PIC16F688 Datasheet, PDF (60/174 Pages) Microchip Technology – 14-Pin Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F688
REGISTER 7-2:
CMCON1 – COMPARATOR CONFIGURATION REGISTER (ADDRESS: 1Ah)
U-0
U-0
U-0
U-0
U-0
U-0
R/W-1 R/W-0
—
—
—
—
—
—
T1GSS C2SYNC
bit 7
bit 0
bit 7-2:
bit 1
bit 0
Unimplemented: Read as ‘0’
T1GSS: Timer1 Gate Source Select bit
1 = Timer1 gate source is T1G pin (RA4 must be configured as digital input)
0 = Timer1 gate source is Comparator 2 Output
C2SYNC: Comparator 2 Synchronize bit
1 = C2 output synchronized with falling edge of Timer1 clock
0 = C2 output not synchronized with Timer1 clock
Legend:
R = Readable bit
- n = Value at POR
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared x = Bit is unknown
7.4 Comparator Outputs
The comparator outputs are read through the
CMCON0 register. These bits are read-only. The
comparator outputs may also be directly output to the
RA2 and RC4 I/O pins. When enabled, multiplexors in
the output path of the RA2 and RC4 pins will switch
and the output of each pin will be the unsynchronized
output of the comparator. The uncertainty of each of
the comparators is related to the input offset voltage
and the response time given in the specifications.
Figure 7-4 and Figure 7-5 show the output block
diagram for Comparator 1 and 2.
The TRIS bits will still function as an output enable/
disable for the RA2 and RC4 pins while in this mode.
The polarity of the comparator outputs can be changed
using the C1INV and C2INV bits (CMCON0<5:4>).
Timer1 gate source can be configured to use the T1G
pin or Comparator 2 output as selected by the T1GSS
bit (CMCON1<1>). This feature can be used to time the
duration or interval of analog events. The output of
Comparator 2 can also be synchronized with Timer1 by
setting the C2SYNC bit (CMCON1<0>). When
enabled, the output of Comparator 2 is latched on the
falling edge of Timer1 clock source. If a prescaler is
used with Timer1, Comparator 2 is latched after the
prescaler. To prevent a race condition, the Comparator
2 output is latched on the falling edge of the Timer1
clock source and Timer1 increments on the rising edge
of its clock source. See (Figure 7-5), Comparator 2
Block Diagram and (Figure 6-1), Timer1 Block Diagram
for more information.
It is recommended to synchronize Comparator 2 with
Timer1 by setting the C2SYNC bit when Comparator 2
is used as the Timer1 gate source. This ensures Timer1
does not miss an increment if Comparator 2 changes
during an increment.
7.5 Comparator Interrupts
The comparator interrupt flags are set whenever there
is a change in the output value of its respective compar-
ator. Software will need to maintain information about
the status of the output bits, as read from
CMCON0<7:6>, to determine the actual change that
has occurred. The CxIF bits, PIR1<4:3>, are the
Comparator Interrupt flags. This bit must be reset in
software by clearing it to ‘0’. Since it is also possible to
write a ‘1’ to this register, a simulated interrupt may be
initiated.
The CxIE bits (PIE1<4:3>) and the PEIE bit
(INTCON<6>) must be set to enable the interrupts. In
addition, the GIE bit must also be set. If any of these
bits are cleared, the interrupt is not enabled, though the
CxIF bits will still be set if an interrupt condition occurs.
The user, in the Interrupt Service Routine, can clear the
interrupt in the following manner:
a) Any read or write of CMCON0. This will end the
mismatch condition.
b) Clear flag bit CxIF
A mismatch condition will continue to set flag bit CxIF.
Reading CMCON0 will end the mismatch condition and
allow flag bits CxIF to be cleared.
Note:
If a change in the CMCON0 register
(CxOUT) should occur when a read
operation is being executed (start of the Q2
cycle), then the CxIF (PIR1<3>) interrupt
flag may not get set.
DS41203B-page 58
Preliminary
 2004 Microchip Technology Inc.