Datasheet

 2010 Microchip Technology Inc. DS39770C-page 147
PIC18F85J90 FAMILY
13.0 TIMER2 MODULE
The Timer2 module incorporates the following features:
• 8-bit Timer and Period registers (TMR2 and PR2,
respectively)
• Readable and writable (both registers)
• Software programmable prescaler
(1:1, 1:4 and 1:16)
• Software programmable postscaler
(1:1 through 1:16)
• Interrupt on TMR2 to PR2 match
• Optional use as the shift clock for the
MSSP module
The module is controlled through the T2CON register
(Register 13-1) which enables or disables the timer and
configures the prescaler and postscaler. Timer2 can be
shut off by clearing control bit, TMR2ON (T2CON<2>),
to minimize power consumption.
A simplified block diagram of the module is shown in
Figure 13-1.
13.1 Timer2 Operation
In normal operation, TMR2 is incremented from 00h on
each clock (F
OSC/4). A 4-bit counter/prescaler on the
clock input gives direct input, divide-by-4 and
divide-by-16 prescale options. These are selected by
the prescaler control bits, T2CKPS<1:0>
(T2CON<1:0>). The value of TMR2 is compared to that
of the Period register, PR2, on each clock cycle. When
the two values match, the comparator generates a
match signal as the timer output. This signal also resets
the value of TMR2 to 00h on the next cycle and drives
the output counter/postscaler (see Section 13.2
“Timer2 Interrupt”).
The TMR2 and PR2 registers are both directly readable
and writable. The TMR2 register is cleared on any
device Reset, while the PR2 register initializes at FFh.
Both the prescaler and postscaler counters are cleared
on the following events:
• a write to the TMR2 register
• a write to the T2CON register
• any device Reset (Power-on Reset, MCLR
Reset,
Watchdog Timer Reset or Brown-out Reset)
TMR2 is not cleared when T2CON is written.
REGISTER 13-1: T2CON: TIMER2 CONTROL REGISTER
U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0
— T2OUTPS3 T2OUTPS2 T2OUTPS1 T2OUTPS0 TMR2ON T2CKPS1 T2CKPS0
bit 7 bit 0
Legend:
R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’
-n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown
bit 7 Unimplemented: Read as ‘0’
bit 6-3 T2OUTPS<3:0>: Timer2 Output Postscale Select bits
0000 = 1:1 Postscale
0001 = 1:2 Postscale
•
•
•
1111 = 1:16 Postscale
bit 2 TMR2ON: Timer2 On bit
1 = Timer2 is on
0 = Timer2 is off
bit 1-0 T2CKPS<1:0>: Timer2 Clock Prescale Select bits
00 = Prescaler is 1
01 = Prescaler is 4
1x = Prescaler is 16