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Philips Semiconductors
User’s Manual - Preliminary -
P89LPC901/902/903
WATCHDOG TIMER
2003 Dec 8 85
MOV WFEED1,#0A5h ; do watchdog feed part 1
MOV WFEED2,#05Ah ; do watchdog feed part 2
SETB EA ; enable interrupt
In timer mode (WDTE = 0), WDCON is loaded to the control register every CCLK cycle (no feed sequence is required to load the
control register), but a feed sequence is required to load from the WDL SFR to the 8-bit down counter before a time-out occurs.
Figure 12-2: Watchdog Timer Control Register
The number of watchdog clocks before timing out is calculated by the following equations:
tclks = (2
(5+PRE)
)(WDL+1)+1
where:
• PRE is the value of prescaler (PRE2-PRE0) which can be the range 0-7, and;
• WDL is the value of watchdog load register which can be the range of 0-255.
The minimum number of tclks is:
tclks = (2
(5+0)
)(0+1)+1 = 33
The maximum number of tclks is:
tclks = (2
(5+7)
)(255+1)+1 = 1,048,577
The following table shows sample P89LPC901/902/903 timeout values.
WDCON
Address: A7h
Not bit addressable
Reset Source(s): See reset value below
Reset Value: 111xx1?1B (Note: WDCON.7,6,5,2,0 - set to ’1’ any reset; WDCON.1 - cleared to ’0’ on Power-on
reset, set to ’1’ on watchdog reset, not affected by any other reset)
BIT SYMBOL FUNCTION
WDCON.7-5 PRE2-PRE0 Clock Prescaler Tap Select. Refer to Table for details.
WDCON.4-3 - Reserved for future use. Should not be set to 1 by user program.
WDCON.2 WDRUN Watchdog Run Control. The watchdog timer is started when WDRUN = 1 and stopped
when WDRUN = 0. This bit is forced to 1 (watchdog running) and cannot be cleared if both
WDTE and WDSE are set to 1.
WDCON.1 WDTOF Watchdog Timer Time-Out Flag. This bit is set when the 8-bit down counter underflows.
In watchdog mode, a feed sequence will clear this bit. It can also be cleared by writing ’0’
to this bit in software.
WDCON.0 WDCLK Watchdog input clock select. When set, the watchdog oscillator is selected. When cleared,
PCLK is selected. (If the CPU is powered down, the watchdog is disabled if WDCLK = 0,
see section "Power down operation"). (Note: If both WDTE and WDSE are set to 1, this
bit is forced to 1.) Refer to section "Watchdog Clock Source" for details.
76543210
PRE2 PRE1 PRE0 - - WDRUN WDTOF WDCLK