CPU (remember only CMOS version!).

My changes are here: https://bitbucket.org/rudolff/ios-z80-mbc2/src/master/
Note: I connected /IORQ z80 to INT0 MCU

Interesting... Thanks for sharing.