BT656 - Interrupts

Two interrupt lines are sent from the BT656 Controller to the host processor on the INT_O[1..0] bus.

Bit 0 of this bus is used for the Frame Interrupt, used to notify the processor that a new frame is being written to the external memory. The frame interrupt is taken High when the sync signal for the start of Field 1 of a frame goes High, provided also that the previous field in the stream was not the last field and that this frame is not being ignored/skipped (as part of T-scaling). The interrupt is cleared automatically after a single cycle of CLK_I, when the sync signal for Field 1 goes Low.

Bit 1 of the bus is used for the Line Interrupt, used to notify the processor that a new line is being written to external memory. The Line interrupt is taken High if the following conditions are all met:

• The sync signal for the start of the new line is High

• The new line is not being ignored/skipped (due to Y-scaling)

• The current frame is not being ignored/skipped (due to T-scaling)

• The sync signals for the start of Field 1 and Field 2 are both Low.

The interrupt is cleared automatically during the 4th byte of the SAV code for the current line.