PIC18F2420/2520/4420/4520
11.0
TIMER0 MODULE
The Timer0 module incorporates the following features:
• Software selectable operation as a timer or
counter in both 8-bit or 16-bit modes
• Readable and writable registers
• Dedicated 8-bit, software programmable
prescaler
• Selectable clock source (internal or external)
• Edge select for external clock
• Interrupt-on-overflow
The T0CON register (Register 11-1) controls all
aspects of the module’s operation, including the
prescale selection. It is both readable and writable.
A simplified block diagram of the Timer0 module in 8-bit
mode is shown in Figure 11-1. Figure 11-2 shows a
simplified block diagram of the Timer0 module in 16-bit
mode.
REGISTER 11-1:
R/W-1
TMR0ON
bit 7
Legend:
R = Readable bit
-n = Value at POR
bit 7
T0CON: TIMER0 CONTROL REGISTER
R/W-1
T0CS
R/W-1
T0SE
R/W-1
PSA
R/W-1
T0PS2
R/W-1
T0PS1
R/W-1
T0PS0
bit 0
R/W-1
T08BIT
W = Writable bit
‘1’ = Bit is set
U = Unimplemented bit, read as ‘0’
‘0’ = Bit is cleared
x = Bit is unknown
TMR0ON:
Timer0 On/Off Control bit
1
= Enables Timer0
0
= Stops Timer0
T08BIT:
Timer0 8-Bit/16-Bit Control bit
1
= Timer0 is configured as an 8-bit timer/counter
0
= Timer0 is configured as a 16-bit timer/counter
T0CS:
Timer0 Clock Source Select bit
1
= Transition on T0CKI pin
0
= Internal instruction cycle clock (CLKO)
T0SE:
Timer0 Source Edge Select bit
1
= Increment on high-to-low transition on T0CKI pin
0
= Increment on low-to-high transition on T0CKI pin
PSA:
Timer0 Prescaler Assignment bit
1
= TImer0 prescaler is not assigned. Timer0 clock input bypasses prescaler.
0
= Timer0 prescaler is assigned. Timer0 clock input comes from prescaler output.
T0PS<2:0>:
Timer0 Prescaler Select bits
111
= 1:256 Prescale value
110
= 1:128 Prescale value
101
= 1:64 Prescale value
100
= 1:32 Prescale value
011
= 1:16 Prescale value
010
= 1:8 Prescale value
001
= 1:4 Prescale value
000
= 1:2 Prescale value
bit 6
bit 5
bit 4
bit 3
bit 2-0
©
2008 Microchip Technology Inc.
DS39631E-page 123
MICROCHIP [ MICROCHIP TECHNOLOGY ]
