Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Posts Tagged ‘ATtiny24’

TFE+ Progress

Posted by retromaster on March 25, 2010

Here is the TFE+ prototype board fully populated and connected to the A500 mainboard. The ribbon cable carries the system clock as well as video color and sync lines. On the mainboard end, it is soldered to appropriate points. This is good for reliability reasons, but I have also ordered some inexpensive test hooks that can be useful when easy installation is desired.

So far, I’ve got the video output, the keyboard control and the buzzer working. There might be some minor problems but hopefully nothing unsolvable. The photo on the left shows the settings screen, which I managed to navigate to using the keyboard. I haven’t tested the SD card access (and floppy emulation) yet, partly because I seem to have misplaced my SD card adapter. There is some new code to be tested there, as the floppy data output is now handled through the ATmega644 USART module (in SPI mode) as opposed to the external shift register in the previous TFE. Once that works, I’ll move on to more advanced features such as write support and on-board MFM decoding.

One of the tricky parts here was the communication between the keyboard MCU and the main MCU. The keyboard MCU detects keypresses for a set of keys (arrows, return, ESC) and when there is a change, it places the new keyboard state (8-bits, one bit per key) in the USI data shift register, which is operated in the external clock mode. The main MCU generates the clock for the USI shift register in software and reads the keyboard state output from the keyboard MCU. This way operation of the keyboard MCU remains independent of the keyboard state reads. To prevent clashes, during shift register content updates, the keyboard MCU asserts the clock line for a while. The main MCU looks out for this situation and restarts the read operation if an update occurs in the middle.

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TFE+ A500 Keyboard Part 2

Posted by retromaster on February 25, 2010

Last night I had some more progress with the TFE+ A500 keyboard interface.

As shown in the schematics, there are two connectors: One for the keyboard, one for the A500 mainboard. All lines of both connectors are directly connected, with the exception of the CLK line. The CLK line goes through the 74LS01 which has one input tied to an ATtiny24 pin. When the TFE+ is supposedly inactive, the keyboard MCU simply monitors for key events, but does not do anything. Once it detects the keys Alt-Amiga-Tab being simultaneously pressed and released (TFE+ is supposed to become active at this point), it lowers the clock enable line, which prevents the clock pulses from the keyboard reaching the Amiga. This means the Amiga does not receive the keypresses, but the ATtiny24 still does. Obviously, in this mode, the acknowledgement must be issued by the ATtiny24 since the Amiga cannot do it.

Right now, the only thing missing here is the communication between the TFE+ main MCU and the keyboard MCU, which I’ll soon implement and test. But before that, I’d like to port the existing TFE firmware from the ATmega32 to the ATmega644.

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TFE+ A500 Keyboard Progress

Posted by retromaster on February 24, 2010

I received the parts I ordered for TFE+ yesterday evening, so I started working on the Amiga keyboard interface right away.

The photo shows an ATtiny24 on a breadboard with the A500 keyboard connected to it. I wrote a few lines of C code to read the output of the keyboard (as described in the Appendix H of the Amiga Hardware Reference Manual) and after a little bit of debugging, got it to work. When I press space, the red LED on the breadboard lights up (I did not connect the keyboard LED lines, that’s why those are not lit up).

Now that I can capture keypresses, the next stage will be to implement additional circuitry to prevent them from going to the Amiga mainboard when TFE+ is activated.

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