Hitachi RAM-60QH4 Manuel De Service page 102

9. Reset circuit
＋
C224
−
• Reset circuit performs initial setting of microcomputer program when power is turned on.
• Microcomputer resets program with reset voltage set to Lo, to enable operation at Hi level.
• Fig 9-1 shows reset circuit, and Fig. 9-2 shows waveforms at each point when power is turned on/off.
• 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.
• 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.
12
6
0
12
6
0
+5V
R290
R291
Q208
Q209
C223
0V
To trip signal
composing circuit
Fig. 9-1
12V line
5V line
Reset voltage
– 114 –
+12V
ZD201
R292
R293
R294
R295
0V
12V line
5V line
Reset voltage
Fig. 9-2
Microcomputer
AX-6M00
48
RES
Time
Time