PIC10F200/202/204/206
FIGURE 6-3:
TIMER0 TIMING: INTERNAL CLOCK/PRESCALE 1:2
PC
(Program
Counter)
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
PC – 1 PC PC + 1 PC + 2 PC + 3 PC + 4 PC + 5 PC + 6
MOVWF TMR0 MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W MOVF TMR0,W
Instruction
Fetch
T0
T0 + 1
NT0
NT0 + 1
Timer0
Instruction
Executed
Read TMR0
reads NT0 + 1
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0
Read TMR0
reads NT0 + 2
Write TMR0
executed
TABLE 6-1:
REGISTERS ASSOCIATED WITH TIMER0
Value on
Power-On
Reset
Value on
All Other
Resets
Address
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
01h
N/A
N/A
TMR0
Timer0 – 8-bit Real-Time Clock/Counter
GPWU GPPU T0CS T0SE PSA
xxxx xxxx
1111 1111
---- 1111
uuuu uuuu
1111 1111
---- 1111
OPTION
TRISGPIO
PS2
PS1
PS0
(1)
—
—
I/O Control Register
—
—
Legend: Shaded cells not used by Timer0. – = unimplemented, x = unknown, u= unchanged.
Note 1: The TRIS of the T0CKI pin is overridden when T0CS = 1.
6.1.1
EXTERNAL CLOCK
SYNCHRONIZATION
6.1
Using Timer0 with an External
Clock (PIC10F200/202)
When no prescaler is used, the external clock input is
the same as the prescaler output. The synchronization
of T0CKI with the internal phase clocks is accom-
plished by sampling the prescaler output on the Q2 and
Q4 cycles of the internal phase clocks (Figure 6-4).
Therefore, it is necessary for T0CKI to be high for at
least 2 TOSC (and a small RC delay of 2 Tt0H) and low
for at least 2 TOSC (and a small RC delay of 2 Tt0H).
Refer to the electrical specification of the desired
device.
When an external clock input is used for Timer0, it must
meet certain requirements. The external clock require-
ment is due to internal phase clock (TOSC) synchroniza-
tion. Also, there is a delay in the actual incrementing of
Timer0 after synchronization.
When a prescaler is used, the external clock input is
divided by the asynchronous ripple counter-type
prescaler, so that the prescaler output is symmetrical.
For the external clock to meet the sampling require-
ment, the ripple counter must be taken into account.
Therefore, it is necessary for T0CKI to have a period of
at least 4 TOSC (and a small RC delay of 4 Tt0H) divided
by the prescaler value. The only requirement on T0CKI
high and low time is that they do not violate the
minimum pulse width requirement of Tt0H. Refer to
parameters 40, 41 and 42 in the electrical specification
of the desired device.
DS41239D-page 30
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
