HT46R01M Datasheet, PDF (22/69 Pages) Holtek Semiconductor Inc – 1.5V Battery 8-Bit OTP MCU

HT46R01M/HT46R02M/HT48R01M/HT48R02M
Watchdog Timer
The Watchdog Timer, also known as the WDT, is pro-
vided to inhibit program malfunctions caused by the pro-
gram jumping to unknown locations due to certain
uncontrollable external events such as electrical noise.
Watchdog Timer Operation
It operates by providing a device reset when the Watch-
dog Timer counter overflows. Note that if the Watchdog
Timer function is not enabled, then any instructions re-
lated to the Watchdog Timer will result in no operation.
Setting up the various Watchdog Timer options are con-
trolled via the configuration options and two internal reg-
isters WDTS and CTRL1. Enabling the Watchdog Timer
can be controlled by both a configuration option and the
WDTEN bits in the CTRL1 internal register in the Data
Memory.
Configuration
Option
Disable
Disable
Enable
CTRL1
Register
Disable
Enable
x
WDT
Function
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, f_SYS/4, or LIRC. It is important to note that when
the system enters the Sleep Mode the instruction clock is
stopped, therefore if the configuration options have se-
lected f_SYS/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 recommended
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 internal 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.
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 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 re-
set. 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.
C L R W D T 1 F la g
C L R W D T 2 F la g
1 o r 2 In s tr u c tio n s
fS Y S /4
LX T
L IR C
W D T C lo c k S o u r c e S e le c tio n
C le a r W D T T y p e
C o n fig u r a tio n O p tio n
C o n fig .
O p tio n
S e le c t
fW D T C K
C LR
1 5 s ta g e c o u n te r
W S 2~W S 0
Watchdog Timer
W D T T im e - o u t
Rev. 1.00
22
December 18, 2009