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UM10316_0
© NXP B.V. 2008. All rights reserved.
User manual
Rev. 00.06 — 17 December 2008
299 of 571
NXP Semiconductors
UM10316
Chapter 20: LPC29xx WatchDog Timer (WDT)
4.2 Watchdog timer counter
The TC represents the timer-count value which is incremented every prescale cycle.
Depending on the prescale register value and the period of CLK_SAFE the contents of
this register can change very rapidly.
Writes to the timer counter register are disabled. Furthermore the timer counter is reset
when the Watchdog keyword is written to the WD_KEY register. The timer counter stops
counting on Watchdog_Time_Out match.
4.3 Watchdog prescale register
The prescale register determines the number of clock cycles as a prescale value for the
Watchdog timer counter. When the value is not equal to zero the internal prescale counter
first counts the number of CLK_SAFE cycles as defined in this register plus one, then
increments the TC_value.
Updates to the prescale register are only possible when the timer and prescale counters
are disabled, see bit COUNTER_ENABLE in the TCR register. It is advisable to reset the
timer counters once a new prescale value has been programmed. Writes to this register
are ignored when the timer counters are enabled (bit COUNTER_ENABLE in the TCR
register is set).
1
COUNTER_RESET
R/W
1
Reset timer and prescale counter. If this bit
is set the counters remain reset until it is
cleared again
0*
0
COUNTER_ENABLE
R/W
1
Enable timer and prescale counter. If this
bit is set the counters are running
0*
Table 252. WTCR register bit description (WTCR, address: 0xE004 0000)
…continued
* = reset value
Bit
Variable name
Access
Value
Description
Table 253. TC register bit description (TC, address: 0xE004 0004)
* = reset value
Bit
Variable name
Access
Value
Description
31 to 0
TC[31:0]
R
Watchdog timer counter. It is advisable not to
access this register, which may change very
rapidly
0000
0000h*
Table 254. PR register bit descritpion (PR, address: 0xE004 0008)
* = reset value
Bit
Variable name
Access
Value
Description
31 to 0
PR[31:0]
R/W
Prescale register. This specifies the maximum
value for the prescale counter. The TC
increments after ‘PR+1’ CLK_SAFE cycles
have been counted
0000
0000h*