Hitachi RAS-25CNH1 Service Manual page 76

11. Reset Circuit
Micro computer
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 L
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
HIC-1
AX-8E21
17
RESET
0V
5V
Q3
Q2
0V
0V
Fig. 11-1
12V line
5V line
Reset voltage
12V line
5V line
Reset voltage
Fig. 11-2
– 85 –
12V
1
R232
R4
0V
Normal: Hi
2
Reset:
until V of microcomputer
0 DD
ZD301
Lo
Time
Time