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HT49C10-1 Datasheet(PDF) 11 Page - Holtek Semiconductor Inc |
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HT49C10-1 Datasheet(HTML) 11 Page - Holtek Semiconductor Inc |
11 / 38 page HT49R10A-1/HT49C10-1 Rev. 1.20 11 July 27, 2007 Oscillator Configuration The device provides three oscillator circuits for system clocks, i.e., RC oscillator, crystal oscillator and 32768Hz crystal oscillator, determined by configuration options. No matter what type of oscillator is selected, the signal is used for the system clock. The HALT mode stops the sys- tem oscillator (RC and crystal oscillator only) and ignores external signals to conserve power. The 32768Hz crystal oscillator (system oscillator) still runs when in the HALT mode. If the 32768Hz crystal oscillator is selected as the system oscillator, the system oscillator is not stopped; but the instruction execution is stopped. Since the 32768Hz oscillator is also designed for timing purposes, the inter- nal timing (RTC, time base, WDT) operation still runs even if the system enters the HALT mode. Of the three oscillators, if the RC oscillator is used, an external resistor between OSC1 and VSS is required, and the range of the resistance should be from 24k W to 1M W. The system clock, divided by 4, is available on OSC2 with a pull-high resistor, which can be used to synchronize external logic. The RC oscillator provides the most cost effective solution. However, the frequency of the oscillation may vary with VDD, temperature, and the chip itself due to process variations. It is therefore, not suitable for timing sensitive operations where accu- rate oscillator frequency is desired. If the crystal oscillator is selected, a crystal across OSC1 and OSC2 is needed to provide the feedback and phase shift required for the oscillator, and no other ex- ternal components are required. A resonator may be connected between OSC1 and OSC2 to replace the crystal and to get a frequency reference, but two exter- nal capacitors on OSC1 and OSC2 are required. There is another oscillator circuit designed for the real time clock. In this case, only the 32.768kHz crystal oscil- lator can be applied. The crystal should be connected between OSC3 and OSC4. The RTC oscillator circuit can be controlled to start up quickly by setting the ²QOSC² bit (bit 4 of RTCC). It is recommended to turn on the quick oscillating function upon power on, and then turn it off after 2 seconds. The WDT oscillator is a free running on-chip RC oscilla- tor, and no external components are required. Although the system enters the power down mode, the system clock stops, and the WDT oscillator still works with a pe- riod of approximately 65 ms at 5V. The WDT oscillator can be disabled by configuration options to conserve power. Watchdog Timer - WDT The WDT clock source is implemented by a dedicated RC oscillator (WDT oscillator) or an instruction clock (system clock/4) or a real time clock oscillator (RTC oscillator). The timer is designed to prevent a software malfunction or se- quence from jumping to an unknown location with unpre- dictable results. The WDT can be disabled by configuration options. But if the WDT is disabled, all exe- cutions related to the WDT lead to no operation. The WDT time-out period is fS/2 15~f S/2 16. If the WDT clock source chooses the internal WDT oscillator, the time-out period may vary with temperature, VDD, and process variations. If the clock source selects the instruc- tion clock and the ²HALT² instruction is executed, the WDT may stop counting and lose its protecting purpose, and the logic can only be restarted by external logic. When the device operates in a noisy environment, using the on-chip RC oscillator (WDT OSC) is strongly recom- mended, since the HALT instruction will stop the system clock. The WDT overflow under normal operation initializes a ²chip reset² and sets the status bit ²TO². In the HALT mode, the overflow initializes a ²warm reset², and only the program counter and stack pointer are reset to zero. To clear the contents of the WDT, there are three meth- ods to be adopted, i.e., external reset (a low level to RES), software instruction, and a ²HALT² instruction. There are two types of software instructions; ²CLR WDT ² and the other set -²CLR WDT1² and ²CLR WDT2 ². Of these two types of instruction, only one type of instruction can be active at a time depending on the options -²CLR WDT² times selection option. If the ²CLR WDT ² is selected (i.e., CLR WDT times equal one), any execution of the ²CLR WDT² instruction clears the WDT. In the case that ²CLR WDT1² and ²CLR WDT2² are cho- sen (i.e., CLR WDT times equal two), these two instruc- tions have to be executed to clear the WDT; otherwise, the WDT may reset the chip due to a time-out. C r y s t a l O s c i l l a t o r R C O s c i l l a t o r O S C 1 f S Y S / 4 O S C 2 V D D 3 2 7 6 8 H z C r y s t a l / R T C O s c i l l a t o r V D D 4 7 0 p F O S C 3 O S C 4 O S C 1 O S C 2 System Oscillator |
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