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This project offers the
very simple VGA interface for PIC24H and dsPIC33 microcontrollers.
As the signal generation is supported by software, it contains a
minimum of hardware. Video memory is located in internal RAM, and
the character generator is a lookup table in internal program
memory. All video signal timings match VGA standard in 800x600 mode,
so the standard VGA monitor can be directly connected. Each
character pixel is represented as 2x2 pixels on the 800x600 screen.
As the whole video
support (including PS2 input and UART RX/TX) is located in the
single interrupt routine, it is easy to embed it in any software
which is not time dependant and which allows time-locked interrupts.
Limitations:
- This project is based
on Microchip's PIC microcontroller which runs at 80 MHz (40 MIPS),
so the selection of limited to 24H or dsPIC33 families. The minimum
of internal Data RAM is 8 Kbytes, as this routine uses 4K for Video
Memory and housekeeping (5.3 Kbytes in Demo
mode).
- It takes a lot of
processor time to generate the VGA signal. In the pure text mode
with black areas between text lines, it requires about 77% of processor
time (which leaves 9.2 MIPS for other tasks in the main
program flow), and in the full-screen mode, which
displays not only characters but also the full-height
frame pseudographics boxes and lines, 95.6% of processor time
(but there is still 1.76 MIPS left for other tasks). Those
are roughly measured values, not calculated.
- Only the text mode is
supported. Full graphics mode is not possible.
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