switching eprom

[rr]

Ron, here's two articles that I wrote to someone a while
back. It's oriented to a c3 ecm, but still relative.

BobR.

P.S. You mentioned wanting ten blocks, think binary, to get
ten, you need to waste the other six. Can get 2x, 4x, 8x,
16x, 32x, each addtional address line doubles the amount
of addressable memory...


(1st article)

I don't know how much detail you need, so let me just type
for a while...

I need 32Kbit of storage (a 2732 EPROM), I have 256Kbit
EPROMS (a 27256). So I take the three highest address
lines, tie them to a 3-pos dip switch, and select one 
of the eight 32Kbit areas within the EPROM.

To do this, I cut those three address lines short enough
that they won't enter a socket. These are pins 2, 26 & 27.
I then place the '256 eprom into a 28 pin socket.

Now you need to do a little soldering on the eprom. Place a
10K resister from each cut eprom pin to ground, pin 14 on 
the eprom. Three pins, three resisters. (Don't cover the 
window).

Tie the three dip switch contacts on one side together, and
run a 1K resister from these to +5v, pin 28 on the eprom.

Now, the other dip switch contacts, run one of each to one
each of the three high address lines. Yep the ones with
the 10K's to ground. Now, if that dip switch position is
in the off or open position, that address line is
grounded (low), through the 10K. If that switch position
is closed or on, that address line will be high, through
the 1K.

The only thing left to do, is to tie pin 26 of the socket,
to pin 28 on the eprom. Then run a 1N914/In4148 diode from
pin 28 of the eprom, to pin 1 of the eprom. This is to place
+5v on the program pin, as spec'd, for use in the read mode.

When the device is programmed, the programming volts will
be set properly, without affecting the normal 5v on pin 28.
When programming, you only program in the 32Kbit block.
The other blocks can be programmed with a change of the dip
switch, or they can be done later.

When you program the eprom, use the '256Kbit algo, but only
have it do a 32Kbit block.

The dip switch is in binary, 3 positions, for 8 different
combinations of those 3 address lines. You can do the same 
general thing with almost any eprom combination. Goto the
National Semi (www.natsemi.com), or Atmel (www.atmel.com),
web sites for data sheets on various eproms and the pin outs.


(2nd article)

Slightly different approach...

First, watch the Bits vs. Bytes. EPROMs are sized in bits, takes
eight bits for a byte. The eproms in this disscussion are byte
addressable. So a 27256 prom is 256Kbits and equals 32KBytes,
or 32768 addressable locations.

The '7730 ECM takes 256KBit eproms, the '7747 ECM takes 32Kbit
eproms.

OK, back to the '7747 ECM, 32Kb (little 'b' is bits, big 'B' is
bytes), is 4KB. To address 4KB locations, you need 12 address
lines: 2^12 = 4096 (4KB).

A 256Kb eprom = 32KB, it has 15 address lines. 2^15 = 32768.

So, if we substitute a '256 eprom for a '32 eprom, we have an
extra 3 address lines. But, they do address usable locations.
So, using these address lines, we dice the eprom up into eight
chunks of 4KB each.

To select which chunk, we put different states (high or low), on
that group of extra address lines. Three lines, eight different
combinations.

If I used a 27128 eprom, I would have four chunks of 4KB. So,
you could use a 1Mb eprom, and have four available chunks.

HTH


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