RAMCAL design options

[Ludis Langens]

"Andrew K. Mattei" wrote:
> 
> We would actually be using the ECM to write to SRAM.

Yes.

> writes the data to the SRAM memory via utilization of the !CS and !OE
> pins (I *assume* these aren't hardwired +/- in the ECM, and are
> available for our usage).

They are hardwired.  They are asserted for EPROM read cycles.  The
Xilinx chip intercepts them.  It routes them to the EPROM, or the SRAM,
or performs a special cycle.  SRAM writes are triggered by back-to-back
reads of $FFFD, $FFFC, and $FD??.  This sequence should not be possible
from 68HC11 code executing out of the EPROM.  During SRAM write cycles,
the CPU provides the address (thinking it is doing a read), the Xilinx
chip provides the data, and the SRAM is told to write.

> Our message to the ECM would consist of a
> special activating function code, then the address, then the data? Also
> a Chip Select function code?

Yes, and yes (actually a bank select function code).

> When you plug the ECM in, is the
> "existing" EPROM copied in to our SRAM, and then the computer reads from
> our SRAM, or is it the ECM starts up reading the EPROM, and then when
> given the command it jumps to reading the SRAM?

It boots out of the EPROM.  It could copy itself to the RAM, but that is
not required.  It would switch to the RAM (or back to EPROM) via an ALDL command.

> So, now, on to which ECM's it would work on (our "market").

1227165, 1227727, 1227730, 1228253, and many others.  It would require
an ECM program which uses the 8192 ALDL - this might knock out a few applications.

> My son and I will bat this around some hopefully this weekend

Think about how to piggyback a MEMCAL onto a board that plugs into the
MEMCAL socket.  Only the '727 seems to have enough vertical space to do
this under a closed MEMCAL access door.

PCB mount female headers exist, but are hard to find.  (I'd get them
from a surplus store, but that may not be a viable option for everyone.)
 Female headers for ribbon cable are common.  Perhaps that's how this
board should plug into an ECM.

ECMs that use EPROMs instead of MEMCALs are a potential problem.  How
should we plug into the EPROM socket?  With a different PCB, or one with
multiple connectors?

ECMs with 32 pin 128KByte EPROMs are also a problem.  The XC9536 has no
pins left for an A16 input.  The A14 output could be deleted and the pin
used for an input.  Should there be two design variations, a jumper
configured universal design, or no support for 27C010 chips?

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/