MEC1322 Datasheet, PDF (305/456 Pages) Microchip Technology – Keyboard and Embedded Controller for Notebook PC

MEC1322
Offset ^0Ah
Bits
Description
15:3 FAN_DRIVE_FAIL_BAND[12:0]
The number of Tach counts used by the Fan Drive Fail detection cir-
cuitry
2:0 RESERVED
Type
RES
R/W
Default
Reset
Event
000000000 VCC1_R
0000b
ESET
000b
-
26.9.11 TACH TARGET REGISTER
The TACH Target Registers hold the target tachometer value that is maintained for the RPM based Fan Control Algo-
rithm.
If the algorithm is enabled, setting the TACH Target Register High Byte to FFh will disable the fan driver (or set the PWM
duty cycle to 0%). Setting the TACH Target to any other value (from a setting of FFh) will cause the algorithm to invoke
the Spin Up Routine after which it will function normally.
These registers only apply if the Fan Speed Control Algorithm is used.
Offset ^0Ch
Bits
Description
Type
Default
Reset
Event
15:3 TACH_TARGET[12:0]
The target tachometer value.
2:0 RESERVED
RES
R/W
1111111111 VCC1_R
111b
ESET
000b
-
26.9.12 TACH READING REGISTER
The TACH Reading Registers’ contents describe the current tachometer reading for the fan. By default, the data rep-
resents the fan speed as the number of 32.768kHz clock periods that occur for a single revolution of the fan.
The Equation below shows the detailed conversion from tachometer measurement (COUNT) to RPM.
RPM = _P-----_o-1--_l--_e---_s- × _C-----_O---⁽--_U-ⁿ---_N-–----_T-1---^)×-----_m--1---- × fTACH × 60
where:
- Poles = number of poles of the fan (typically 2)
- fTACH = the frequency of the tachometer measurement clock
- n = number of edges measured (typically 5 for a 2 pole fan)
- m = the multiplier defined by the RANGE bits
- COUNT = TACH Reading Register value (in decimal)
The following equation shows the simplified translation of the TACH Reading Register count to RPM assuming a 2-pole
fan, measuring 5 edges, with a frequency of 32.768kHz.
RPM= ^3---9---_C-3---2_O---1--_U-6---0_N----^×-_T----^m---
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DS00001719D-page 305