HT48R063B/064B/065B/066B
Under normal program operation, a Watchdog Timer
time-out will initialise a device reset and set the status bit
TO. However, if the system is in the Idle/Sleep Mode,
when a Watchdog Timer time-out occurs, the device will
be woken up, the TO bit in the status register will be set
and only the Program Counter and Stack Pointer will be
reset. Three methods can be adopted to clear the con-
tents of the Watchdog Timer. The first is an external
hardware reset, which means a low level on the external
reset pin, the second is using the Clear Watchdog Timer
software instructions and the third is when a HALT in-
struction is executed. There are two methods of using
software instructions to clear the Watchdog Timer, one
of which must be chosen by configuration option. The
first option is to use the single ²CLR WDT² instruction
while the second is to use the two commands ²CLR
WDT1² and ²CLR WDT2². For the first option, a simple
execution of ²CLR WDT² will clear the Watchdog Timer
while for the second option, both ²CLR WDT1² and
²CLR WDT2² must both be executed to successfully
clear the Watchdog Timer. Note that for this second op-
tion, if ²CLR WDT1² is used to clear the Watchdog
Timer, successive executions of this instruction will have
no effect, only the execution of a ²CLR WDT2² instruc-
tion will clear the Watchdog Timer. Similarly after the
²CLR WDT2² instruction has been executed, only a suc-
cessive ²CLR WDT1² instruction can clear the Watch-
dog Timer.
Configuration
Option
CTRL1
WDT
Register
Function
Disable
Disable
Enable
Disable
Enable
x
OFF
ON
ON
Watchdog Timer On/Off Control
The Watchdog Timer will be disabled if bits
WDTEN3~WDTEN0 in the CTRL1 register are written
with the binary value 1010B and WDT configuration op-
tion is disable. This will be the condition when the device
is powered up. Although any other data written to
WDTEN3~WDTEN0 will ensure that the Watchdog
Timer is enabled, for maximum protection it is recom-
mended that the value 0101B is written to these bits.
The Watchdog Timer clock can emanate from three dif-
ferent sources, selected by configuration option. These
are LXT, fSYS/4, or LIRC. It is important to note that when
the system enters the Idle/Sleep Mode the instruction
clock is stopped, therefore if the configuration options
have selected fSYS/4 as the Watchdog Timer clock
source, the Watchdog Timer will cease to function. For
systems that operate in noisy environments, using the
LIRC or the LXT as the clock source is therefore the rec-
ommended choice. The division ratio of the prescaler is
determined by bits 0, 1 and 2 of the WDTS register,
known as WS0, WS1 and WS2. If the Watchdog Timer in-
ternal clock source is selected and with the WS0, WS1
and WS2 bits of the WDTS register all set high, the
prescaler division ratio will be 1:128, which will give a
maximum time-out period.
·
WDTS Register
Bit
Name
R/W
7
6
5
4
3
2
1
0
WS2
R/W
1
WS1
R/W
1
WS0
R/W
1
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
¾
POR
Bit 7~3 :
Bit 2~0
unimplemented, read as ²0²
WS2, WS1, WS0: WDT time-out period selection
000: 28 tWDTCK
001: 29 tWDTCK
010: 210 tWDTCK
011: 211 tWDTCK
100: 212 tWDTCK
101: 213 tWDTCK
110: 214 tWDTCK
111: 215 tWDTCK
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Watchdog Timer
Rev. 1.00
29
April 7, 2011
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
