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PIC17C75X Datasheet, PDF (85/320 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers | |||
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11.0 OVERVIEW OF TIMER
RESOURCES
The PIC17C75X has four timer modules. Each module
can generate an interrupt to indicate that an event has
occurred. These timers are called:
⢠Timer0 - 16-bit timer with programmable 8-bit
prescaler
⢠Timer1 - 8-bit timer
⢠Timer2 - 8-bit timer
⢠Timer3 - 16-bit timer
For enhanced time-base functionality, four input Cap-
tures and three Pulse Width Modulation (PWM) out-
puts are possible. The PWMs use the Timer1 and
Timer2 resources and the input Captures use the
Timer3 resource.
11.1 Timer0 Overview
The Timer0 module is a simple 16-bit overï¬ow counter.
The clock source can be either the internal system
clock (Fosc/4) or an external clock.
When Timer0 uses an external clock source, it has the
ï¬exibility to allow user selection of the incrementing
edge, rising or falling.
The Timer0 module also has a programmable pres-
caler. The PS3:PS0 bits (T0STA<4:1>) determine the
prescale value. TMR0 can increment at the following
rates: 1:1, 1:2, 1:4, 1:8, 1:16, 1:32, 1:64, 1:128, 1:256.
Synchronization of the external clock occurs after the
prescaler. When the prescaler is used, the external
clock frequency may be higher then the deviceâs fre-
quency. The maximum external frequency, on the
T0CKI pin, is 50 MHz, given the high and low time
requirements of the clock.
11.2 Timer1 Overview
The Timer1 module is an 8-bit timer/counter with an
8-bit period register (PR1). When the TMR1 value rolls
over from the period match value to 0h, the TMR1IF
ï¬ag is set, and an interrupt will be generated if
enabled. In counter mode, the clock comes from the
RB4/TCLK12 pin, which can also be selected to be the
clock for the Timer2 module.
TMR1 can be concatenated with TMR2 to form a
16-bit timer. The TMR1 register is the LSB and TMR2
is the MSB. When in the 16-bit timer mode, there is a
corresponding 16-bit period register (PR2:PR1). When
the TMR2:TMR1 value rolls over from the period
match value to 0h, the TMR1IF ï¬ag is set, and an
interrupt will be generated if enabled.
PIC17C75X
11.3 Timer2 Overview
The Timer2 module is an 8-bit timer/counter with an
8-bit period register (PR2). When the TMR2 value rolls
over from the period match value to 0h, the TMR2IF
ï¬ag is set, and an interrupt will be generated if
enabled. In counter mode, the clock comes from the
RB4/TCLK12 pin, which can also provide the clock for
the Timer1 module.
TMR2 can be concatenated with TMR1 to form a
16-bit timer. The TMR2 register is the MSB and TMR1
is the LSB. When in the 16-bit timer mode, there is a
corresponding 16-bit period register (PR2:PR1). When
the TMR2:TMR1 value rolls over from the period
match value to 0h, the TMR1IF ï¬ag is set, and an
interrupt will be generated if enabled.
11.4 Timer3 Overview
The Timer3 module is a 16-bit timer/counter with a
16-bit period register. When the TMR3H:TMR3L value
rolls over to 0h, the TMR3IF bit is set and an interrupt
will be generated if enabled. In counter mode, the
clock comes from the RB5/TCLK3 pin.
When operating in the four capture mode, the period
registers become the second (of four) 16-bit capture
registers.
11.5 Role of the Timer/Counters
The timer modules are general purpose, but have ded-
icated resources associated with them. TImer1 and
Timer2 are the time-bases for the three Pulse Width
Modulation (PWM) outputs, while Timer3 is the
time-base for the four input captures.
© 1997 Microchip Technology Inc.
Preliminary
DS30264A-page 85
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