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diff --git a/Documentation/hwmon/kbatt.rst b/Documentation/hwmon/kbatt.rst new file mode 100644 index 000000000000..b72718c5ede3 --- /dev/null +++ b/Documentation/hwmon/kbatt.rst @@ -0,0 +1,60 @@ +.. SPDX-License-Identifier: GPL-2.0 + +Kernel driver kbatt +=================== + +Supported chips: + + * KEBA battery monitoring controller (IP core in FPGA) + + Prefix: 'kbatt' + +Authors: + + Gerhard Engleder <eg@keba.com> + Petar Bojanic <boja@keba.com> + +Description +----------- + +The KEBA battery monitoring controller is an IP core for FPGAs, which +monitors the health of a coin cell battery. The coin cell battery is +typically used to supply the RTC during power off to keep the current +time. E.g., the CP500 FPGA includes this IP core to monitor the coin cell +battery of PLCs and the corresponding cp500 driver creates an auxiliary +device for the kbatt driver. + +This driver provides information about the coin cell battery health to +user space. Actually the user space shall be informed that the coin cell +battery is nearly empty and needs to be replaced. + +The coin cell battery must be tested actively to get to know if its nearly +empty or not. Therefore, a load is put on the coin cell battery and the +resulting voltage is evaluated. This evaluation is done by some hard wired +analog logic, which compares the voltage to a defined limit. If the +voltage is above the limit, then the coin cell battery is assumed to be +ok. If the voltage is below the limit, then the coin cell battery is +nearly empty (or broken, removed, ...) and shall be replaced by a new one. +The KEBA battery monitoring controller allows to start the test of the +coin cell battery and to get the result if the voltage is above or below +the limit. The actual voltage is not available. Only the information if +the 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 the +coin cell battery is reduced directly by the duration of each test. To +limit the negative impact to the lifetime the test is limited to at most +once every 10 seconds. The test load is put on the coin cell battery for +100ms. Thus, in worst case the coin cell battery lifetime is reduced by +1% of the uptime or 3.65 days per year. As the coin cell battery lasts +multiple years, this lifetime reduction negligible. + +This driver only provides a single alarm attribute, which is raised when +the coin cell battery is nearly empty. + +====================== ==== =================================================== +Attribute R/W Contents +====================== ==== =================================================== +in0_min_alarm R voltage of coin cell battery under load is below + limit +====================== ==== =================================================== |