SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 235

• Manufacturer Part Number: SCH3112I-NE
• Description: LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
• Manufacturer: SMSC [SMSC Corporation]

LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
Datasheet
SMSC SCH311X
23.14.2.5
Internal PWM
Window for
Valid Tach
Tach
Pulses
Tach
Pulses
Pulses
Signal
ASSUMPTIONS (REFER TO FIGURE 4 - PWM AND TACHOMETER CONCEPT):
The Tachometer pulse generates 5 transitions per fan revolution (i.e., two fan tachometer periods per
revolution, edges 2 → 6). One half of a revolution (one tachometer period) is equivalent to three edges
(2 → 4 or 3 → 5). One quarter of a revolution (one-half tachometer period) is equivalent to two edges.
To obtain the fan speed, count the number of 90Khz pulses that occurs between 2 edges i.e., 2 → 3,
between 3 edges i.e., 2 → 4, or between 5 edges, i.e. 2 → 6 (the case of 9 edges is not shown). The
time from 1-2 occurs through the guard time and is not to be used. For the discussion below, an edge
is a high-to-low or low-to-high transition (edges are numbered – refer to Figure 4 - PWM and
Tachometer Concept
The Tachometer circuit begins monitoring the tach when the associated PWM output transitions high
and the guard time has expired. Each tach circuit will continue monitoring until either the “ON” time
ends or the programmed number of edges has been detected, whichever comes first.
The Fan Tachometer value may be updated every 300ms, 500ms, or 1000ms.
FAN TACHOMETER OPTIONS FOR MODE 2
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■
■
■
■
Fan Tachometer Reading Registers:
The Tachometer Reading registers are 16 bits, unsigned. When one byte of a 16-bit register is read,
the other byte latches the current value until it is read, in order to ensure a valid reading. The order
An interrupt will only be generated if no valid readings were made during the programmed update
time.
2, 3, 5 or 9 “edges” to calculate the fan speed (Figure 4)
Guard time A is programmable (8-63 clocks) to account for delays in the system (Figure 4)
Suggested PWM frequencies for mode 2 are: 11.0 Hz, 14.6 Hz, 21.9 Hz, 29.3 Hz, 35.2 Hz, 44.0
Hz, 58.6 Hz, 87.7Hz
Option to ignore first 3 tachometer edges after guard time
Option to force tach reading register to FFFEh to indicate a slow fan.
1
A
Figure 23.11 PWM and Tachometer Concept
B
2
Guard time A
C
3
DATASHEET
PWM “ON”
219
D
4
E
5
F
6
Rev 0.2 (09-28-04)