DS1780 Datasheet, PDF (11/28 Pages) Dallas Semiconductor – CPU Peripheral Monitor

DS1780
maximum possible input up to V_DD for best noise immunity. Alternatively, use a shunt reference or Zener
diode to clamp the input level.
Tables 7 and 8 describe the format of the data stored in the FAN reading registers (Internal Address
Registers 28h and 29h).
FAN TACHOMETER INPUT OPTIONS Figure 3
VOLTAGE/DATA RELATIONSHIPS FOR FAN INPUTS (FAN1, FAN2) UNDER
DEFAULT (÷2) MODE Table 7
RPM
4400
3080
2640
Timer Per Revolution
13.64 ms
19.48 ms
22.73 ms
Counts for “Divide by 2” (default mode) in decimal
153
219
255 (maximum)
Comments
Typical RPM
70% RPM
60% RPM
VOLTAGE/DATA RELATIONSHIPS FOR FAN INPUTS (FAN1, FAN 2) Table 8
MODE
SELECT
Divide by 1
Divide by 2
Divide by 4
Divide by 8
NOMINAL
RPM
8800
4400
2200
1100
TIME PER
REVOLUTION
6.82 ms
13.64 ms
27.27 ms
54.54 ms
COUNTS FOR THE
NOMINAL
SPEED IN DECIMAL
153
153
153
153
70% RPM
6160
3080
1540
770
TIME PER
REVOLUTION FOR
70% RPM
9.74 ms
19.48 ms
38.96 ms
77.92 ms
In general, the value stored in the FAN registers (in decimal) follows the equation:
Count (RPM, Divisor) ≅
1.35 x 10⁶
RPM Divisor
If fans can be powered while the power to the DS1780 is off, the DS1780 inputs will provide diode
clamping. Limit input current to the Input Current at Any Pin specification shown in the ABSOLUTE
MAXIMUM RATINGS section. In most cases, open collector outputs with pullup resistors inherently
limit this current. If this maximum current could be exceeded, either a larger pullup resistor should be
used or resistors connected in series with the fan inputs.
The Fan Inputs gate an internal 22.5 kHz oscillator for one period of the Fan signal into an 8-bit counter
(maximum count = 255). The default divisor, located in the Fan Divisor/RST_Register, is set to 2
(choices are 1, 2, 4, and 8) providing a nominal count of 153 for a 4400 rpm fan with two pulses per
revolution. Typical practice is to consider 70% of normal RPM a fan failure, at which point the count will
be 219.
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