.. Original article at http://text.maze.io/2010/02/23/micro-electonics:-parallax-propeller
   
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   (c) 2010 Wijnand 'maze' Modderman - http://maze.io/


Micro electonics: Parallax Propeller
====================================
  
.. image:: http://i.maze.io/32a9e945.jpg
   :align: right
   :alt: Parallax Propeller
   :width: 200
   :height: 150

Recently I have acquired a `Parallax Propeller <http://www.parallax.com/propeller/>`_ development board. I am investigating how to emulate DOS text mode (80x25 @ 16 colors) on this device, and ran into a few problems.

The specs of the Parallax Propeller:

  * 8 x 10 MHz RISC processor

  * 32 kB EEPROM

  * 32 kB RAM

  * 512 x 32 bit RAM per core

  * USB/Serial interface

  * 32 digitial I/O ports

  * VGA-ready

  * PS2-ready

So, a quick calculation shows that this is by far not enough for even simple 16 color ANSI rendition. This would require at least (80 * 25 * 16 * 2 = 64000 =~ ) 64 kB of memory to buffer the characters and their foreground and background colors. 

I have ordered 256 kB of RAM which will serve as a video buffer, and 128 kB of EEPROM to store the Code Page 437 character sets. The video output will be using approximately 5 to 6 CPU cores, which leaves 1 core for interfacing the SD card and 1 core for handling the PS/2 inputs.

Secondly, I have never done any true multi-processor programming! This nifty little device has 8 cores or `cogs` which are signalled by the `hub`. The `hub` takes care of invoking each active `cog` in a round-robin fashion, allowing all cogs to share 32 I/O pins, 2 shared video memory buffers and the built-in clock.

It's nice to learn about VGA signaling, but it's quite complex. Here are the parameters I try to emulate on my Parallax Propeller board::

           ______________________          ________
  ________|        VIDEO         |________| VIDEO (next line)
      |-C-|----------D-----------|-E-|
  __   ______________________________   ___________
    |_|                              |_|
    |B|
    |---------------A----------------|

    A: scanline time
    B: sync pulse length
    C: back porch
    D: active video time
    E: front porch

The default `Propeller OBEX <http://obex.parallax.com/>`_ `VGA Tile Driver <http://obex.parallax.com/objects/176/>`_ delivers 1280x768 pixels and is a good basis for writing my own version of a VGA driver.

Hopefully this project results in a working demo, I will keep you posted :)