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avr-teletext ------------ 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: https://plus.google.com/117474986382867317779/posts/eKBn5ePYWbc Hardware -------- 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 http://www.rapidonline.com * 13.875 MHz crystal. For AVR clock. This is double teletext frequency. Extremely hard to find. I got it from http://quarndon.co.uk/ * 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 Bits 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 .endm "com" being complement, or invert, a register.