Legacy-Free Keyboard/Embedded Controller with SPI and LPC Docking Interface
19.2.2 Fan Pulse Counter and Read Latch
The fan pulse counter measures the fan pulse time, T shown in Figure 19.1. The fan pulse counter is
p,
reset by the rising edge of each fan tachometer input pulse and by writing the counter preload register.
The fan pulse counter does not wrap. For example, when the counter reaches 0xFF, it remains at 0xFF
until the counter is reset by the next input pulse or by writing the Preload register. The host can read
the last maximum fan pulse counter value using the FAN1 and FAN2 Read Latch registers (see Section
19.3, "Example" and Section 19.4, "FAN1 Read Latch Register", below). The fan pulse counter equation
is shown in Table 19.1. The factor of ½ in first term of the equation accounts for the fan Revolution
Time T (i.e., two pulses per revolution as shown in Figure 19.1). The numerator of the second term
R
is derived by multiplying the 32.768kHz timebase by 60sec/min. The denominator of the second term
is the product of the fan RPM and the timebase prescaler.
19.2.3 Fan Pulse Counter Threshold Detector
The fan pulse counter threshold detector consists of the two AND’ed MSB outputs of the fan pulse
counter. This corresponds to an upper limit for the fan pulse counter of 192. The outputs of the fan
pulse counter threshold detectors are always asserted when the fan pulse count equals or exceeds 192.
The outputs of the fan pulse counter threshold detectors are always deasserted when the fan pulse
count is less than 192. If enabled, the 8051 receives an interrupt when the outputs of the fan pulse
counter threshold detectors are asserted. For a description of the FAN TACH1 and FAN TACH2 8051
interrupt registers see Section 19.9, "8051 FAN Tachometer Interrupt Registers".
Table 19.1 Fan Pulse Counter Equation
6
FAN PULSE COUNT =
1
2
X
1.966 x 10
RPM x PRESCALER
19.2.4 Fan Pulse Counter Preload
The fan pulse counter preload is the initial value for the fan pulse counter which is used to scale the
count so that the value of 192 corresponds to the “lower limit” of the threshold RPM. The fan pulse
counter is initialized with the preload on the rising edge of the fan tachometer input pulse. Typically, the
fan pulse counter preload value will be 192 minus the fan pulse count of the desired RPM trigger
threshold. The counter preload value is programmable for each fan tachometer via the FAN1 and FAN2
Preload Registers (see Section 19.5, "FAN2 Read Latch Register" and Section 19.7, "FAN2 Preload
Register"). By setting the fan pulse counter preload value and the timebase prescaler appropriately, the
8051 can be interrupted when the fan speed reaches the desired percentage of the nominal RPM to
indicate fan failures for a wide range of fan types and speeds (see Section 19.3, "Example").
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