and ROM could refer to a chip that has some data in it

that you can only read - there is no way to write it or change it

so that the chip is manufactured in a way that it has that

data in it but there are other types of

ROMs that are programmable and

those are called PROMs or "Programmable

Read-Only Memory and those usually

give you some way of programming it

once, so you get the chip and it's blank

and you can program it once and

once you've programmed it then you can't

change what's programmed in it but you

can read it as much as you want and of

course that can be inconvenient so they

also make any EPROMs which are

erasable programmable read only memories

and usually can be erased by exposing

them to ultraviolet light so this here

is an EPROM and you see

there's a little window on the top of

the chip and you can see the the die and

so what you can program this and then

it's a read-only memory at that point

but you can also erase it by exposing

the the die here to ultraviolet light so

you need some kind of ultraviolet eraser

thing to to erase these otherwise you

know you can also you leave them out in

the Sun for a few hours sometimes that

works which is which is why they also

have these little stickers that go over

top so they don't accidentally erase

themselves so that's a erasable

programmable read-only memory but the

most convenient kind of the electrically

erasable programmable read-only memory

which is what these guys are which

allows you to program them and then of

course they acted normal read-only

memory at that point but then you can

also erase them and reprogram them

electronically without needing to expose

ultraviolet light or do anything special.

So the 28C16 that's what I've got here

I've got two different 28C16s. This is

made by Catalyst, this one's made by Exel.

I also have the data sheet over here for

the Atmel AT28C16, they're all

basically the same. So this is sixteen thousand bits,

which is organized as two thousand 8-bit words or

"bytes" essentially, two thousand

bytes. Parallel EEPROMs so if we take a

look on the data sheet at the pin-out

here this is our pin out comes in a

couple different packages. We have the

plastic dual inline package

that's this guy here. It's fairly

straightforward - there's basically

eight I/O pins, so 0, 1, 2, 3, 4, 5, 6, 7 so eight I/O pins that's for

our data. They're input for

programming and normally they'd be

output for reading from it and then there are

11 address lines so zero through seven

here and eight nine and ten over here

and those address lines are used for

telling it which byte we want to read, or

write if we're writing to it for programming it.

And other than that pretty

straightforward. There's ground and power for

powering the thing and then there's a

"Write Enable", "Output Enable" and "Chip Enable",

which we will take a look at in a

moment. I'll start by hooking up power

and ground. The next pin I'll hook up is

pin 18 which is the "Chip Enable", and I'll hook

that to ground, and so chip enable is

active low so when this is low the chip

is enabled and we always want the chip to be

enabled so we'll just tie that directly

to ground. Next to note is there's are 8 I/O lines, so

I/O 0, 1, 2, 3, 4, 5, 6 and 7 so I'm going to hook those up to

some LEDs so we can see what's in the

chip so when we we look at a particular

address will be able to see what data is

stored in the chip. So I'm gonna add some LEDs

here that we'll use for looking at what

data is in here.

Alright, so there's 8 LEDs that we'll use for

looking at the data. Now I'll connect the data

pins here so I/0 0 (pin 9). I'll connect

that. I'll start connecting those to the

LEDs. So that's I/O 0 (pin 9) going to

the first LED, pin 10 to the next LED

here, and pin 11, and I/O 3 I'll connect over

here to the other side of these LEDs.

I/O 4

So I've connected all these I/O lines to the LEDs so we'll be

able to see the output of the chip, but we

need to connect the other side of the

LEDs over to ground. So I'm connecting

these LEDs to ground through these 330

ohm resistors, and that's important because

this chip doesn't have any current

limiting on its outputs so if we don't

have these current limiting resistors

here, then it's going to drive as much

current can through the chip through the

LED, potentially damaging both the chip

and the LED in the process so we don't

want that we need to do some kind of

current limiting. So I've got these 330 ohm

resistors that are that are going from

ground through the LED and then into the

chip into the output of the chip will

who will go through the LED into ground

and these are a little bit asymmetrical

because the the ones on this side are

going to the LEDs this way into ground

the ones on the this side are going through

the LEDs that way and then the ground and

so these first 5 LEDs are the other way

around

so this should allow us to see what's

coming out of these I/O lines, but in

order to do that we've got to first tell

it what address we want to look

at. So if we want to read from address 0 for

example have to set all these address

lines to 0. We want to be able to select

which address we're reading from so what

I want to do is hook these address lines

up to some switches so we can select

whichever address we want.

