Maybe this should be [OT]??
Hi, i've been thinking about a "perfect" ram chip
for a PIC, small, simple, but with a big capacity
(1 megabyte) and very fast and PIC-friendly data
transfer. Something many times faster and simpler
than I2C with a basic parallel format.

A 1Mb static ram in a 12-pin DIP, which needs
only 10 PIC pins:

* D0-D7 (8 data lines)
* Read strobe (control line)
* Write strobe (control line)

The PIC can read/write two full bytes to the ram
at a time with each control pulse:
when read strobe goes / ram shows the next byte,
when read strobe goes \ ram shows another byte,
giving the ability to read twice as many sequential
bytes in a fast block read. The ram address counter
always increments to the next byte position.

The same for writing sequential bytes. With each
strobe pulse you can write 2 sequential bytes
to the ram. Or continue strobing to write a block.

By using the extra control sequences available
from our two strobe lines we can also write a
one or two byte address to the ram. Giving 65536
adressable locations of 16 bytes each, and the
ability to do fast contiguous block read/writes
from anywhere to anywhere.

Bytes are read in pairs, no big problem considering
the speed gains. The main advantage would be a
SINGLE SMALL CHIP with a very high capacity of ram
which can be read by the minimum PIC code (and
pins) at very high block data rates.

To send a 16bit address to the ram chip would
probably be about 10 or 12 PIC instructions.
Or you could send just the low address byte,
in about 6 instructions.

How quick for a block load from the 1Mb ram to
our 16f628? 1Mb/second?

block_load
        bsf PORTA,rstrobe       ; send / read_strobe pulse to ram
        movf PORTB,w            ; get first ram byte!
        movwf INDF              ; save byte to PIC internal ram
        incf FSR,f              ; move our pointer

        bcf PORTA,rstrobe       ; send \ read_strobe pulse to ram
        movf PORTB,w            ; get second ram byte!
        movwf INDF              ; save byte to PIC internal ram
        incfsz FSR,f            ; move pointer again and test
        goto block_load         ; keep looping
                                ; block must be done here.

That's 10 pic instructions for 2 bytes of ram
loaded, or a data transfer rate of 1 million
bytes per second on a 20MHz PIC! The ability to
read/write full banks of PIC ram very quickly
to a HUGE external static ram opens up a lot of
possibilities, especially with just one small
PIC and one 12-pin ram chip. Now Microchip have
the 16f628 and similar small powerful chips
this would be a dream ram, suitable for
high speed high capacity datalogging or DSP,
or maybe a PIC + 1Mb ram + graphics lcd?
Or a very powerful "interactive talking" pcb
with only 2 chips and some buttons?

I wish someone was making these ram chips.
:o)
-Roman

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