Rainbow Electronics DS1780 User Manual
Page 11
DS1780
11 of 28
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
Timer Per Revolution
Counts for “Divide by 2” (default mode) in decimal
Comments
4400
13.64 ms
153
Typical RPM
3080
19.48 ms
219
70% RPM
2640
22.73 ms
255 (maximum)
60% RPM
VOLTAGE/DATA RELATIONSHIPS FOR FAN INPUTS (FAN1, FAN 2) Table 8
MODE
SELECT
NOMINAL
RPM
TIME PER
REVOLUTION
COUNTS FOR THE
NOMINAL
SPEED IN DECIMAL
70% RPM
TIME PER
REVOLUTION FOR
70% RPM
Divide by 1
8800
6.82 ms
153
6160
9.74 ms
Divide by 2
4400
13.64 ms
153
3080
19.48 ms
Divide by 4
2200
27.27 ms
153
1540
38.96 ms
Divide by 8
1100
54.54 ms
153
770
77.92 ms
In general, the value stored in the FAN registers (in decimal) follows the equation:
Count (RPM, Divisor)
≅
Divisor
RPM
10
x
1.35
6
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.