Hitachi RAM-18QH5E, Service Manual

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9. Reset Circuit
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Reset circuit performs initial setting of the microcomputer program when power is turned on.
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Microcomputer resets program with reset voltage set to Lo, to enable operation at Hi level.
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Fig. 9-1 shows the reset circuit, and Fig. 9-2 shows waveform at each point when power is turned on/
off.
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After power is turned on, 12V line and 5V line voltages rise: When 12V line voltage reaches 7.2V (Zener
voltage of ZD201), ZD201 turns ON and Q208 and Q209 turn on, and reset voltage becomes Hi. Reset
voltage is not set to Hi until VDD of microcomputer rises to 5V, enabling operation, due to ZD201.
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After power turns off, when 12V line voltage drops, ZD201 also turns OFF. However, Q208 is left ON since
reset voltage is fed back by R294, until 12V line drops to about 7.6V. This prevents chattering of reset
voltage due to voltage change in 12V line.
12V line
12V line
5V line
5V line
Fig. 9-2
Reset voltage
Reset voltage
12
6
0
V
oltage when power is turned ON
12
6
0
V
oltage when power is turned ON
Time
Time
Fig. 9-1