Watchdog Clock Source - Philips P89LPC938 User Manual

Philips Semiconductors
Table 104: Watchdog timeout vales
PRE2 to PRE0 WDL in decimal)
110 0
255
111 0
255

16.3 Watchdog clock source

The watchdog timer system has an on-chip 400 KHz oscillator. The watchdog timer can
be clocked from either the watchdog oscillator or from PCLK (refer to
configuring the WDCLK bit in the Watchdog Control Register WDCON. When the
watchdog feature is enabled, the timer must be fed regularly by software in order to
prevent it from resetting the CPU.
After changing WDCLK (WDCON.0), switching of the clock source will not immediately
take effect. As shown in
sequence. In addition, due to clock synchronization logic, it can take two old clock cycles
before the old clock source is deselected, and then an additional two new clock cycles
before the new clock source is selected.
Since the prescaler starts counting immediately after a feed, switching clocks can cause
some inaccuracy in the prescaler count. The inaccuracy could be as much as 2 old clock
source counts plus 2 new clock cycles.
Note: When switching clocks, it is important that the old clock source is left enabled for
two clock cycles after the feed completes. Otherwise, the watchdog may become disabled
when the old clock source is disabled. For example, suppose PCLK (WCLK = 0) is the
current clock source. After WCLK is set to logic 1, the program should wait at least two
PCLK cycles (4 CCLKs) after the feed completes before going into Power-down mode.
Otherwise, the watchdog could become disabled when CCLK turns off. The watchdog
oscillator will never become selected as the clock source unless CCLK is turned on again
first.
User manual
...continued
Timeout Period
(in watchdog clock
cycles)
2,049
524,289
4097
1,048,577
Figure 50, the selection is loaded after a watchdog feed
Rev. 03 — 7 June 2005
P89LPC938 User manual
Watchdog Clock Source
400 KHz Watchdog 12 MHz CCLK (6 MHz
CCLK
Oscillator Clock
(Nominal) Clock)
5.12 ms 341.5 µs
1.31 s 87.4 ms
10.2 ms 682.8 µs
2.62 s 174.8 ms
© Koninklijke Philips Electronics N.V. 2005. All rights reserved.
UM10119
⁄
Watchdog
2
Figure 48) by
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