Power supply control

Power

The power supply is controlled using an LPC915 accessed via I2C/SMBUS. See: https://style.nxp.com/docs/en/user-guide/UM10107.pdf

Controller Registers

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.

However, 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	battery voltage level

It is possible that these values are coincidentally similar to the internal microcontroller registers.

The schematic shows the P89LPC915 microcontroller.

In the legacy (letux-400) kernel, the I2C address of the microcontroller is given as (0x50 >> 1) or 0x28, with registers as follows:

Register	Purpose
0xd8	shutdown - bit 0 written for power off
0xd9	battery charge
0xdb	battery status

The status register appears to provide the battery level.

See: https://web.archive.org/web/20090226101357/http://projects.kwaak.net/twiki/bin/view/Epc700/LPC915F

Battery Status

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

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