Although the cause cannot be specified without seeing the actual symptom, it appears that there is a problem with the sync signal.
To use the uPD6464A in the external video mode, a composite sync signal synchronous with the video signal must be input from the CSYIN pin (pin 17).
When such a problem occurs, there are two likely causes for the external video signal and character signals misaligning out of synchronization.
(1) There is a problem with the sync signal input to CSYIN.
- Monitor the CSYIN signal to check for abnormalities in level and phase.
- Check if noise is superimposed on the CSYIN signal.
The uPD6464A recognizes a pulse of 0.2 us or more as Hsync.
Therefore, if a noise pulse of 0.2 us or more is mixed with the CSYNC signal it is recognized as Hsync, causing the vertical position counter to advance.
(2) Sync separation operation cannot be performed inside the uPD6464A.
- To use the uPD6464A in the external video signal mode, a composite video signal input from CSYIN is separated into a horizontal sync signal and a vertical sync signal by the synchronization separator.
As the clock for separation, a countdown clock from the 4fsc oscillator is used.
Therefore, if the operation of the 4fsc oscillator is abnormal, the sync separation operation is abnormal.
If the above problem occurs, check if the 4fsc oscillation is correct.
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