[sdiy] TC5514 polysix memory...upgrade?

[ASSI]

On Samstag 03 Mai 2008, Bob Weigel wrote:
> ANyway I was thinking it'd be cool to design even a simple daughter
> board that has leads for a switch that could be accessed somewhere
> non-intrusive that would give the ps6 more memory by switching a
> couple more address lines.  Does anyone know a compatible 4 bit data
> out chip that would pull this off with 16K or something? -Bob

I just had to look up in the schematics (thanks, oldcrow!) why you would 
absolutely need a 4bit SRAM.  Hilarious that they saved the second RAM 
and really just use the lower 4bit of the data bus...  Anyway, Cypress 
still makes 5V x4 SRAM, the 16kx4 part is the CY7C164/166.  However the 
lower-density parts have rather high active and standby power and I'd 
think about using a 64kx4 instead (CY7C194BN) even if you end up 
wasting some of it's capacity.  The data sheet doesn't say anything 
about a data retention mode, the only part where this is explicitly 
specified is the 256x4 CY7C1006.  Even then, the max. data retention 
current is specified at more than 1000x the current for the TC5514-APL.  
The typical values would likely be much lower, but it's something to 
check.  While you're at it, replace the NiCad with a lithium block and 
the D15 diode and the R91 resistor with a Schottky diode in antiserial 
arrangement.

The Simtek nvSRAM are also a strong consideration even though they're 
not available as x4: STK12C68 (8kx8) or STK14C88 (32kx8).  They store 
the SRAM content into a non-volatile shadow array on power down and 
recall the information on power up, so there is no need for a battery 
at all.  They'd be fast enough so you could rig a read-modify-write 
state machine with a GAL for stuffing two x4 nibbles into one x8 byte.


Achim.
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