
2004 Microchip Technology Inc.
Preliminary
DS39637A-page 159
PIC18F2480/2580/4480/4580
14.0
TIMER3 MODULE
The Timer3 module timer/counter incorporates these
features:
Software selectable operation as a 16-bit timer or
counter
Readable and writable 8-bit registers (TMR3H
and TMR3L)
Selectable clock source (internal or external) with
device clock or Timer1 oscillator internal options
Interrupt-on-overflow
Module Reset on CCP special event trigger
A simplified block diagram of the Timer3 module is
shown in Figure 14-1. A block diagram of the module’s
operation in Read/Write mode is shown in Figure 14-2.
The Timer3 module is controlled through the T3CON
register (Register 14-1). It also selects the clock source
options for the CCP modules (see
Section 15.1.1
“CCP Modules and Timer Resources”
for more
information).
REGISTER 14-1:
T3CON: TIMER3 CONTROL REGISTER
R/W-0
R/W-0
R/W-0
RD16
T3ECCP1
(1)
T3CKPS1 T3CKPS0 T3CCP1
(1)
bit 7
R/W-0
R/W-0
R/W-0
T3SYNC
R/W-0
TMR3CS TMR3ON
R/W-0
bit 0
bit 7
RD16:
16-bit Read/Write Mode Enable bit
1
= Enables register read/write of Timer3 in one 16-bit operation
0
= Enables register read/write of Timer3 in two 8-bit operations
T3ECCP1:T3CCP1:
Timer3 and Timer1 to CCPx Enable bits
(1)
1x
= Timer3 is the capture/compare clock source for both CCP and ECCP modules
01
= Timer3 is the capture/compare clock source for ECCP1;
Timer1 is the capture/compare clock source for CCP1
00
= Timer1 is the capture/compare clock source for both CCP and ECCP modules
Note 1:
These bits are available on PIC18F4X80 devices only.
bit 6,3
bit 5-4
T3CKPS1:T3CKPS0
: Timer3 Input Clock Prescale Select bits
11
= 1:8 prescale value
10
= 1:4 prescale value
01
= 1:2 prescale value
00
= 1:1 prescale value
T3SYNC:
Timer3 External Clock Input Synchronization Control bit
(Not usable if the device clock comes from Timer1/Timer3.)
When TMR3CS =
1
:
1
= Do not synchronize external clock input
0
= Synchronize external clock input
When TMR3CS =
0
:
This bit is ignored. Timer3 uses the internal clock when TMR3CS =
0
.
TMR3CS:
Timer3 Clock Source Select bit
1
= External clock input from Timer1 oscillator or T13CKI (on the rising edge after the first
falling edge)
0
= Internal clock (F
OSC
/4)
TMR3ON:
Timer3 On bit
1
= Enables Timer3
0
= Stops Timer3
bit 2
bit 1
bit 0
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