External SRAM on PIC (or AVR)
Dan Michaels email (remove spam text)
> There have been lots of discussions about serial SRAM on the list over
>the years. As far as I can tell, there aren't any of any significant size
>out there. I WAS using the Dallas RamPort (DS1380 and DS1381) 'til they
>discontinued them on us... I'm now moving designs over the the PIC18c452
>which has enough RAM for several products. On one product I'm using the
>18c452 with several latches to drive a 128kbyte SRAM (capacitor backed).
>That takes several chips. Seems like SOMEONE would make a reasonably
>large serial SRAM, but that doesn't seem to be the case.
Bob Ammerman wrote:
>Exactly what my search turned up - nada nil nothing none :-(
You can always go to a PIC with a large 68-84 pinout, like Harold
is doing, but adequate RAM definitely *is* a problem for small PICs.
Maybe nada [from practical perspective] but --> Solutions Cubed has
the RAMPack, which has 8Kx8, RS-232 addressable, designed originally
for use with Basic Stamp, but expensive at $29.95 from Jameco. Can
expand to 32K.
I recently designed my own spin-off from the RAMPack. Uses 2 32Kx8
SRAMs, controlled by a PIC64 on a small 2"x2" pcb. Addressable via
RS-232, and also in byte-mode with address/data lines multiplexed.
Can also be addressed in nybble-mode, so you can run off a PIC using
as few as 8 interface lines. I just received the pcbs, but haven't
finished the s.w. yet. I may eventually add I2C access. Estimated
max read/write rate is 500KB/sec in burst-mode/byte-mode.
This is a more difficult approach [and more expensive] than going
to a larger pinout PIC, but I think a reasonable compromise for
drop-in to a system with a small PIC.
I had also considered going with a '4040, like Octavio, but you can't
have multiple data buffers or random access that way. An alternative
is 4 '161' chips, but that's a "real" PITA.
- Dan Michaels
See also: www.piclist.com/techref/microchip/memory.htm?key=sram
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