Personally I'd put down a CPLD and do everything in one device, that way you
can change the function as it pleases you. For example you might decide that
you want individual bits allocated to individual addresses so as to avoid
bit shifts and masks in the software, or you might decide that you want
whole words of data so that the changes on each bit happen at the same time.
With a CPLD this will be easy to configure.

However I'm not sure why you need an external latch. Assuming that you are
not using the "SAMOSA bus", just the pins, since the SAMOSA signals come
from the FPGA you can use the FPGA to address decode and latch the data from
the processor bus.

You might want to buffer the output so that when you accidentally connect IO
pins to 12v, as we all do from time to time, you only blow up the buffer
chip (if you're lucky). In which case any HCT device will do.

If you *are* trying to use the SAMOSA bus then you will be better off using
a CPLD, as the address and data are multiplexed over the same pins and there
are a number of control signals to decode to get the bus data flow in the
right direction.

Hope this helps

David B


-----Original Message-----
From: Peter Clifton [mailto:pcjc2@cam.ac.uk] 
Sent: Monday, February 04, 2008 10:54 PM
To: david@itechnic.co.uk
Cc: balloon
Subject: Re: [Balloon] Balloon 3 FPGA code?


On Thu, 2008-01-24 at 16:36 +0000, David Bisset wrote:

> a) Use the SAMOSA bus. This can be used as a word bus with muxed address
> latches (actually this is a NAND FLASH type bus)

Hi David,

Just on the off-chance you had a part-number for such a latch, could you
send it my way please? Were you thinking of something like the 74HCT373?
(With some address decode chip).

Reusing the Samosa bus as an FPGA controlled word bus sounds like it may
be the best way if I'm to use the Balloon board. Sacrificing the CF
address and data-bus could also yield plenty of IO lines, but that has
obvious drawbacks.

Best wishes,

-- 
Peter Clifton

Electrical Engineering Division,
Engineering Department,
University of Cambridge,
9, JJ Thomson Avenue,
Cambridge
CB3 0FA

Tel: +44 (0)7729 980173 - (No signal in the lab!)