Driver for the TLC5940 chip for use on the Parallax Propeller
 Adapted from the Brilldea Propeller Library

 Written by Heath Paddock and Colin Harrington


Description:
This program sends grey scale data to TI TLC5940
LED control chips wired in series.

The Start function starts 2 cogs. The first runs the SendData
function which constantly checks to see if there is any
data to send to the TLC5940(s), and if there is, sends it.
The second cog is handles the GSCLK and BLANK pins on the
TLC5940(s).
The program uses a two buffer system. The OffScreen
Buffer is populated by the SetChannel and SetAllChannels functions.
Each channel takes up a 16-bit word in the buffer, although
only the lower 12-bits of each word are utilized. After setting
any number of channels, a call to the Update function will trigger
the "SendData" cog to copy the OffScreen Buffer to the OnScreen Buffer
and then send the data to the TLC5940(s).
Dot correction can also be set at any after Start. A call to SetDC or
SetAllDC will update the TLC5940(s) immediately. No call to Update
is required. The dot correction data is double buffered like the
grey scale data. Although, if one was not making consecutive changes
to dot correction values, one could probably get by with a single buffer.
If your project does not require dot correction, you can connect the
VPRG pin on the TLC(s) to ground.

reference:
      http://focus.ti.com/lit/ds/symlink/tlc5940.pdf (datasheet)
      A big head start from Timothy D. Swieter, E.I. www.brilldea.com

To Do:
-convert serial shifting routine to ASM
-code to do multiplexing

**********************************************************************

    Basic Pin setup:
    
    PROPELLER                                     TLC5940
    ------------                                  ---u----
                |                           OUT1 |1     28| OUT channel 0
                |                           OUT2 |2     27|-> VPRG (pin 21)
                |                           OUT3 |3     26|-> SIN (pin 19)
                |                           OUT4 |4     25|-> SCLK (pin 20)
                |                             .  |5     24|-> XLAT (pin 17)
                |                             .  |6     23|-> BLANK (pin 18)
              23|                             .  |7     22|-> GND
              22|                             .  |8     21|-> VCC (+5V)
              21|-> VPRG (pin 27)             .  |9     20|-> 2K Resistor -> GND
              20|-> SCLK (pin 25)             .  |10    19|-> +5V (DCPRG)
              19|-> SIN (pin 26)              .  |11    18|-> GSCLK (pin 16)
              18|-> BLANK (pin 23)            .  |12    17|-> SOUT
              17|-> XLAT (pin 24)             .  |13    16|-> XERR
              16|-> GSCLK (pin 18)          OUT14|14    15| OUT channel 15
    ------------                                  --------