External EEPROM can't read/write

[coyotegd]

I am using a 16F877 with a 24LC256 in an effort to display a logo on a graphic LCD. The following code should write the logo to the 24LC256 and read 10 of the written addresses from the 24LC256. However, the read retrieves FFH for each address.

The 24LC256 has pins 1, 2, 3, 4, and 7 at gnd. Pin 8 is 5V. Pin 5 to PORTC.4 with a 4.7k pull-up to 5V. Pin 6 to PORTC.3 with a 4.7k pull-up to 5V. I've tried multiple 24LC256's and other resistor values to no avail.

Any suggestions? Thank you.

@ DEVICE HS_OSC, WDT_OFF, LVP_OFF, BOD_OFF, PWRT_ON, PROTECT_OFF

DEFINE	LOADER_USED	1
DEFINE    OSC 20

ADCON1 = 7

sda   var   PORTC.4     'I2C SDA for ext eeprom
scl    var   PORTC.3     'I2C SCL  for ext eeprom 
ctl    con  160	     'EEPROM control code	
addr  var  WORD         'eeprom word address
i       var   byte           'Index counter

edata     var    BYTE        'Data byte to be written and read
out_pin	var    PORTC.6   'TX pin of 16F877
ser_baud  con  32            '19200

addr   = 0
edata  = 0
pause 100
serout2 out_pin,ser_baud,["Writing to EEPROM . . .",10,13]
for i = 0 to 249	  
lookup i,[_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$3F,$C0,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$0F,$FA,$60,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$7C,$04,$38,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$D2,$60,$C8,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$01,$D0,$22,$1C,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$03,$A2,$80,$0C,$00,$00,$00],edata
I2CWRITE sda,scl,ctl,addr.highbyte,addr.lowbyte,[edata]
pause 10
addr = addr + 1
next

. . .

for i = 0 to 249	  
lookup i,[_
$00,$00,$00,$01,$FE,$1F,$9F,$E0,$7C,$07,_
$FE,$3C,$1F,$03,$80,$3F,$C0,$3C,$E0,$00,_
$7F,$DF,$FC,$14,$97,$1D,$FD,$77,$00,$1C,_
$00,$00,$00,$03,$0F,$03,$E1,$F0,$FC,$0E,_
$1E,$38,$0E,$03,$80,$61,$E0,$FD,$E0,$00,_
$77,$CD,$FC,$80,$95,$8D,$EF,$7B,$80,$1C,_
$00,$00,$00,$00,$07,$01,$C0,$F0,$1C,$1C,_
$04,$38,$0E,$03,$80,$00,$E0,$1E,$00,$00,_
$7F,$DF,$6E,$2E,$BC,$CE,$FF,$BE,$80,$1C,_
$00,$00,$00,$00,$07,$01,$C0,$70,$1C,$18,_
$00,$38,$0E,$03,$80,$00,$E0,$1C,$00,$00,_
$7D,$4F,$FF,$81,$37,$3B,$FB,$F8,$80,$1C,_
$00,$00,$00,$00,$07,$01,$C0,$70,$1C,$38,_
$00,$38,$0E,$03,$80,$00,$E0,$1C,$00,$00,_
$5F,$E5,$BB,$CA,$2F,$FE,$FF,$F4,$80,$1E,_
$00,$00,$00,$00,$3F,$01,$C0,$70,$1C,$38,_
$00,$38,$0E,$03,$80,$07,$E0,$1C,$00,$00,_
$6F,$AF,$FF,$C1,$7E,$FE,$BF,$C7,$00,$0E,_
$00,$00,$00,$01,$E7,$01,$C0,$70,$1C,$38,_
$00,$38,$0E,$03,$80,$3C,$E0,$1C,$00,$00,_
$3D,$E3,$F7,$E0,$6B,$57,$F7,$C3,$00,$0F,_
$00,$00,$00,$43,$07,$01,$C0,$70,$1C,$38,_
$00,$38,$0E,$03,$80,$60,$E0,$1C,$00,$00,_
$2F,$EA,$DD,$FE,$D9,$BF,$BD,$F3,$00,$07,_       '1231 to 1240
$80,$00,$01,$CE,$07,$01,$C0,$70,$1C,$3C],edata
I2CWRITE sda,scl,ctl,addr.highbyte,addr.lowbyte,[edata]
pause 10
addr = addr + 1
if addr > 1230 and addr < 1241 then
     serout2 out_pin,ser_baud,[dec addr,44,32,hex2 edata,72,10,13]
endif
next

. . .

