I2C Read/Write problems

[Dispersion123]

Hello all,

I did a search but didn't find anything that quite explains my problem, so here it goes:

I'm attempting to do I2C communications with another (slaved) microcontroller. The other microcontroller is a purchased product so I have little control over the code loaded on it. The device ID (device address?) is $10, I'm attempting to read a 16bit word on the microcontroller that is stored at $22 and $23 on the microcontroller. The microcontroller uses I2C communications at 400Khz, but for some reason my clock is at 122Khz (PIC16f676, 20MHz crystal). I'm not 100% sure I'm using the I2C read command correctly :P. Here is my code thus far:

define OSC 20

'lines to use i2cread
SDA var PORTA.0
SCK var PortA.1
'end lines to use i2cread

'LCDOUT DEFINES
define LCD_DREG PORTC
define LCD_DBIT 0
define LCD_RSREG PORTC
define LCD_RSBIT 4
define LCD_EREG PORTC
DEFINE LCD_EBIT 5 'Register enable bit on portc.5
define lcd_lines 4
'END LCDOUT DEFINES


Scntrl var byte
sadd var byte
Scntrl = $10
sadd = $22
myvar var word
pause 2000
main:
lcdout $fe,1,"Dubugging..."
i2cread PortA.0, PortA.1, Scntrl,sadd,[myvar.lowbyte, myvar.highbyte],fail

'last:
lcdout $fe,$c0,"Success!!!"
lcdout $fe,$94, "myvar = ", #myvar
goto done
fail:
LCDOUT $fe,$c0,"READ FAILED!!!"
done:
end

NOTE: THE LCD Works fine.

[skimask]

uses I2C communications at 400Khz

Probably not 400Khz all the time, but a maximum of 400Khz...

(PIC16f676, 20MHz crystal).

Look carefully at the datasheet for the 'F676, specifically the RA0, RA1, and section 6 of the datasheet...

[JD123]

I don't think you can use the myval.Highbyte etc. in the I2C instruction.

From the manual:

"If a word-sized Var is specified, 2 bytes are read and stored into the Var high byte first, followed by the low byte. This order is different than the way variables are normally stored, low byte first."

Looks like you just use the variable (WORD) as is. Pay attention to the loading order.

[Dispersion123]

I have tried just putting myvar instead of myvar.lowbyte and myvar.highbyte in there. So, I'm guessing I need to turn the comparators off?

ANSEL = %00000000
CMCON = %00000111

???

Any suggestions on how to get the correct device address sent ($10), and read the address $22?

[mister_e]

You already have a LCD attached to it... why not monitor the HEX value of the WORD variable using HEX4

Not sure if the optional label jump work with I2CREAD and your external device...

[JD123]

Any suggestions on how to get the correct device address sent ($10), and read the address $22?

If the device address is $10 then doesn't the address have to be left shifted one place?

$10 = %00010000

PBP handles the R/W bit (bit.0), so your sending out address %0001000R (Read or Write bit) and the address "seen" by the receiving device is %00001000

Remember that I2C device addresses are shifted left one place to allow for the read/write bit. You'll never (edit: never say never - there are 10 bit addressing in some devices, though this is not part of the original I2C design data) see an I2C device address over $7F, or %01111111 or DEC127.

So, I think you should be using the DEVICE ADDRESS of $20

[mister_e]

Doesn't sounds right to me... how about most I2C EEPROM with $A0?

[JD123]

Doesn't sounds right to me... how about most I2C EEPROM with $A0?

Sorry, I overlooked your post. Are you seeing what I'm saying?

[skimask]

If the device address is $10 then doesn't the address have to be left shifted one place?

I thought PBP handled that part for you. 'cause with a generic serial eeprom, the address is something like $a0, and that's how you address it...$a0, without the left shift, PBP handles the R/W bit.
But then again, we don't know exactly what kind of secret magic device is being addressed here...

[JD123]

I thought PBP handled that part for you. 'cause with a generic serial eeprom, the address is something like $a0, and that's how you address it...$a0, without the left shift, PBP handles the R/W bit.
But then again, we don't know exactly what kind of secret magic device is being addressed here...

$A0 is the LOWER 7 bit's of the address, shifted left one place. The DEVICE address being sent out and 'seen' by the reciever is %01010000, or $50. Check the specs of I2C, not just EEPROMS. The device address is specified as a 7 bit address, not 8. But since we are adding the R/W bit to the 8 bit address byte, we have to left shift the DEVICE address one place.

The confusion comes in dealing with 2 different data "specs" or "types" combined into one 8 bit Byte being send. Then suming the two values to "use" one value. That "one" value is not the device address.

You ever do this: (I know I have)
Gang 8 64KBit EEproms and use the device address (bit 4:1) by just rolling the long address over directly into the device address. It won't work. Trust me, just shift the address left once to embed the address into the device address byte.