Rainbow Electronics W78C51D - Manual (Page 8)

Preliminary W78C51D
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The time-out period is obtained using the following formula:
1
OSC
2 PRESCALER 1000 12 mS 14 Ч Ч Ч Ч
Before Watchdog time-out occurs, the program must clear the 14-bit timer by writing 1 to WDTC.6
(CLRW). After 1 is written to this bit, the 14-bit timer , prescaler and this bit will be reset on the next
instruction cycle. The Watchdog timer is cleared on reset.
OSC 1/12 PRESCALER
14-BIT TIMER
CLEAR
CLRW
EXTERNAL
RESET
INTERNAL
RESET
WIDL
IDLE
ENW
Watchdog Timer Block Diagram
Typical Watchdog time-out period when OSC = 20 MHz
PS2 PS1 PS0 WATCHDOG TIME-OUT PERIOD
0 0 0 19.66 mS
0 1 0 39.32 mS
0 0 1 78.64 mS
0 1 1 157.28 mS
1 0 0 314.57 mS
1 0 1 629.14 mS
1 1 0 1.25 S
1 1 1 2.50 S
Reduce EMI Emission
Because of the on-chip ROM, when a program is running in internal ROM space, the ALE will be
unused. The transition of ALE will cause noise, so it can be turned off to reduce the EMI emission if it
is not needed. Turning off the ALE signal transition only requires setting the bit 0 of the AUXR SFR,
which is located at 08Eh. When ALE is turned off, it will be reactivated when the program accesses
external ROM/RAM data or jumps to execute an external ROM code. The ALE signal will turn off
again after it has been completely accessed or the program returns to internal ROM code space.
AUXR - Auxiliary Register
Bit: 7 6 5 4 3 2 1 0
- - - - - - - AO
Mnemonic: AUXR Address: 8Eh
AO: Turn off ALE signal.