Chapter 5: Functional Circuit Description
Type 150/150G/400/400G Course Computers Service Manual 83156-1
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Type 150/400 Course Computers
Watchdog
Transistors TR67a and TR67b monitor the “health” of the microcontroller IC15. Drive
pulses from the microcontroller on signal line WATCHDOG-DRIVE keep capacitor C67
charged up which maintains the output signal (WATCHDOG) in a high logic state. If the
microcontroller crashes, the input signal WATCHDOG-DRIVE will cease, causing the
output signal WATCHDOG to go low. This Output signal is fed into the H-Bridge control
logic to shut it down in the event of any problem occurring. Link LK3 (normally not fitted)
connects capacitor C67 to the 5V-DIG power rail, keeping it charged at all times. This is
used for diagnostic purposes only.
Flash Memory Programming
FET TR72 is used to connect the VPP (programming voltage) pin of the microcontroller to
the 7.8V power rail during Flash memory programming.
Non-Volatile Memory
Calibration data, hours spent in Auto modes etc. is stored in EEPROM memory IC17. This
chip also has a secondary function of providing the reset signal to the microcontroller and
the H-Bridge shut down circuit.
