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Platinum7 Electronics

TRS-80 Model 1 Clone Blog #3

VGA Timings and information

This is the timing information for a 800x600@60Hz signal taken from TinyVGA.  There are things called front porch and back porch for the horizontal scans and the vertical retrace, as well as the sync(hronising) pulses needed by the monitor.  Remember that there used to be things called Cathode Ray Tube monitors and these used a beam of electrons to excite phosphor on the back of the glass to make it glow.  This beam was "steered" by electromagnets to scan the face of the monitor, left to right and "flashing" on or off for each pixel, then turn off the beam, move down one pixel and back to the left, scan again left to right flashing as it goes, down one pixel, etc until the last row which at the right hand end the beam needs to turn off and move back to the top left to start again.  The move back to the left is controlled by the horizontal sync pulse and the back to the top is the vertical sync pulse.  These take time and this is what is shown by the timing diagrams.  Everything is measured in pixels and the pixel clock is what determines how long a pixel is.  This is known as a Raster Scan.

Raster Scan

General timing: 40MHz pixel clock, Screen Refresh rate 60Hz

Horizontal timing

Horizontal sync pulse is positive polarity

Scanline part   Pixels     Time [µs]

Display area     800        20

Front porch       40         1

Sync pulse       128         3.2

Back porch        88         2.2


Whole line      1056        26.4

Vertical timing

Vertical sync pulse is positive polarity

Frame part      Lines      Time [ms]

Display area     600        15.84

Front porch        1         0.0264

Sync pulse         4         0.1056

Back porch        23         0.6072


Whole frame      628        16.5792

The diagram on the left shows the "proper" way the timing works.  My modified version on the right is to make it a bit easier in the CUPL code.  They are the same, just that we start with the display area first and then move to the back porch, then the sync and then the front porch last...  The monitor will work it out as it uses the sync pulses to do that, and they still occur in the same place relative to the display area.  My visible area is a bit smaller as per the conversation in the last blog about fonts.

VGA Screen Timings

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