Basic I2C question

[jseidmann]

I have a basic question of how to interface with I2C components. I am trying to interface with a real time clock (RTC) where I have to be able to read/write to specific registers. My question is this: what is the code to read a specific register or write to a specific register. I have something like this, but it doesnt work:

i2c_start();
i2c_write(0xD1); //Read command for RTC address
i2c_write(0x00); //Read register 0
data1 = i2c_read();
i2c_stop();

This doesnt seem to work. I know this is probably a very basic question, but a short example routine would be greatly appreciated. I guess, I dont know how the slave device knows that I am looking at a specific register, I would think the second 'write' command would be taken as an enable command for a different address, no?

Anyways, if there is some code to make it clear how to write to a register or read to a register, I would greatly appreciate it. I already understand that the last bit of the address needs to change for a read/write function. Also, should I wait any time between the start and the write, or any of the writes and the read commands?

Thanks!

[rnielsen]

What is the part number of the component you are trying to talk to? Many of them have different sequences that need to be followed in order to establish the proper communications with them.

Ronald

[jseidmann]

I am looking at the Dallas Semiconductor DS1307 64x8 RTC.

Thanks!

[Ttelmah]

The 'key', is that the chip does not allow you to send a 'register address', for the read command. It only allows the register address to be selected for a 'write' (quite a lot of I2C chips are like this).
To set the register address for a read, you have to use a different sequence:

[PCM programmer]

And the last read operation does a "NACK" (a 0 parameter):

[jseidmann]

Thanks everyone! I will not be able to test this out for a few days, but I appreciate all your input and help!

Thanks!