Semiconductor Group
8-2
1997-10-01
Fail Safe Mechanisms
C541U
8.1.1 Input Clock Selection
The input clock rate of the watchdog timer is derived from the system clock of the C541U. There is
a prescaler available, which is software selectable and defines the input clock rate. This prescaler
is controlled by bit WDTPSEL in the SFR WDTREL. Tabel 8-1 shows resulting timeout periods at
f
OSC
= 12 MHz.
Special Function Register WDTREL (Address 86H)
Reset Value : 00H
Immediately after start, the watchdog timer is initialized to the reload value programmed to
WDTREL.0-WDTREL.6. After an external hardware reset, an oscillator watchdog power on reset,
or a watchdog timer reset, register WDTREL is cleared to 00H. The lower seven bits of WDTREL
can be loaded by software at any time.
Bit
Function
WDTPSEL
Watchdog timer prescaler select bit.
If WDTPSEL=0, the watchdog timer is clocked by
f
OSC
/12 (default after
reset).
If WDTPSEL=1, the watchdog timer is clocked by
f
OSC
/192
WDTREL.6 - 0
Seven bit reload value
for the high-byte of the watchdog timer. This value is loaded to WDTH
when a refresh is triggered by a consecutive setting of bits WDT and
SWDT.
Table 8-1
Watchdog Timer Time-Out Periods (WDTPSEL = 0)
WDTREL
Time-Out Period
f
OSC
= 12 MHz
Comments
00H
32.768 ms
This is the default value
80H
0.55 s
Maximum time period
7FH
256
µ
s
Minimum time period
MSB
LSB
Bit No.
WDT
86H
WDTREL
7
6
5
4
3
2
1
0
Reload Value
PSEL
Содержание C541U
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