PIC18FXX39 Datasheet, PDF (101/322 Pages) Microchip Technology – Enhanced FLASH Microcontrollers with Single Phase Induction Motor Control Kernel

10.1 Timer0 Operation
Timer0 can operate as a timer or as a counter.
Timer mode is selected by clearing the T0CS bit. In
Timer mode, the Timer0 module will increment every
instruction cycle (without prescaler). If the TMR0L reg-
ister is written, the increment is inhibited for the follow-
ing two instruction cycles. The user can work around
this by writing an adjusted value to the TMR0L register.
Counter mode is selected by setting the T0CS bit. In
Counter mode, Timer0 will increment, either on every
rising or falling edge of pin RA4/T0CKI. The increment-
ing edge is determined by the Timer0 Source Edge
Select bit (T0SE). Clearing the T0SE bit selects the ris-
ing edge. Restrictions on the external clock input are
discussed below.
When an external clock input is used for Timer0, it must
meet certain requirements. The requirements ensure
the external clock can be synchronized with the internal
phase clock (TOSC). Also, there is a delay in the actual
incrementing of Timer0 after synchronization.
10.2 Prescaler
An 8-bit counter is available as a prescaler for the Timer0
module. The prescaler is not readable or writable.
The PSA and T0PS2:T0PS0 bits determine the
prescaler assignment and prescale ratio.
Clearing bit PSA will assign the prescaler to the Timer0
module. When the prescaler is assigned to the Timer0
module, prescale values in power-of-2 increments,
from 1:2 through 1:256, are selectable.
When assigned to the Timer0 module, all instructions
writing to the TMR0L register (e.g., CLRF TMR0,
MOVWF TMR0, BSF TMR0,etc.) will clear the prescaler
count.
Note:
Writing to TMR0L when the prescaler is
assigned to Timer0 will clear the prescaler
count, but will not change the prescaler
assignment.
PIC18FXX39
10.2.1 SWITCHING PRESCALER
ASSIGNMENT
The prescaler assignment is fully under software
control; it can be changed “on-the-fly” during program
execution.
10.3 Timer0 Interrupt
The TMR0 interrupt is generated when the TMR0 reg-
ister overflows from FFh to 00h in 8-bit mode, or FFFFh
to 0000h in 16-bit mode. This overflow sets the TMR0IF
bit. The interrupt can be masked by clearing the
TMR0IE bit. The TMR0IE bit must be cleared in soft-
ware by the Timer0 module Interrupt Service Routine
before re-enabling this interrupt. The TMR0 interrupt
cannot awaken the processor from SLEEP, since the
timer is shut-off during SLEEP.
10.4 16-bit Mode Timer Reads and
Writes
TMR0H is not the high byte of the timer/counter in
16-bit mode, but is actually a buffered version of the
high byte of Timer0 (see Figure 10-2). The high byte of
the Timer0 counter/timer is not directly readable nor
writable. TMR0H is updated with the contents of the
high byte of Timer0 during a read of TMR0L. This pro-
vides the ability to read all 16 bits of Timer0 without
having to verify that the read of the high and low byte
were valid, due to a rollover between successive reads
of the high and low byte.
A write to the high byte of Timer0 must also take place
through the TMR0H buffer register. Timer0 high byte is
updated with the contents of TMR0H when a write
occurs to TMR0L. This allows all 16 bits of Timer0 to be
updated at once.
TABLE 10-1: REGISTERS ASSOCIATED WITH TIMER0
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Value on
POR, BOR
Value on
All Other
RESETS
TMR0L Timer0 Module Low Byte Register
xxxx xxxx uuuu uuuu
TMR0H Timer0 Module High Byte Register
0000 0000 0000 0000
INTCON GIE/GIEH PEIE/GIEL TMR0IE INT0IE RBIE TMR0IF INT0IF RBIF 0000 000x 0000 000u
T0CON TMR0ON T08BIT T0CS T0SE PSA T0PS2 T0PS1 T0PS0 1111 1111 1111 1111
TRISA
—
PORTA Data Direction Register
-111 1111 -111 1111
Legend: x = unknown, u = unchanged, - = unimplemented locations read as '0'. Shaded cells are not used by Timer0.
 2002-2013 Microchip Technology Inc.
Preliminary
DS30485B-page 101