So I'll put some DIP-switches here that we'll use for setting the address,

and I'll hook all the address lines up to the switches

So that's the first eight address lines here

and from zero through address seven are hooked

up to these switches when the

switches are off we want the address

lines to be low so i'm gonna tie them

low with a, this is a 10k resistor I think, brown black

and I think that's orange, so it's a 10k resistor so

I'm going to tie these these all low

with these 10k resistors. So when the switches

are off these will all be tied low through

these resistors when we turn the switch

on then we want to connect these to

five volts. So if we hook the other side of the

switch to five volts then when the switch is

on the pin will be high and when the switch is

off then it's pulled up by that

resistor or pulled down i guess by

that resistor to ground so we want

this side of the switches to all be tied to five volts

So that should take care of the

first 8 address lines over here now we've still

got address 8, 9 and 10 over on this side

so i'm going to add another set of

switches here and we're going to use the

the, i guess the bottom three of these

for address 8, 9 and 10 and I'm gonna

hook them up pretty much the same way.

and so that's address 8, 9 and 10 here;

8, 9 and 10 hooked up to this side

of the switch and so again when these

switches are off we want to pull them

low, so I'll do that with the 10k resistors

and then when the switches are on we

want to be pulled high

so now we can use these first 11 switches

everything but the top one here to set our address

and of course 11 bits of address gives us the

2048 (two thousand fourty eight) or approximately 2,000

different memory locations

So the only pins we haven't hooked up yet

are the right enable and output enable

the output enable is... let's keep pin 20

and if we set that too low its active

low so if we set that to low by tying

that to ground then that will enable the

output which means that whatever address

we set here we'll see the contents down

here on the I/O lines

so we try that we can hook this up to

power now so I'm gonna hook this up to 5 volts

and what we see is all ones and so that's

address location 0 and if we change this

to address 1 we see all ones if we go

to address 2 we still see all ones, address 3

all ones and you might be noticing a pattern

here that's because when these chips are

brand-new or they're erased, they are

erased with one's at every location so

all one's named the chip is erased and

there's there's nothing stored there and

that makes sense because we haven't

programmed anything here yet.

So to program stuff that's where the right enable pin

comes into play and understand how that

works we're gonna have to take a closer

look at the data sheet if we go to page

three there's the section about byte

write and it says a few things. it says

you can use a low pulse on the right

enable or chip enable input with the

output enable high and either chip

enable write enable low respectively and

that initiates a byte write. So what does

that mean? Well you can read that a few

times and there's some more details in

there

it also helps to look over on page six

theres some timing diagrams usually the

best way to to understand these data

sheets have to read them through you

know at least a couple times and you

know when you're reading it through it

may not make sense entirely what's going

on but as you see other parts of the

data sheet you'll start to kind of get a

picture of what's going on and so here

it is helpful to look at the timing

diagram which tells you how to write and

then it turns out there's there's two

ways you can write you can either write

to the chip using the write enable

controlled option or the chip enable

controlled option

we're going to go with write enable

controlled and basically what that means

is that means that its the write enable

pin going low and then high that controls the

write, that the tell that went to write

data and the way you read these timing

diagrams is you imagine time going from

left to right and there's all these

different transitions and things that

are going on here and really what

they're trying to do is we're trying to

show you the the parameters for the

timing and all of these parameters are

given in the table above and so for

example you know there's this tWP which

is the time of the write pulse so that's

the time from here to here which is the

time that the write enable goes low and