Description¶

The KEBA battery monitoring controller is an IP core for FPGAs, whichmonitors the health of a coin cell battery. The coin cell battery istypically used to supply the RTC during power off to keep the currenttime. E.g., the CP500 FPGA includes this IP core to monitor the coin cellbattery of PLCs and the corresponding cp500 driver creates an auxiliarydevice for the kbatt driver.

This driver provides information about the coin cell battery health touser space. Actually the user space shall be informed that the coin cellbattery is nearly empty and needs to be replaced.

The coin cell battery must be tested actively to get to know if its nearlyempty or not. Therefore, a load is put on the coin cell battery and theresulting voltage is evaluated. This evaluation is done by some hard wiredanalog logic, which compares the voltage to a defined limit. If thevoltage is above the limit, then the coin cell battery is assumed to beok. If the voltage is below the limit, then the coin cell battery isnearly empty (or broken, removed, ...) and shall be replaced by a new one.The KEBA battery monitoring controller allows to start the test of thecoin cell battery and to get the result if the voltage is above or belowthe limit. The actual voltage is not available. Only the information ifthe voltage is below a limit is available.

The test load, which is put on the coin cell battery for the health check,is similar to the load during power off. Therefore, the lifetime of thecoin cell battery is reduced directly by the duration of each test. Tolimit the negative impact to the lifetime the test is limited to at mostonce every 10 seconds. The test load is put on the coin cell battery for100ms. Thus, in worst case the coin cell battery lifetime is reduced by1% of the uptime or 3.65 days per year. As the coin cell battery lastsmultiple years, this lifetime reduction negligible.

This driver only provides a single alarm attribute, which is raised whenthe coin cell battery is nearly empty.