The Watchdog Circuit

The watchdog circuit was designed to reset the microprocessor if it detects an error in system timing or on the address and data buses. In older games without watchdog circuits static discharge, noise or vibration would introduce false information into the system which sometimes caused the game to lock-up until it was manually reset. In later games the watchdog will detect the false information or lock-up condition and will deliver an automatic reset pulse to the microprocessor. This automatic...

The Z80 Sync Bus Controller

The component at location 6D will be either an NVC285 custom integrated circuit or a small printed circuit board with TTL chips. This is the Z80 SYNC bus controller. Its purpose is to control the passage of data between the CPU data bus and the DB (buffered data) bus. Information can pass through component 6D from the CPU data bus to the DB bus or from the DB bus to the CPU. When a component can pass data in either direction it is called bidirectional. To see how this is accomplished look at...