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I2C for beginners

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Hi all,


Is there a fundamental flaw(s) with the attached code. I know it isn't elegant :-). New to this and I'm trying to write the word ABCDh to address location 2004h of an RFID chip with serial EEPROM. The device address is Ah but bits b3-b1 allow for multiple devices on the same bus hence 0xA2. I'm using p1.7 = SDA and P1.6 = SCL and have the necessary pull-ups. Any help/pointers/advice appreciated whilst I still have some hair.




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#include <Wire.h>
void setup()
  Wire.begin(); // join i2c bus
byte val =   0xAB;
byte val_1 = 0xCD;
void loop()
  Wire.beginTransmission(0xA2); // transmit to device 0xA2 10100010
  Wire.write(byte(0x20));            // sends address hi byte
  Wire.write(byte(0x04));            // sends address lo byte
  Wire.write(val);               // sends value into memory
  Wire.write(val_1);             // sends value into memory
  Wire.endTransmission();     // stop transmitting


Sorry, found the paste function.

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First of all, I2C has this oddity of using 7 bit addresses. When representing 7 bits in an 8 bit byte, it's never clear where to add the extra zero bit.
Your device address is


But does this translate to


or to


try using 89h instead of A2h, it might just work.


Next thing, when writing 16 bit values over an 8 bit bus, make sure you have thew byte order (also called endianness) correct.

Does 00FFh


translate to 00h, FFh

00000000 11111111

or to FFh, 00h

11111111 00000000

so you could try either (or read back both addresses).

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First time I've been able to use this forum for a few days as all I've been getting is an error page.


I'm using the 430G2553. I had a look on data and clock pins and they were just sat high. Thought I'd try the code on an Arduino and I had data and clock activity. Sorting the I2C address out form A2h to a 7 bit address which became 51h (cheers for the reminder roadrunner84) I was able to communicate over I2C to my device :-) Out of interest I tried an MSP430FR5969 with my bit of code and that worked fine too. It would seem it there's an issue when using the 2553. I do have another 2553 which I tried and still no joy. I'm using Energia 17 by the way as 18 wouldn't work for me!

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