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PIC17C4X
1996 Microchip Technology Inc.
DS30412B-page 77
12.1.3.3.1 MAX RESOLUTION/FREQUENCY FOR
EXTERNAL CLOCK INPUT
The use of an external clock for the PWM time-base
(Timer1 or Timer2) limits the PWM output to a maxi-
mum resolution of 8-bits. The PWxDCL<7:6> bits must
be kept cleared. Use of any other value will distort the
PWM output. All resolutions are supported when inter-
nal clock mode is selected. The maximum attainable
frequency is also lower. This is a result of the timing
requirements of an external clock input for a timer (see
the Electrical Specification section). The maximum
PWM frequency, when the timers clock source is the
RB4/TCLK12 pin, is shown in Table 12-3 (standard res-
olution mode).
12.2
Timer3
Timer3 is a 16-bit timer consisting of the TMR3H and
TMR3L registers. TMR3H is the high byte of the timer
and TMR3L is the low byte. This timer has an associ-
ated 16-bit period register (PR3H/CA1H:PR3L/CA1L).
This period register can be software configured to be a
second 16-bit capture register.
When the TMR3CS bit (TCON1<2>) is clear, the timer
increments every instruction cycle (Fosc/4). When
TMR3CS is set, the timer increments on every falling
edge of the RB5/TCLK3 pin. In either mode, the
TMR3ON bit must be set for the timer to increment.
When TMR3ON is clear, the timer will not increment or
set the TMR3IF bit.
Timer3 has two modes of operation, depending on the
CA1/PR3 bit (TCON2<3>). These modes are:
One capture and one period register mode
Dual capture register mode
The PIC17C4X has up to two 16-bit capture registers
that capture the 16-bit value of TMR3 when events are
detected on capture pins. There are two capture pins
(RB0/CAP1 and RB1/CAP2), one for each capture reg-
ister. The capture pins are multiplexed with PORTB
pins. An event can be:
a rising edge
a falling edge
every 4th rising edge
every 16th rising edge
Each 16-bit capture register has an interrupt flag asso-
ciated with it. The flag is set when a capture is made.
The capture module is truly part of the Timer3 block.
Figure 12-7 and Figure 12-8 show the block diagrams
for the two modes of operation.
TABLE 12-4:
REGISTERS/BITS ASSOCIATED WITH PWM
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Value on
Power-on
Reset
Value on
all other
resets
(Note1)
16h, Bank 3
TCON1
CA2ED1
CA2ED0
CA1ED1
CA1ED0
T16
TMR3CS
TMR2CS
TMR1CS
0000 0000
0000 0000
17h, Bank 3
TCON2
CA2OVF
CA1OVF
PWM2ON
PWM1ON
CA1/PR3
TMR3ON
TMR2ON
TMR1ON
0000 0000
0000 0000
10h, Bank 2
TMR1
Timer1 register
xxxx xxxx
uuuu uuuu
11h, Bank 2
TMR2
Timer2 register
xxxx xxxx
uuuu uuuu
16h, Bank 1
PIR
RBIF
TMR3IF
TMR2IF
TMR1IF
CA2IF
CA1IF
TXIF
RCIF
0000 0010
0000 0010
17h, Bank 1
PIE
RBIE
TMR3IE
TMR2IE
TMR1IE
CA2IE
CA1IE
TXIE
RCIE
0000 0000
0000 0000
07h, Unbanked
INTSTA
PEIF
T0CKIF
T0IF
INTF
PEIE
T0CKIE
T0IE
INTE
0000 0000
0000 0000
06h, Unbanked
CPUSTA
—
—
STKAV
GLINTD
TO
PD
—
—
--11 11--
--11 qq--
10h, Bank 3
PW1DCL
DC1
DC0
—
—
—
—
—
—
xx-- ----
uu-- ----
11h, Bank 3
PW2DCL
DC1
DC0
TM2PW2
—
—
—
—
—
xx0- ----
uu0- ----
12h, Bank 3
PW1DCH
DC9
DC8
DC7
DC6
DC5
DC4
DC3
DC2
xxxx xxxx
uuuu uuuu
13h, Bank 3
Legend:
PW2DCH
= unknown,
shaded cells are not used by PWM.
DC9
DC8
DC7
DC6
DC5
= value depends on conditions,
DC4
DC3
DC2
xxxx xxxx
uuuu uuuu
x
u
= unchanged,
-
= unimplemented read as '0',
q