參數(shù)資料
型號(hào)： AMC6821
英文描述： Analog Monitor and Control Circuit
中文描述： 模擬監(jiān)視和控制電路
文件頁(yè)數(shù)： 15/47頁(yè)
文件大?。?/td> 547K
代理商： AMC6821
www.ti.com
P
TACH-DATA Register
RPM =
(10000 x 60)
(Value of TACH-DATA Register)
(2)
Clock
Measurement Period for 4 Pulses Per Revolution
TACH Pulse
PWM
Measurement Period
for 2 Pulses/Revolution
TACH Pulse
Measurement
Starts
Clock
Measurement Period for 4 Pulses Per Revolution
Measurement Period
for 2 Pulses/Revolution
RPM Measurement for TACH-MODE = 0
RPM Measurement for TACH-MODE = 1
b) Measuring the Period of TACH Pulses to Determine the Fan Speed
Signal Conditioning
FAN Speed Counter
TACH
PWM
Control
PWM-Out
TACH Data
TACH
Output
+5V
+5V
START
TACH-EN
AMC6821
a) Block Diagram of Fan Speed Monitoring
AMC6821
SBAS386A–MAY 2006–REVISED MAY 2006
Two fan tach pulse periods (PSPR = 0) or four tach pulse periods (PSPR = 1) are measured and the result is
stored in the
TACH-DATA Register
, as shown in
Figure 8
. Counting stops if the counter is over-range; the
measurement cycle repeats until monitoring is disabled, and the fan speed (RPM) can be calculated as shown in
Equation 2
:
Reading the TACH Data Register
To read the fan speed, both TACH-DATA-LByte and TACH-DATA-HByte must be read. TACH-DATA-LByte
must be read first. This reading causes TACH-DATA-HByte to be frozen until both the high and low byte
registers have been read from; this method prevents erroneous TACH readings.
RPM Measurement Rate
The TACH-FAST bit of
Configuration Register 4
determines the rate. When TACH-FAST = 1, the
TACH-DATA
Register
is updated every 250ms (fast monitoring). When TACH-FAST = 0 (default), the reading is updated
every second (standard monitoring period).
Select Number of Pulses/Revolution
The speed sensor of most common fans provides two or four TACH pulses per revolution. The PSPR bit of
Configuration Register 4
specifies how many pulses per revolution are generated. PSPR = 1 indicates four
pulses/revolution and PSPR = 0 (default) indicates two pulses/revolution.
TACH Mode Selection
The TACH-MODE bit of
Configuration Register 2
specifies the TACH pulse output mode of the fan. Some fans
(such as three- and two-wire) are powered directly by the PWM, and must be
PWM-On
to provide a TACH pulse
output. When the PWM-Out pin switches these fans ON/OFF directly, the PWM-Out must be kept ON to power
the fan during the measurement. In this case, the TACH-MODE bit of
Configuration Register 2
must be cleared
('0'). When TACH-MODE = 0, the PWM-Out pin is kept ON during the measurement period. Clearing the TACH
mode ('0') also enables the internal correction circuitry to correct the error caused by the extra duty cycle applied
in the measurement period. The power-on default value of the PWM mode is 0.
Figure 8. Fan Speed Measurement
15
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