MEC1404 Datasheet, PDF (370/572 Pages) Microchip Technology – Keyboard and Embedded Controller Products for Notebook PC

MEC140X/1X
FIGURE 26-2:
FAN GENERATED 50%DUTY CYCLE WAVEFORM
one revolution
In typical systems, the fans are powered by the main power supply. Firmware may disable this block when it detects that
the main power rail has been turned off by either clearing the <enable> TACH_ENABLE bit or putting the block to sleep
via the supported Low Power Mode interface (see Low Power Modes).
26.10.1 MODES OF OPERATION
The Tachometer block supports two modes of operation. The mode of operation is selected via the TACH_READING_-
MODE_SELECT bit.
26.10.1.1 Free Running Counter
In Mode 0, the Tachometer block uses the TACHx input as the clock source for the internal TACH pulse counter (see
TACHX_COUNTER). The counter is incremented when it detects a rising edge on the TACH input. In this mode, the
firmware may periodically poll the TACHX_COUNTER field to determine the average speed over a period of time. The
firmware must store the previous reading and the current reading to compute the number of pulses detected over a
period of time. In this mode, the counter continuously increments until it reaches FFFFh. It then wraps back to 0000h
and continues counting. The firmware must ensure that the sample rate is greater than the time it takes for the counter
to wrap back to the starting point.
Note: Tach interrupts should be disabled in Mode 0.
26.10.1.2 Mode 1 -- Number of Clock Pulses per Revolution
In Mode 1, the Tachometer block uses its 100kHz_Clk clock input to measure the programmable number of TACHx
pulses. In this mode, the internal TACH pulse counter (TACHX_COUNTER) returns the value in number of 100kHz_Clk
pulses per programmed number of TACH_EDGES. For fans that generate two square waves per revolution, these bits
should be configured to five edges.
When the number of edges is detected, the counter is latched and the COUNT_READY_STATUS bit is asserted. If the
COUNT_READY_INT_EN bit is set a TACH interrupt event will be generated.
26.10.2 OUT-OF-LIMIT EVENTS
The Tachometer Block has a pair of limit registers that may be configured to generate an event if the Tachometer indi-
cates that the fan is operating too slow or too fast. If the TACHX_COUNTER exceeds one of the programmed limits, the
TACHx High Limit Register and the TACHx Low Limit Register, the bit TACH_OUT_OF_LIMIT_STATUS will be set. If
the TACH_OUT_OF_LIMIT_STATUS bit is set, the Tachometer block will generate an interrupt event.
26.11 EC-Only Registers
The registers listed in the EC-Only Register Summary table are for a single instance of the Tachometer. The addresses
of each register listed in this table are defined as a relative offset to the host “Base Address” defined in the EC-Only
Register Base Address Table.
DS00001956D-page 370
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