MEC1322 Datasheet, PDF (294/456 Pages) Microchip Technology – Keyboard and Embedded Controller for Notebook PC

MEC1322
26.8.3.2 Tachometer Measurement
In both modes of operation, the tachometer measurement operates independently of the mode of operation of the fan
driver and RPM based Fan Speed Control algorithm. Any tachometer reading that is higher than the Valid TACH Count
(see Section 26.9.9, "Valid TACH Count Register") will flag a stalled fan and trigger an interrupt.
When measuring the tachometer, the fan must provide a valid tachometer signal at all times to ensure proper operation.
The tachometer measurement circuitry is programmable to detect the fan speed of a variety of fan configurations and
architectures including 1-pole, 2-pole (default), 3-pole, and 4-pole fans.
APPLICATION NOTE: The tachometer measurement works independently of the drive settings. If the device is put
into manual mode and the fan drive is set at a level that is lower than the fan can operate
(including zero drive), the tachometer measurement may signal a Stalled Fan condition and
assert an interrupt.
STALLED FAN
If the TACH Reading Register exceeds the user-programmable Valid TACH Count setting, it will flag the fan as stalled
and trigger an interrupt. If the RPM based Fan Control Algorithm is enabled, the algorithm will automatically attempt to
restart the fan until it detects a valid tachometer level or is disabled.
The FAN_STALL Status bit indicates that a stalled fan was detected. This bit is checked conditionally depending on the
mode of operation.
• Whenever the Manual Mode is enabled or whenever the drive value is changed from 00h, the FAN_STALL inter-
rupt will be masked for the duration of the programmed Spin Up Time (see Table 26-17, “Spin time,” on page 303)
to allow the fan an opportunity to reach a valid speed without generating unnecessary interrupts.
• In Manual Mode, whenever the TACH Reading Register exceeds the Valid TACH Count Register setting, the
FAN_STALL status bit will be set.
• When the RPM based Fan Control Algorithm, the stalled fan condition is checked whenever the Update Time is
met and the fan drive setting is updated. It is not a continuous check.
26.8.3.3 Spin Up Routine
The RPM-PWM Interface also contains programmable circuitry to control the spin up behavior of the fan driver to ensure
proper fan operation. The Spin Up Routine is initiated under the following conditions:
• The TACH Target High Byte Register value changes from a value of FFh to a value that is less than the Valid
TACH Count (see Section 26.9.9, "Valid TACH Count Register").
• The RPM based Fan Control Algorithm’s measured tachometer reading is greater than the Valid TACH Count.
• When in Manual Mode, the Drive Setting changes from a value of 00h.
When the Spin Up Routine is operating, the fan driver is set to full scale for one quarter of the total user defined spin up
time. For the remaining spin up time, the fan driver output is set a a user defined level (30% to 65% drive).
After the Spin Up Routine has finished, the RPM-PWM Interface measures the tachometer. If the measured tachometer
reading is higher than the Valid TACH Count Register setting, the FAN_SPIN status bit is set and the Spin Up Routine
will automatically attempt to restart the fan.
APPLICATION NOTE: When the device is operating in manual mode, the FAN_SPIN status bit may be set if the
fan drive is set at a level that is lower than the fan can operate (excluding zero drive which
disables the fan driver). If the FAN_SPIN interrupt is unmasked, this condition will trigger an
errant interrupt.
Figure 26-3, "Spin Up Routine" shows an example of the Spin Up Routine in response to a programmed fan speed
change based on the first condition above.
DS00001719D-page 294
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