Teletext inserter for AVR microcontroller and lm1881 sync separator
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This is a teletext inserter for AVR microcontroller. It adds teletext to an
existing composite signal. It is controlled on I2C and features two modes.
In passthrough mode whole teletext packets are received on I2C and inserted
into the video signal, allowing the broadcast of an interleaved magazine with
multiple pages. In console mode the I2C port receives individual characters
and emulates a framebuffer by generating a single page repeatedly. The console
supports accelerated scrolling.

A video of the device operating is here:


The hardware schematic is in schematic.png. General parts list:

 * AVR microcontroller. I used Atmega 168. Using a part with more RAM would
   allow for multiple consoles are other fancy stuff.

 * LM1881 sync splitter. For generating horizontal and vertical sync pulses.
   Hard to find in through hole package. I got it at

 * 13.875 MHz crystal. For AVR clock. This is double teletext frequency.
   Extremely hard to find. I got it from

 * 74LS244N Buffer. This is used in a slightly odd way. See below.

 * Various resistors and capacitors. Especially you need some filter caps 
   between ground and vcc or the output signal will be too dirty to work.

I2C addresses

The AVR implements two I2C addresses: data and control.

0x00 - Data address

Bits    Function
0-6     7 bit characters written here will be placed into the output buffer.
7       Unused.

0x01 - Control address

0       Set mode. 0 = console, 1 = passthrough.
1-7     Unused.

Console mode

In this mode each character received will be written into a character
framebuffer, staring in the top left corner and moving to the right with each 
character. When the end of the line is reached the cursor position moves to the
beginning of the next line. When the bottom of the display is reached the row
pointers into the buffer are shifted by 1 line, and the line at the top of the 
display becomes the line at the bottom. This line is cleared and new characters
go here. Essentially it is a scrolling text console. Newlines are supported.
Setting the cursor position is not supported. The whole buffer is output on a 
loop regardless of whether new data is available.

Passthrough mode

In this mode each received character is written into the output buffer and then
outputted only once. The buffer is used with a head and tail pointer here. When
a row fills up the head pointer is incremented and when the row is output the 
tail pointer is incremented. If there are no lines to output a filler line is
output instead. This mode is intended to be used to broadcast normal teletext
magazines with multiple interleaved pages.

About that 74LS244N

This is a tristate buffer, but it isn't used as a buffer. The AVR uses SPI to 
output teletext signal. This signal needs to be off for most of the display
period. When the AVR SPI has no data to output it's output goes high. When
outputting a zero it goes low. In order to insert the teletext in an existing 
signal we need for the SPI signal to go high impedence (ie disconnected) when
there is nothing to output. The 74LS244N is used to do this. The signal from 
the AVR is connected to the enable input on the 74LS244N. Since that signal is 
active low, whenever the AVR outputs high the outputs of the 74LS244N will 
go high impedence. When the AVR outputs low (zero), the output on the 74LS244N 
will go high as that is their default state when unconnected. A side effect of
this is that the signal ends up being inverted. This is countered by inverting 
the data in the AVR before writing it:

.macro serial_send
        com     r16
        sts     UDR0, r16
        com     r16

"com" being complement, or invert, a register.