Figure 3.2.1.1 System reset circuit diagram
MST718
68
3.2.1 System control circuit
System control circuit is mainly composed of reset circuit, clock circuit t, serial/parallel conversion
circuit, software program, EEPROM and MCU (MST718 is built inside). The built-in MCU of MST718 is
mainly responsible for the coordination and control of system of the player. This MCU is connected with
a 2M ROM externally, which is used to store software and control function of the player. Since I/O port of
MST718 is limited, a serial/parallel conversion circuit (CD4094)is added to extend I/O port. Functions
may be adjusted through software. MST718 is connected with a 32k EEPROM externally, used to store
user-set information.
1. System reset circuit
(1) system reset circuit is shown in the figure 3.2.1.1:
(2) Working principle: this circuit is a typical low level effective reset circuit. After power on, “+5V
EXT” changes from 0V to 5V,and charge R282 through CE101,Voltage of two ends of CE101cannot
change suddenly, now Q103 is saturated on, C-E electrode is connected, MST RESET is low level to
provide pin 68 of U101 with a low level reset signal to reset MST718. After capacitor CE101 is fully
charged, voltage on two ends is near to 5V, now Q13 cuts off, voltage of pin 68 of MST718 restores to
high level to finish reset action.Reset time of this circuit is about 100MS, which is decided by the value
of R282 and Ce101. Function of D100 is to discharge Ce101 quickly to ensure the reliable reset when
power on for the second time. The trouble that appears in this circuit is that machine cannot enter
standby state after power on, and machine fault after power on.
Section Two Unit Circuit Principle
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