Detecting Multiple Voltages Using the PIC10F204/206 Comparator

TB083
Detecting Multiple Voltages Using the
PIC10F204/206 Comparator
Author:
Roy Sasaki
Michael Waldron
Microchip Technology Inc.
INTRODUCTION
The PIC10F204/206 microcontrollers have a built-in
comparator which can be utilized to detect different
discrete voltage levels. This technical brief describes
one technique for detecting two or more such voltages
as different trip points.
THEORY
This technique takes advantage of the properties of an
RC network. An RC network, which is charged up to a
known voltage, will have a voltage decay governed by
an RC time constant, as shown in Equation 1.
EQUATION 1:
VOUT = VIN * e -t/RC
VOUT = Volts, R = Ohms, t = Seconds, VIN = Volts,
C = Farads
If the voltages and values of R and C are known, then
the time t may be determined by re-evaluating the
equation shown in Equation 2.
EQUATION 2:
t = -1*RC * In (VOUT/VIN)
HARDWARE TECHNIQUE
Pins GP0/CIN+ and GP1/CIN-, of the PIC10F204/206
microcontrollers, can be switched between Digital
mode and Analog mode under firmware control. By
placing a known RC network on GP0/CIN+ and the
voltage to be sampled on GP1/CIN-, firmware can
detect discrete voltages.
Application firmware sets pin GP0/CIN+ to Digital mode
and applies a PWM signal to the pin to charge up the
RC network to a known voltage higher than the sample
voltage to be detected. Once the RC network is
charged, pin GP0/CIN+ is switched to Analog mode
and a comparator read made. By starting a timer and
keeping track of the time interval for the comparator to
trip, a time is determined which can be referenced to a
look-up table in firmware for various voltage values.
The size of the look-up table should be adjusted to
cover the voltage regions of interest.
Application firmware will take different actions at
different trip points such as providing a warning and
then a shutdown.
In order to switch pin GP0/CIN+ from Analog mode to
Digital mode and back again, use the CMPON control
bit in the CMCON0 register (CMCON0<3>). By turning
the comparator on, the pin is set to Analog mode. By
turning the comparator off, the pin is set to Digital
mode.
Note:
The sampled voltage is assumed to be
stable during the sampling period. Users
should setup their firmware and look-up
tables to search for time ranges rather
than exact time intervals to adjust for
sample voltages which drift.
By knowing the time required for a given VOUT, a lookup table, (see AN556, “Implementing A Table Read”), in
firmware, equates a given time to a given voltage.
Using discrete times of interest, trip points are created
which a microcontroller acts upon.
 2004 Microchip Technology Inc.
DS91083A-page 1
TB083
FIGURE 1:
EXAMPLE CIRCUIT
VDD
10K
R2
CIN+
GP0
C1
R1
VSAMPLE
470Ω
VDD
Red
CINGP1
GP2
Yellow
GP3
470Ω
VSS
PIC10F204/206
Button
EXAMPLE CIRCUIT
CONCLUSION
In Figure 1, GP2 is used to drive both a warning yellow
LED as well as a shut-down LED. A push button is
present on GP3 to trigger a sample to take place.
Detecting multiple discrete voltages is easily accomplished using the hardware and firmware technique
described above. The capability to switch the comparator input pins from Analog Input mode to Digital
Output mode and back again allow for a polled
sampling scheme.
The appropriate values for R1, R2 and C1 should be
determined for the sample voltages and timings of
interest. Correspondingly, the timer used to track the
decay time and look-up table will need to be sized.
DS91083A-page 2
Note:
Voltage detection during Sleep is possible,
but requires the user to cycle through the
process of charging up the RC network
compensating for the time delays in Reset,
Reset service routine and re-establishing
wake-on-comparator.
 2004 Microchip Technology Inc.
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DS91083A-page 3
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DS91083A-page 4
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