Thread: Ortonview PCB
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Old 15th Sep 2021, 11:40 pm   #512
Slothie
Octode
 
Join Date: Apr 2018
Location: Newbury, Berkshire, UK.
Posts: 1,287
Default Re: Ortonview PCB

Quote:
Originally Posted by Mark1960 View Post
I was trying to figure out the basics of the video output, so there are three output levels controlled by two PIC outputs. Both outputs low for sync level, then the sync output is raised at the start of the line, then the second output switches between black and white levels. When the UART is initialised there is an unwanted burst at the white level.

If the synch output is changed to open collector/drain, connected to the base of the transistor, then the series resistor changed to pull up to 5v, and initialise the UART during the line synch. Then the synch output could override the high level during UART initialisation.
But, I remember something about that in order for the black to remain black it seems you have to keep feeding the serial port bytes every 8 instruction cycles. The software already has to output dummy "0" bytes after sync and turning on the serial port (or the white bar would go all the way up to the screen image) - it needs that time before starting the serial port to set up for the new scan line.The program needs the time at the end of each scan line to read in 2 more bytes so that after 8 scan lines it has the next 16 bytes to display. So it has to turn off the serial port so it can use the time to read those bytes in. That is why after vertical sync there are 2 bursts of 16 byte reads (to pre-fill the top and bottom buffers for the screen halves) and 2 reads on every scan line.
(The serial port also has to be off at the beginning of the page so that the P4 input can be read at the beginning of the page because the pin it is connected to is the clock output of the serial port - its synchronous)
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