for i = 0 to 169	  
lookup i,[_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$03,$C0,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,_
$00,$00,$00,$00,$00,$00,$00,$00,$00,$00],edata
I2CWRITE sda,scl,ctl,addr.highbyte,addr.lowbyte,[edata]
pause 10
addr = addr + 1
next
serout2 out_pin,ser_baud,["Reading EEPROM . . .",10,13]
for addr = 1231 to 1240
     i2cread sda,scl,ctl,addr.highbyte,addr.lowbyte,[edata]
     serout2 out_pin,ser_baud,[dec addr,44,32,hex2 edata,72,10,13]
next
serout2 out_pin,ser_baud,["Finished",10,13]
end

[skimask]

First thoughts... Get rid of the fat...
Break it down...Make it as simple as possible.
Read and write a single value to a single location, then build your way up to the final result.
Then try this:

eepromaddress var word
eepromdata var byte
i2cd var portc.0
i2cc var portc.1
eepromctrl var byte
eepromctrl = $a0
i2cread i2cd , i2cc , eepromctrl , eepromaddress , [ eepromdata ]
i2cwrite i2cd , i2cc , $a0 , eepromaddress , [ eepromdata ]

Notice anything different?


(besides the fact that there isn't a single colon up there )
(I could've done this

a var word:d var byte:c var portc.0:k var portc.1:n var byte:n=160:i2cread c,k,n,a,[d]:i2cwrite c,k,n,a,[d]

[Jerson]

I am using a 16F877 with a 24LC256 in an effort to display a logo on a graphic LCD. The following code should write the logo to the 24LC256 and read 10 of the written addresses from the 24LC256. However, the read retrieves FFH for each address.

addr = 0
write 0+(0..249)
write 249+(0..249)
write 498+(0..169)

read 1231 ????

You seem to write somewhere and read elsewhere. No wonder it reads FF

[skimask]

You seem to write somewhere and read elsewhere. No wonder it reads FF

Which is why I said to 'break it down'.
You know why it doesn't work, I know why it doesn't work...
Hopefully, now the O/P knows why it doesn't work...

[coyotegd]

I don't mean to be rude but the ". . ." means that I omitted code from the post or did you want three pages of the same thing over and over?

I also have no idea what colons have to do with anything.

[skimask]

I don't mean to be rude but the ". . ." means that I omitted code from the post or did you want three pages of the same thing over and over?

But have you tried the above suggestions at all?

I also have no idea what colons have to do with anything.

That's sort of a semi-inside-joke. Don't worry about it. (I have a tendency to write code with a load of colons in it to make more of it fit on each screen)

[Bruce]

Instead of using addr.highbyte,addr.lowbyte in your I2CREAD / I2CWRITE lines, try using
just addr. PBP already knows addr is a word and will handle both high & low bytes of the
address for you.

[coyotegd]

I wrote a very simple program but with the same output result:

5, 57
5, 255

DEFINE	LOADER_USED	1
DEFINE    OSC 20

edata     var	  BYTE

pause 100
i2cwrite portc.4,portc.3,160,5,[57]
pause 10
serout2 portc.6,32,[dec 5,44,32,dec 57,10,13]
i2cread portc.4,portc.3,160,5,[edata]
serout2 portc.6,32,[dec 5,44,32,dec edata,10,13]
end

I tried using portc.0 as SDA and portc.1 at SCL with the same result. I used a different 16F877. It just doesn't make any sense to me anymore. I've checked the datasheets and my circuit many, many times.

[Jerson]

Have you tried scoping the SCL and SDA lines to see if it is really communicating? I2C is a no-brainer - if you get it working the first time.

The address should be declared in a WORD sized variable for the 24LC256. You then use this variable to hold the addresses for access.

Address  var  Word             ' this will hold the 16 bit word address
EData     var  Byte             '  Eprom data byte for read/write

Address = 5                      ' address to access
Edata    = $55                   ' data to write
I2Cwrite SCL,SDA,$A0, Address, [EData]   ' write it
Edata = 0               ' destroy the value before read
I2Cread SCL,SDA,$A0, Address , [EData]   ' read it back
if Edata = $55 then Success                   ' if it compares, You Succeeded

[coyotegd]

It's the fact that this should be a "no brainer" that is so baffling. I've breadboarded much more complicated circuits, yet this simple EEPROM circuit won't work. I thought perhaps someone else had encountered this type of problem before and knew of a solution or could identify a source for troubleshooting.

A digital probe pulses away on both SCL and SDA during read/write ops.

BTW, I scoped portc.4 and c.3 outputs and SDA and SCL appear to be working. See Scope.jpg with data on top and clock on bottom. Both high prior to write/read operation and data high, clock low after write/read op. However, after a couple minutes, the clock goes high. I have no idea why.

The write-data signal doesn't change thoughout the write process and it's the same for the read-data signal, but the read signal is not the same as the write signal. See Scope2.jpg.