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VOLTAGE COMPARATORS
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The simplest way of inputting analog voltages in the PIC microcontroller is to use the optional comparators that indicate when a voltage is greater than another voltage. The inputs compared can be switched between different I/O pins as well as ground or a reference voltage that can be generated inside the PIC microcontroller chip. Enabling built-in comparators is a very straightforward operation, with the only prerequisite being that the pins used for the analog compare must be in input mode. Comparator response is virtually instantaneous, which allows alarm or other fast responses from changes in the comparator inputs. The comparator works very conventionally, as shown in Fig. 16.24. If the value of the +input is greater than the input, the output is high. There are two comparators in the PIC16C82X controlled by the CMCON register, which is de ned in Table 16.13. The CIS and CM2 CM0 bits work together to select the operation of the comparators, and the resulting I/O pin con gurations are listed in Table 16.14. On power-up, the comparator CM bits are all reset, which means that RA0 to RA3 are in analog input mode. If you want to disable analog input, the CM bits must be set (write 0x007 to CMCOM). Interrupts can be enabled that will interrupt the processor when one of the comparator output changes. This is enabled differently for each PIC microcontroller with built-in comparators. Like the PORTB change on interrupt, after a comparator change interrupt request has been received, the CMCOM register must be read to reset the interrupt handler.
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ANALOG I/O
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Input Signal Vref
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+ Comparator
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Comparator Output Input Signal
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Vref
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Comparator Output
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Figure 16.24 A comparator returns a digital value each time an analog voltage is at a speci c value.
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Along with comparing to external values, the PIC16C62x also can generate a reference voltage (Vref in the preceding table) using its own built-in 4-bit digital-to-analog converter. The digital-to-analog converter circuit is shown in Fig. 16.25. The Vref control bits are found in the VRCON register and are de ned in Table 16.15. The Vref output depends on the state of the VRR bit. The Vref voltage output can be expressed mathematically if VRR is set as
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Vref = Vdd*(Vfcon & 0x00F)/24
or if it is reset as
Vref = Vdd*(8 + (Vrcon & 0x00F))/32
Note that when VRR is set, the maximum voltage of Vref is 15/24 of Vdd, or just less than two-thirds Vdd. When VRR is reset, Vref can be almost three-quarters of Vdd.
TABLE 16.13 COMPARATOR CMCON REGISTER BIT DEFINITIONS BIT DESCRIPTION
7 6 5 4 3 2 0
C2OUT Comparator 2 output (high if + > ). C1OUT Comparator 1 output (high if + > ). Unused CIS Comparator input switch. CM2 CM0 Comparator mode.
PIC MCU OPTIONAL HARDWARE FEATURES
TABLE 16.14
COMPARATOR MODULE I/O SPECIFICATION COMP +INPUT COMP INPUT COMP 2 INPUT COMP 2 INPUT
000 001 001 010 010 011 100 101 110 111
X 0 1 0 1 X X X X X
RA0 RA2 RA2 Vref Vref RA2 RA3 Don t care RA2 RA3
RA3a RA0 RA3 RA3 RA3 RA0
RA2b RA2 RA2 Vref Vref RA2 RA2 RA2 RA2 RA2
RA1c RA1 RA1 RA1 RA2 RA1 RA1 RA1 RA1f RA1h
Don t care RA0
RA0g
For CM2 CM0 equal to 000, RA3 through RA0 cannot be used for digital I/O. For CM2 CM0 equal to 000, RA2 and RA1 cannot be used for digital I/O. c RAO and RA3 can be used for digital I/O. d RA3 can be used for digital I/O. e RA3 is a digital output, same as comparator 1 output. f RA4 is the open-drain output of comparator 2. g RA0 and RA3 can be used for digital I/O. h RA1 and RA2 can be used for digital I/O.
Figure 16.25 The comparator Vref circuit selects an analog voltage from a resistor divider.
ANALOG I/O
TABLE 16.15 COMPARATOR VRCON REGISTER BIT DEFINTIONS BIT DESCRIPTION
7 6 5
VREN Vref, enable when set. VROE Vref output, enable when set RA2 Vref. VRR Vref range select. 1 Low range. 0 High range.
4 3 0
Unused. VR3 VR0 Voltage selection bits.
Vref can be output on RA2, but you probably will want it to be used internally only with the comparators because using it to drive RA2 requires extra current from the application. If an analog voltage output is required, I have used the comparator module s Vref in several applications, and I also have used a PWM output driving a resistor and capacitor network. In either case, the resulting analog voltage will have to be buffered using something like a unity gain op-amp circuit because the current output capabilities are very modest.
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