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HT46R01B-1 Datasheet(PDF) 24 Page - Holtek Semiconductor Inc |
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HT46R01B-1 Datasheet(HTML) 24 Page - Holtek Semiconductor Inc |
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24 / 71 page  HT46R01B-1/HT46R01N-1 HT48R01B-1/HT48R01N-1 Rev.1.00 24 June 9, 2011 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 CTRL1 Register WDT Function Disable Disable OFF Disable Enable ON Enable x 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 Sleep Mode the instruction clock is stopped, therefore if the configuration options have se- lected 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 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 WDT1 ² 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. 1 5 s t a g e c o u n t e r W D T T i m e - o u t W S 2 ~ W S 0 C o n f i g . O p t i o n S e l e c t W D T C l o c k S o u r c e S e l e c t i o n C l e a r W D T T y p e C o n f i g u r a t i o n O p t i o n C L R W D T 1 F l a g C L R W D T 2 F l a g 1 o r 2 I n s t r u c t i o n s C L R f S Y S / 4 L I R C L X T f W D T C K Watchdog Timer |
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