Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Posted by retromaster on March 1, 2010

Having tested all the new circuitry over the weekend, I finished the schematics and board layout for the TFE+ Amiga version. The board is single-sided, with a few jumper wires (mostly for power distribution). This week, I plan to make and assemble a prototype and if all goes well, start with the modification of existing TFE firmware to match the new board.

The transition to ATmega644 turned out to be even better than I expected, as a simple comparison with the old TFE schematics will quickly reveal. First of all, the mega644 has a new “USART in Master SPI mode” functionality that allowed me to eliminate the 74HC166 shift register (and a potential 74HC164 for write support). Previously, I had experimented with the ATmega32 SPI module for video and floppy read support. It was not usable for this purpose since it was not able to continuously shift out data at a constant rate (most likely due to the data not being buffered). In contrast, all USART transactions in ATmega644 are double-buffered, and a breadboard test confirmed that I could obtain exactly the same video output as that with the external shift register.

Another useful feature of the mega644 is the “System Clock Prescaler”. With the mega32 breadboard tests in previous posts, I had to use a clock divider circuit to be able to use the Amiga system clock. With the prescaler in ATmega644, I was able to eliminate all that circuitry and simplify the board design even further. In fact, this makes it possible to (optionally) overclock the mega644. I’ve seen some projects that do this, so it is certainly possible. It could be useful for High Density disk emulation or on-board (or on-the-fly) image conversion support.

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