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'[PIC]: SRAM TO PIC16F84 interfacing?'
2002\01\19@020330 by Prince Anamalech

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Hello PicSters!

I was wondering how i could connect either a 6264 SRAM (64k) or a 6116LP-4 (16k) SRAM to my pic16f84?

Otherwise i have some old 30pin 1 mb ram modules out of a 486 computer, can i use these in any way?

 

Thankyou for your time

Anamalech

Web: www.anamalech.multiservers.com

 
Email: anamalech@hotmail.com


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2002\01\19@031227 by Jinx

face picon face
> I was wondering how i could connect either a 6264 SRAM (64k)
> or a 6116LP-4 (16k) SRAM to my pic16f84?

Different options, depending on how many i/o pins you want
to use and what access time you want. 6264 (8k x 8) has 8
data lines and 13 address lines (A0 to A12). 6116 (2k x 8)
has 11 address lines. Both plus /WE /CS and /OE

For addressing - shift registers, counters, latches or even a
combination. Using a counter like the 4040 is simple, but is
sequential (=slow). If you wanted to access the last byte of the
RAM, you'd have to count all the way up to it. Shift registers
(eg 74HC164) will give you faster access as any address can
be set up in around about the same time as any other. i/o used
in this way is pretty much committed to the interface chips.

Using latches instead lets you claw back some i/o pins, as the
address can be set up and then those i/o pins used for data
transfer. 74HC295, 74HC573 or similar could be used. As
most buss-orientated logic is only 8-bits, you could use PIC
i/o as a kind of "bank select". In a system with the lower 8
address bits covered by a latch IC, use a counter IC (74HC393)
to set to the 5 high order address bits.

Data i/o can be a little messy. Unlike the address pins, the
RAM data pins are obviously bidirectional. What is often
done is to use a tri-state buffer (74HC241 or similar) that
is controlled in simultaneously with /OE and /WE. The other
possibility is a 74HC251 (addressable data selector) or
74HC165 to read and a 74HC164 to write, chips separated
by a buffer chip (or even 8 isolating diodes in series with the
164's output)

If this is a new design, I'd personally go for an EEPROM or
serial RAM. For using up recycled memory chips and a bit
of s/w and EE practice I'd still tinker about with the options
above. Some are better than others of course. If you can get
your hands on a 4000 or 74 series hand book you'll soon
come up with a few ideas of your own

> Otherwise i have some old 30pin 1 mb ram modules out of a
> 486 computer, can i use these in any way?

http://www.piclist.com/techref/microchip/picsimm.htm

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2002\01\19@092728 by Byron A Jeff

face picon face
Prince:

Please, pretty please, do not send HTML mailings to the PIC list. There are
quite a few people here that use plain text E-mail readers.
I ended up having to go to the PICLIST archive simply to get your question.

>Hello PicSters!
>
>I was wondering how i could connect either a 6264 SRAM (64k) or a 6116LP-4
>(16k) SRAM to my pic16f84?
>
>Otherwise i have some old 30pin 1 mb ram modules out of a 486 computer, can i
>use these in any way?
>

Jinx covered a lot of ground in his reply. I'm going to add another. You can
save yourself a lot of grief and real estate by changing PICs. Parallel
interfaces is one area where size does matter. An extra port of three can
greatly simplify your interfaces.

These are the PICs I believe almost everyone needs in their arsenal:

Extra small: 12C509. 8 pins. few I/O. enough memory to be useful.
     Small: 16F628. 18 pins. Superior in every way to the 16F84, which I now
            consider to be an obsolete part.
     Large: 16F872 or 16F876 depending on if you want a bootloader or not.
            These 28 pin parts adds an extra port to the mix and have true
            A/D. At 8K the 16F876 has enough firepower to have a bootloader
            installed. One could be squeezed into the 16F872, but may require
            using the onbaord USART to accomplish it. Maybe.
Extra Large: 16F877 hands down. Just like the 16F876 with two extra ports
            attached.

Note that the 16F84 isn't in the list. Some days I think Microchip should
stamp the data sheet with "Do not use this part in new designs.". The fact
of the matter is that you're going to have to attach so much support circuitry
to it to accomplish the task, that you would have been better off picking a
better part.

BTW I would have picked a 16F877 and probably 2 74HCT573 octal latches.
I'd attach the latches to PORTB or PORTD and then control from the 3 bits
of PORTE.

But a 16F84 is a bit lightweight for the task.

BAJ
>
>
>

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