From: Maciej Witkowiak (ytm_at_elysium.pl)
Date: 2003-09-16 23:51:08
Hello!
Last few days I have been trying to put together an interface to emulate 1351
mouse basing on input from PS/2 mouse. There is similar project on
ftp.funet.fi but it uses RS232 mouse, a kind that becomes rare.
I used Amtel AVR 90S8515[1] and so far I have implemented bidirectional PS/2
protocol using built-in serial port (SPI, not UART) and I have working
joystick emulation mode. I have also functions which (I think) should transfer
bytes over POTX/Y to C64 more or less accurately.
Now, the only thing missing is the detection of when SID starts the measuring
cycle. In the document from funet the POTX line is connected to uC interrupt
input so when it changes from high to low there is an interrupt triggered.
I tried using it in my design, but it failed to work.
There is also this quote from funet document:
"Unfortunately, the POTX signal that was enough for the 16C84 to sense an
interrupt request, is too off for this Schmitt-trigger input :-(. What you see
from this, that you can use the 16F84 as a 16C84 substitute - but the
interface won't work in native 1351 mode at all! The same applies, BTW, for
the 16CR84 chip."
While I don't understand what exactly he meant, I guess that this is the
cause why I don't get any interrupts on my AVR. From its datasheet I
learned that it has Schmitt-trigger input.
In such situation the author suggests using unused I/O from MAX232 chip for
"conditioning the POTX signal to meet the requirements". Obviously I don't
want to add MAX232 to my simple design just for that.
Are there any other options?
ytm
[1] *8515 for development, finished project should fit on a smaller device
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