29-09-2010, 04:57 PM
[attachment=4630]
fan speed control system
INTRODUCTION
Fan TachometerThe embedded controller registers 0x84 (LSB), 0x85 (MSB) are the main fan tachometer, and report fan speed in RPM in everything since the A31 and maybe a little earlier. Not much is know about the tachometer in earlier models, or even whether they had one or not.
On the X61/X61s, one must select through EC register 0x31 bit 0 which fan the tachometer registers will expose (Firmware 7M). Beware: this is the same register used for brightness control in other models.
Automatic mode
In automatic mode, the embedded controller sets the fan speed automatically according to system temperatures and some unknown algorithm.
Note that the ACPI DSDT may supplement this in some models. The X40, for example, changes the profile of speeds the automatic mode should use depending on battery status.
Manual mode
In manual mode, the fan level is forced to the given value and the EC will auto-regulate the fan to maintain at a (roughly) constant RPM, which is model-dependent. Manual speed levels 8-63 yield the same behavior as level 7, and the the ACPI DSDT uses level 7 for the emergency mode it enters upon critical CPU/GPU temperature, so apparently 7 is the real maximum level.
Disengaged (full-speed) mode
In disengaged mode, the embedded controller does not monitor the fan speed. It "disengages" the closed-loop control function that keeps track of fan speed, and uses an open-loop control function that ramps up the fan to its maximum speed (100% duty-cycle). The end speed is not stable, but it is often much faster than the maximum speed manual and automatic modes would set the fan to.
Most newer ThinkPads take quite a while (in excess of one minute) to fully enter disengaged mode. Exiting it is much faster. The A31 acts differently, and switches to disengaged mode as fast as it switches to other modes.
For some reason, the embedded controller may stop updating the tachometer registers while entering or exiting disengaged mode in some ThinkPad firmware versions (hence the EC tachometer registers will not be updated on these models while entering/exiting disengaged mode). Once it arrives at maximum speed, or once it gets back at closed-loop cruise speed, the embedded controller starts updating the tachometer registers again. Later T models such as the T43 have this problem, while the A31 does not.
On the X61/X61s, one must select through EC register 0x31 bit 0 which fan the tachometer registers will expose (Firmware 7M). Beware: this is the same register used for brightness control in other models.
Automatic mode
In automatic mode, the embedded controller sets the fan speed automatically according to system temperatures and some unknown algorithm.
Note that the ACPI DSDT may supplement this in some models. The X40, for example, changes the profile of speeds the automatic mode should use depending on battery status.
Manual mode
In manual mode, the fan level is forced to the given value and the EC will auto-regulate the fan to maintain at a (roughly) constant RPM, which is model-dependent. Manual speed levels 8-63 yield the same behavior as level 7, and the the ACPI DSDT uses level 7 for the emergency mode it enters upon critical CPU/GPU temperature, so apparently 7 is the real maximum level.
Disengaged (full-speed) mode
In disengaged mode, the embedded controller does not monitor the fan speed. It "disengages" the closed-loop control function that keeps track of fan speed, and uses an open-loop control function that ramps up the fan to its maximum speed (100% duty-cycle). The end speed is not stable, but it is often much faster than the maximum speed manual and automatic modes would set the fan to.
Most newer ThinkPads take quite a while (in excess of one minute) to fully enter disengaged mode. Exiting it is much faster. The A31 acts differently, and switches to disengaged mode as fast as it switches to other modes.
For some reason, the embedded controller may stop updating the tachometer registers while entering or exiting disengaged mode in some ThinkPad firmware versions (hence the EC tachometer registers will not be updated on these models while entering/exiting disengaged mode). Once it arrives at maximum speed, or once it gets back at closed-loop cruise speed, the embedded controller starts updating the tachometer registers again. Later T models such as the T43 have this problem, while the A31 does not.
