Basics of EEPROM - Can someone help?

[Guest_X44]

I'm trying to learn to use the eeprom as shown in the CCS Exercise book.

The code does not have any comment so I wonder if anyone can try to make sense out of it as I'm new to eeprom.

I don't know the basics of it, how it's addressed, how it's initialised, and so on.

I have included the code so maybe, someone can put comments for me so I can understand it. I know the C language, so I only need an understanding of the eeprom. I'd like to start with a simple eeprom, like the one in the example.

#ifndef EEPROM_SELECT

#define EEPROM_SELECT PIN_B4
#define EEPROM_CLK PIN_B2
#define EEPROM_DI PIN_B1
#define EEPROM_DO PIN_B0

#endif

#define EEPROM_ADDRESS BYTE
#define EEPROM_SIZE 256

void init_ext_eeprom() {
BYTE cmd[2];
BYTE i;

output_low(EEPROM_DI);
output_low(EEPROM_CLK);
output_low(EEPROM_SELECT);

cmd[0]=0x80;
cmd[1]=0x9;

for(i=1;i<=4;++i)
shift_left(cmd,2,0);
output_high(EEPROM_SELECT);
for(i=1;i<=12;++i) {
output_bit(EEPROM_DI, shift_left(cmd,2,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);
}
output_low(EEPROM_DI);
output_low(EEPROM_SELECT);
}


void write_ext_eeprom(EEPROM_ADDRESS address, BYTE data) {
BYTE cmd[3];
BYTE i;

cmd[0]=data;
cmd[1]=address;
cmd[2]=0xa;

for(i=1;i<=4;++i)
shift_left(cmd,3,0);
output_high(EEPROM_SELECT);
for(i=1;i<=20;++i) {
output_bit(EEPROM_DI, shift_left(cmd,3,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);

}
output_low(EEPROM_DI);
output_low(EEPROM_SELECT);
delay_ms(11);
}


BYTE read_ext_eeprom(EEPROM_ADDRESS address) {
BYTE cmd[3];
BYTE i,data;

cmd[0]=0;
cmd[1]=address;
cmd[2]=0xc;

for(i=1;i<=4;++i)
shift_left(cmd,3,0);
output_high(EEPROM_SELECT);
for(i=1;i<=20;++i) {
output_bit(EEPROM_DI, shift_left(cmd,3,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);

if(i>12)
shift_left(&data,1,input(EEPROM_DO));
}
output_low(EEPROM_SELECT);
return(data);
}

[Guest_X44]

By the way, the eeprom is 93LC56A.

[Ttelmah]

[neil]

Isn't the 93x series of EEPROMS an SPI/microwire interface? These won't need pullups surely? This is a 3 wire bus.

[Ttelmah]

[Neutone]

[rwyoung]

Neutone's comments are good but I would add that you should get a copy of the data sheet for the EEPROM you are trying to use.

Programming for microcontrollers means you need to learn how to read a datasheet so no time like the present...
_________________
Rob Young
The Screw-Up Fairy may just visit you but he has crashed on my couch for the last month!

[Guest_X44]

Thanks to everyone, specially to Neutone.

I'm still coming to grips on this.

In the init_ext_eeprom() constaing the following code:

cmd[0]=0x80;
cmd[1]=0x9;

for(i=1;i<=4;++i)
shift_left(cmd,2,0);
output_high(EEPROM_SELECT);
for(i=1;i<=12;++i)
{
output_bit(EEPROM_DI, shift_left(cmd,2,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);
}
output_low(EEPROM_DI);
output_low(EEPROM_SELECT);

QUESTION: In looping 12 times it will output_bit the value 0x980 (prior to left-shifting cmd[1] = 0x09 and cmd[0] = 0x80 ). What's this 0x980 value then?


***
Going through the read_ext_eeprom(EEPROM_ADDRESS address) function :

for(i=1;i<=20;++i)
{ BYTE cmd[3];
BYTE i,data;

cmd[0]=0;
cmd[1]=address;
cmd[2]=0xc;

for(i=1;i<=4;++i)
shift_left(cmd,3,0);
output_high(EEPROM_SELECT);
for(i=1;i<=20;++i) {
output_bit(EEPROM_DI, shift_left(cmd,3,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);

if(i>12)
shift_left(&data,1,input(EEPROM_DO));
}
output_low(EEPROM_SELECT);
return(data);
}

QUESTION: Why write this and what's it got to do with reading? And why cycle 20 times? Also, the variable data is declared but not initialised.

[Guest_X44]

Thanks to everyone, specially to Neutone.

I'm still coming to grips on this.

In the init_ext_eeprom() constaing the following code:

cmd[0]=0x80;
cmd[1]=0x9;

for(i=1;i<=4;++i)
shift_left(cmd,2,0);
output_high(EEPROM_SELECT);
for(i=1;i<=12;++i)
{
output_bit(EEPROM_DI, shift_left(cmd,2,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);
}
output_low(EEPROM_DI);
output_low(EEPROM_SELECT);

QUESTION: In looping 12 times it will output_bit the value 0x980 (prior to left-shifting cmd[1] = 0x09 and cmd[0] = 0x80 ). What's this 0x980 value then?


***
Going through the read_ext_eeprom(EEPROM_ADDRESS address) function :

for(i=1;i<=20;++i)
{ BYTE cmd[3];
BYTE i,data;

cmd[0]=0;
cmd[1]=address;
cmd[2]=0xc;

for(i=1;i<=4;++i)
shift_left(cmd,3,0);
output_high(EEPROM_SELECT);
for(i=1;i<=20;++i) {
output_bit(EEPROM_DI, shift_left(cmd,3,0));
output_high(EEPROM_CLK);
output_low(EEPROM_CLK);

if(i>12)
shift_left(&data,1,input(EEPROM_DO));
}
output_low(EEPROM_SELECT);
return(data);
}

QUESTION: Why write this and what's it got to do with reading? And why cycle 20 times? Also, the variable data is declared but not initialised.

[Guest_X44]

Now I get it. I know why it's looped. I studied the timing diagram of both the Read and Write Cycles.

But what is still not clear to me is during the initialisation. It's looped 12 times and its basically sending 12 bits. The first 3 bits are 1, 0, 0.
The second and third bits represent the opcode. I'm still confused because this is the opcode for either ERASE ALL or ERASE/WRITE ENABLE.

So, what could it be? And what should it be?

[Guest_X44]

Is it necessary to put a delay after a write as shown in the CCS Exercise?
As you can see, the write function has a delay_ms(11), but the read function does not have any delay at all.

If it's necessary to put a delay, then what is the minimum required?