TLC156 Datasheet, PDF (18/50 Pages) List of Unclassifed Manufacturers – EPROM/ROM-Based 8-Bit Microcontroller Series

TLC
TLC156
2.3 Timer0/WDT & Prescler
2.3.1 Timer0
The Timer0 is a 8-bit timer/counter. The clock source of Timer0 can come from the internal clock or by an external
clock source (T0CKI pin).
2.3.1.1 Using Timer0 with an Internal Clock : Timer mode
Timer mode is selected by clearing the T0CS bit (OPTION<5>). In timer mode, the timer0 register (TMR0) will
increment every instruction cycle (without prescaler). If TMR0 register is written, the increment is inhibited for the
following two cycles.
2.3.1.2 Using Timer0 with an External Clock : Counter mode
Counter mode is selected by setting the T0CS bit (OPTON<5>). In this mode, Timer0 will increment either on every
rising or falling edge of pin T0CKl. The incrementing edge is determined by the source edge select the rising edge.
The external clock requirement is due to internal phase clock (Tosc) synchronization. Also, there is a delay in the
actual incrementing of Timer0 after synchronization.
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 accomplished by sampling the prescaler output on the T2 and T4 cycles of
the internal phase clocks. Therefore, it is necessary for T0CKI to be high for at least 2 TOSC and low for at least 2
Tosc.
When a prescaler is used, the external clock input is divided by the asynchronous prescaler. For the external clock
to meet the sampling requirement, the ripple counter must be taken into account. Therefore, it is necessary for
T0CKI to have a period of at least 4Tosc divided by the prescaler value.
2.3.2 Watchdog Timer (WDT)
The Watchdog Timer (WDT) is a free running on-chip RC oscillator which does not require any external components.
So the WDT will still run even if the clock on the OSCI and OSCO pins is turned off, such as in SLEEP mode. During
normal operation or in SLEEP mode, a WDT time-out will cause the device reset and the TO bit (STATUS<4>) will
be cleared.
The WDT can be disabled by clearing the control bit WDTE (PCON<7>) to “0”.
The WDT has a nominal time-out period of 18 ms (without prescaler). If a longer time-out period is desired, a
prescaler with a division ratio of up to 1:128 can be assigned to the WDT controlled by the OPTION register. Thus,
the longest time-out period is approxmately 2.3 seconds.
The CLRWDT instruction clears the WDT and the prescaler, if assigned to the WDT, and prevents it from timing out
and generating a device reset.
The SLEEP instruction resets the WDT and the prescaler, if assigned to the WDT. This gives the maximum SLEEP
time before a WDT Wake-up Reset.
2.3.3 Prescaler
An 8-bit counter (down counter) is available as a prescaler for the Timer0, or as a postscaler for the Watchdog Timer
(WDT). Note that the prescaler may be used by either the Timer0 module or the WDT, but not both. Thus, a
prescaler assignment for the Timer0 means that there is no prescaler for the WDT, and vice-versa.
The PSA bit (OPTION<3>) determines prescaler assignment. The PS<2:0> bits (OPTION<2:0>) determine
prescaler ratio.
When the prescaler is assigned to the Timer0 module, all instructions writing to the TMR0 register will clear the
prescaler. When it is assigned to WDT, a CLRWDT instruction will clear the prescaler along with the WDT.
The prescaler is neither readable nor writable. On a RESET, the prescaler contains all ‘1’s.
To avoid an unintended device reset, CLRWDT or CLRR TMR0 instructions must be executed when changing the
prescaler assignment from Timer0 to the WDT, and vice-versa.
Rev0.95 Nov 20, 2003
P.2/TLC156