MB86290A Datasheet, PDF (57/184 Pages) Fujitsu Component Limited. – Graphics Controller Hardware Specifications

The vertical synchronization is controlled by the following state transitions:
Otherwis
Disp
Raster counter
reaches VDP
Otherwis
Fporch
Bporch
Otherwis
Negation of external
vertical synchronization
pulse detected
Counter initialized when raster
counter reaches VTP
Sync
Otherwis
State transitions are mainly controlled using the count values of the raster
counter. The display duration is equivalent to the Disp state. When the
value of the raster counter reaches the setting of the VDP register, the
display duration ends, causing a transition from the Disp state to the Fporch
state (front porch). In the Fporch state, the processing waits for the external
vertical synchronization pulse to be asserted. When assertion of the external
vertical synchronization pulse is detected, a transition is made to the Sync
state. In the Sync state, the processing waits for the external vertical
synchronization signal to be negated. When the negation is detected, a
transition is made to the Bporch state (back porch). With the Bporch state
established, when the value of the raster counter reaches the setting of the
VTR register, the raster counter is reset and a transition is made to the Disp
state, starting display of the next field.
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