– 85 –
11. Reset Circuit
● Reset circuit performs initial setting of the microcomputer program before power is turned on.
● Microcomputer resets program with reset voltage set to Lo, and program can be operated with Hi.
● Fig. 11-1 shows the reset circuit and Fig. 11-2 shows waveform at each point when power is turned on
and off.
● When power is turned on, 12V line and 5V line voltages rise and 12V line voltage reaches 7.2V (Zenor
voltage of ZD501), ZD301 is turned ON, Q2 and Q3 are turned ON and reset voltage input to pin & of
microcomputer is set to Hi. By ZD501, reset voltage maintains input of pin & at L0 until VDD of microcomputer
rises to 5V to obtain operable status.
● When power is shut off and potential of 12V is lowered, ZD501 is shifted to OFF. However, since reset
voltage is feed back to Q2 by R4, Q2 maintains ON state until 12V line voltage drops to about 7.6V. This
prevents reset voltage from chattering due to voltage change in 12V line.
12
6
0
12
6
0
12V line
5V line
12V line
5V line
Reset voltage
Reset voltage
Fig. 11-2
Time
Time
Power is ON
Voltage (V)
Power is OFF
Voltage (V)
2
1
17
0V
0V
0V
0V
5V
Q3
Q2
R4
R232
12V
ZD301
HIC-1
Micro computer
AX-8E21
RESET
Normal: Hi
Reset: Lo
Fig. 11-1
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