Legacy-Free Keyboard/Embedded Controller with SPI and LPC Docking Interface
PRELOAD
PRESCALER
32 kHz
FAN PULSE
COUNTER
FAN_TACHn
↑ LOAD
MSB
SYNC
8051
READ LATCH
Figure 19.2 Fan Tachometer Block Diagram
19.3
Example
Table 19.2 illustrates a fan tachometer interface programming example for a 4400 RPM nominal fan. In
this example, the system designer has specified a fan counter preload value of 33 so that the 8051 will
be alerted when the fan speed drops below 70% nominal RPM.
Note: The values in Table 19.2 are based on a 2 pulse/revolution fan tachometer output with the
default timebase prescaler of 2.
Table 19.2 4400 RPM Fan Tachometer Example
PULSE
TIME
FAN
PULSE
COUNT
Note 19.2
RPM
(TR)
Note 19.1
(TP)
FAN PULSE COUNT
+ PRELOAD
8051
PRELOAD
DESCRIPTION
INTERRUPT
4400
3960
6.8 ms
7.6 ms
111
124
33
144
157
Nominal RPM
NO
90% Nominal
RPM
3520
3080
2640
2200
8.5 ms
9.7 ms
139
159
186
223
172
192
219
80% Nominal
RPM
70% Nominal
RPM
YES
11.4 ms
13.6 ms
60% Nominal
RPM
>255 (maximum
count)
50% Nominal
RPM
Note 19.1 There are 2 fan tachometer pulses per fan revolution: T = 60 ÷ (2 × RPM).
P
Note 19.2 The timebase prescaler = 2.
SMSC LPC47N350
213
Revision 1.1 (01-14-03)
DATASHEET
SMSC [ SMSC CORPORATION ]
