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HT48RA3 Datasheet(PDF) 9 Page - Holtek Semiconductor Inc |
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HT48RA3 Datasheet(HTML) 9 Page - Holtek Semiconductor Inc |
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9 / 36 page  HT48RA3 Rev. 1.20 9 May 12, 2003 shift required for the oscillator, and no other external components are demanded. Instead of a crystal, the resonator can also be connected between OSC1 and OSC2 to get a frequency reference, but two external ca- pacitors in OSC1 and OSC2 are required. The WDT oscillator is a free running on-chip RC oscilla- tor, and no external components are required. Even if the system enters the power down mode, the system clock is stopped, but the WDT oscillator still works with a period of approximately 90 ms. The WDT oscillator can be disabled by ROM code option to conserve power. Watchdog Timer - WDT The WDT clock source is implemented by a dedicated RC oscillator (WDT oscillator), instruction clock (system clock divided by 4), determines the ROM code option. This timer is designed to prevent a software malfunction or sequence from jumping to an unknown location with unpredictable results. The Watchdog Timer can be dis- abled by ROM code option. If the Watchdog Timer is dis- abled, all the executions related to the WDT result in no operation. Once the internal WDT oscillator (RC oscillator with a period of 90 ms/3V normally) is selected, it is first divided by 256 (8-stage) to get the nominal time-out period of 23ms/3V. This time-out period may vary with tempera- tures, VDD and process variations. By invoking the WDT prescaler, longer time-out periods can be realized. Writing data to WS2, WS1, WS0 (bit 2,1,0 of the WDTS) can give different time-out periods. If WS2, WS1, and WS0 are all equal to 1, the division ratio is up to 1:128, and the maximum time-out period is 2.9s/3V seconds. If the WDT oscillator is disabled, the WDT clock may still come from the instruction clock and operates in the same manner except that in the HALT state the WDT may stop counting and lose its protecting purpose. In this situation the logic can only be restarted by external logic. The high nibble and bit 3 of the WDTS are re- served for user s defined flags, which can be used to in- dicate some specified status. If the device operates in a noisy environment, using the on-chip RC oscillator (WDT OSC) is strongly recom- mended, since the HALT will stop the system clock. WS2 WS1 WS0 Division Ratio 000 1:1 001 1:2 010 1:4 011 1:8 1 0 0 1:16 1 0 1 1:32 1 1 0 1:64 1 1 1 1:128 WDTS register The WDT overflow under normal operation will initialize ²chip reset² and set the status bit ²TO². But in the HALT mode, the overflow will initialize a ²warm reset² and only the PC and SP are reset to zero. To clear the contents of WDT (including the WDT prescaler), three methods are adopted; external reset (a low level to RES), software in- struction and a ²HALT² instruction. The software instruc- tion include ²CLR WDT² and the other set -²CLR WDT1 ² and ²CLR WDT2². Of these two types of instruc- tion, only one can be active depending on the ROM code option -²CLR WDT times selection option².Ifthe ²CLR WDT² is selected (i.e. CLRWDT times equal one), any execution of the ²CLR WDT² instruction will clear the WDT. In the case that ²CLR WDT1² and ²CLR WDT2 ² are chosen (i.e. CLRWDT times equal two), these two instructions must be executed to clear the WDT; otherwise, the WDT may reset the chip as a result of time-out. Power Down Operation - HALT The HALT mode is initialized by the ²HALT² instruction and results in the following... · The system oscillator will be turned off but the WDT oscillator remains running (if the WDT oscillator is se- lected). · The contents of the on chip RAM and registers remain unchanged. · WDT and WDT prescaler will be cleared and re- counted again (if the WDT clock is from the WDT os- cillator). · All of the I/O ports maintain their original status. · The PD flag is set and the TO flag is cleared. S y s t e m C l o c k / 4 8 - b i t C o u n t e r W D T P r e s c a l e r 7 - b i t C o u n t e r 8 - t o - 1 M U X W D T T i m e - o u t W S 0 ~ W S 2 R O M C o d e O p t i o n S e l e c t W D T O S C Watchdog Timer |
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Similar Description - HT48RA3 |
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