2. Synchronization

A. Horizontal Timing

A hardware counter on this chip produces all horizontal timing (such as sync, blank, burst) independent of the microprocessor, This counter is driven from an external 3.58 Mhz oscillator and has a total count of 228. Blank is decoded as 68 counts and sync and color burst as 16 counts.

B. Vertical Timing

There are one bit, addressable registers on this chip for vertical sync and vertical blank. The timing for these functions is established by the microprocessor by writing zero or one into these bits. (VSYNC, VBLANK )

C. Composite Sync

Horizontal sync and the output of the vertical sync bit are combined together to produce composite sync. This composite sync signal drives a chip output pad to an external composite video resistor network.

D. Microprocessor Synchronization

The 3.58 MHz oscillator also clocks a divide by three counter on this chip whose output (1.19 Mhz) is buffered to drive an output pad called 00. This pad provides the input phase zero clock to the microprocessor which then produces the system 02 clock (1.19 Mhz). Software program loops require different lengths of time to run depending on branch decisions made within the program. Additional synchronization between the software and hardware. This is done with a one bit latch called WSYNC (wait for sync). When the microprocessor finishes a routine such as loading registers for a horizontal line, or computing new vertical locations during vertical blank, it can address WSYNC, setting this latch high. When this latch is high, it drives an output pad to zero connected to the microprocessor ready line (RDY). A zero on this line causes the microprocessor to halt and wait. As shown in figure 2, WSYNC latch is automatically reset to zero by the leading edge of the next horizontal blank timing signal, releasing the RDY line, allowing the microprocessor to begin its computation and register writing for this horizontal television line or line pair.

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