Microchip Technology MA300015 데이터 시트

DS70150E-page 86

© 2011 Microchip Technology Inc.

Each output compare channel can select between one
of two 16-bit timers; Timer2 or Timer3.

The selection of the timers is controlled by the OCTSEL
bit (OCxCON<3>). Timer2 is the default timer resource
for the Output Compare module.

When control bits OCM<2:0> (OCxCON<2:0>) = 001,
010

 or 011, the selected output compare channel is

configured for one of three simple output compare
match modes:

• Compare forces I/O pin low

• Compare forces I/O pin high

• Compare toggles I/O pin

The OCxR register is used in these modes. The OCxR
register is loaded with a value and is compared to the
selected incrementing timer count. When a compare
occurs, one of these compare match modes occurs. If
the counter resets to zero before reaching the value in
OCxR, the state of the OCx pin remains unchanged.

When control bits OCM<2:0> (OCxCON<2:0>) = 100
or 101, the selected output compare channel is config-
ured for one of two Dual Output Compare modes,
which are:

• Single Output Pulse mode

• Continuous Output Pulse mode

13.3.1

SINGLE PULSE MODE

For the user to configure the module for the generation
of a single output pulse, the following steps are
required (assuming timer is off):

• Determine instruction cycle time T

CY

.

• Calculate desired pulse-width value based on 

T

CY

.

• Calculate time to start pulse from timer start value 

of 0x0000.

• Write pulse-width start and stop times into OCxR 

and OCxRS Compare registers (x denotes 
channel 1, 2, ...,N).

• Set Timer Period register to value equal to, or 

greater than, value in OCxRS Compare register.

• Set OCM<2:0> = 100.

• Enable timer, TON (TxCON<15>) = 1.

To initiate another single pulse, issue another write to
set OCM<2:0> = 100.

13.3.2

CONTINUOUS PULSE MODE

For the user to configure the module for the generation
of a continuous stream of output pulses, the following
steps are required: 

• Determine instruction cycle time T

CY

.

• Calculate desired pulse value based on T

CY

.

• Calculate timer to start pulse width from timer start 

value of 0x0000.

• Write pulse-width start and stop times into OCxR 

and OCxRS (x denotes channel 1, 2, ...,N) 
Compare registers, respectively.

• Set Timer Period register to value equal to, or 

greater than, value in OCxRS Compare register.

• Set OCM<2:0> = 101.

• Enable timer, TON (TxCON<15>) = 1.

When control bits OCM<2:0> (OCxCON<2:0>) = 110
or 111, the selected output compare channel is config-
ured for the PWM mode of operation. When configured
for the PWM mode of operation, OCxR is the main latch
(read-only) and OCxRS is the secondary latch. This
enables glitchless PWM transitions.

The user must perform the following steps in order to
configure the output compare module for PWM
operation:

1.

Set the PWM period by writing to the appropriate
period register.

2.

Set the PWM duty cycle by writing to the OCxRS
register.

3.

Configure the output compare module for PWM
operation.

4.

Set the TMRx prescale value and enable the
Timer, TON (TxCON<15>) = 1.

13.4.1

INPUT PIN FAULT PROTECTION 
FOR PWM

When control bits OCM<2:0> (OCxCON<2:0>) = 111,
the selected output compare channel is again
configured for the PWM mode of operation, with the
additional feature of input Fault protection. While in this
mode, if a logic ‘0’ is detected on the OCFA/B pin, the
respective PWM output pin is placed in the high-
impedance input state. The OCFLT bit (OCxCON<4>)
indicates whether a Fault condition has occurred. This
state will be maintained until both of the following
events have occurred:

• The external Fault condition has been removed. 

• The PWM mode has been re-enabled by writing 

to the appropriate control bits.