Well Hans, I'm quite sure you could reproduce the unit given just the
information in the previous mail, but for documentation purposes and for
the benefit of the non-technically inclined, here's how it works.
It's a very simple mod; all it does is convert the pinout of the 28 pin
1Mbit mask ROM socket to fit the 32 pin Flash ROM chip (requires
re-routing of A16 and Vcc, and OE# held low) and adds the top two bits
using the switches. I have a single side of A4 on which I drew the 28 pin
socket pinout and marked in the pins which needed to be changed and a
small sketch of the switch circuit, and together with Sean's pinouts.txt
that's all I used for the project. I'll attempt to summarise the switch
circuit using ASCII art.
+5V
|
/ +------- A17
\ R | (on SCC chip)
/ ..o
| ..
A17 -------+-----o o
(on 32 pin socket)
o
|
_
-
GND
That's my first ever "useful" ASCII diagram, by the way. The switch is a 3
position with centre position "off". When it is in the upper position, A17
is connected to the SCC A17 output (it is pulled up by R but this
shouldn't matter as long as R is large enough). When it is in the middle
position A17 is pulled high. When it is in the lower position it is
grounded. This gives the functionality described in the previous mail.
There is an identical circuit for the A18 address line.
The choice of R caused me the most difficulty (!) - at the moment it is
2K2 and this works fine but probably places an unnecessary load on the SCC
chip outputs (and the MSX power supply). 4mA worst case is just wasted.
After some discussion on the IRC channel, I decided 10K was a good
trade-off between low noise and power consumption, but you are of course
free to choose any value you think appropriate.
I am going to make one or two minor "tweaks" to this project in the coming
weeks, to make it easier to use. You are welcome to include these ideas in
your designs from the outset. Firstly, I will obtain a 32 pin ZIF socket
and put it in the existing 32 pin socket so that Flash ROM chips are
easier to exchange. Secondly, I will investigate ways of mounting the
switches that are a bit stronger (at the moment they are physically
supported by the pull-up resistors!). Thirdly, I will think about adding a
pull-up resistor to OE# and putting a switch in for that line, so that the
cartridge is switchable in the usual SCC sense. I will almost certainly
replace the existing 2K2 pull-ups with 10K ones when I make these changes.
Richard
--
For info, see http://www.stack.nl/~wynke/MSX/listinfo.html
