Power supply control
Power
The power supply is controlled using an LPC915 accessed via I2C/SMBUS. The schematic shows a P89LPC915 microcontroller specifically. See: https://style.nxp.com/docs/en/user-guide/UM10107.pdf
Of more direct relevance to drivers interacting with this microcontroller is the I2C-accessible interface it is programmed to expose, described as follows.
I2C Interface
In the legacy (letux-400) kernel, the I2C address of the microcontroller is given as (0x50 >> 1) or 0x28.
The mipsbook-mcu.c driver seems to reference the following I2C-accessible addresses, registers or "commands" (as Linux calls them):
| Register | Purpose |
|---|---|
| 0xd8 | control (bit 0 written for power off) |
| 0xd9 | status (bit 0 provides status) |
| 0xdb | voltage level |
It is possible that the register numbers are coincidentally similar to the internal microcontroller registers (described in a section below), but they should not be confused.
See: https://web.archive.org/web/20090226101357/http://projects.kwaak.net/twiki/bin/view/Epc700/LPC915F
Voltage Level
LIPO battery with usable range 7.0V - 8.4V.
Battery level scaled by resistor divider to 3V from 8.4V with LPC915 powered by 3.3V (presumably used as reference), measuring the level in 255 increments (255 corresponds to 3.3V).
level in V = (3.3V / 255) * (8.4V / 3.0V) * value
level in µV = (3300000µv / 255) * (8400000µV / 3000000µV) * value
level in µV = 36235 * value
Without a battery installed, the voltage level merely reflects the supply voltage.
Shutdown (Power Down/Off)
Writing a byte with bit 0 set (for example, a value of 1) to I2C address 0xd8 will cause power to be disconnected after a short delay.
Microcontroller Internals
Registers related to I2C defined in the NXP documentation are given as follows:
| Register | Purpose |
|---|---|
| 0xd8 | control |
| 0xd9 | status (bits 7..3 provide status code) |
| 0xda | data |
| 0xdb | address |
These are special function registers in the microcontroller itself, not I2C-accessible addresses or registers.
Resources
The following files in the vendor kernel are informative:
- drivers/char/jzchar/minipc-misc.c
In modern kernels, the following files are pertinent:
- drivers/power/supply/mipsbook-mcu.c