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I have a doubt related to 10-bit mode of I2C protocol for multiple transactions with repeated start:

I would like to know how an i2c master / slave has to be designed to support 10 bit addressing for multiple transactions with repeat start (say to change the direction of the transfer) in between transactions.

Following is the scenario :

start + Read from a ten bit slave + repeat start + write to a ten bit slave + stop

As per I2C protocol,

  • Master-transmitter transmits to slave-receiver with a 10-bit slave
    address. When a 10-bit address follows a START condition, each slave compares the first seven bits of the first byte of the slave address (1111 0XX) with its own address and tests if the eighth bit (R/W direction bit) is 0. It is possible that more than one device will find a match and generate an acknowledge (A1). All slaves that found a match will compare the eight bits of the second byte of the slave address (XXXXXXXX) with their own addresses, but only one slave will find a match and generate an acknowledge (A2). The matching slave will remain addressed by the master until it receives a STOP condition (P) or a repeated START condition (Sr) followed by a different slave address.

  • Master-receiver reads slave-transmitter with a 10-bit slave address. The transfer direction is changed after the second R/W bit. Up to and
    including acknowledge bit A2, the procedure is the same as that described for a master-transmitter addressing a slave-receiver. After the repeated START condition (Sr), a matching slave remembers that it was addressed before. This slave then checks if the first seven bits of the first byte of the slave address following Sr are the same as they were after the START condition (S), and tests if the eighth (R/W) bit is 1. If there is a match, the slave considers that it has been addressed as a transmitter and generates acknowledge A3. The slave-transmitter remains addressed until it receives a STOP condition (P) or until it receives another repeated START condition (Sr) followed by a different slave address. After a repeated START condition (Sr), all the other slave devices will also compare the first seven bits of the first byte of the slave address (1111 0XX) with their own addresses and test the eighth (R/W) bit. However, none of them will be addressed because R/W = 1 (for 10-bit devices), or the 1111 0XX slave address (for 7-bit devices) does not match.

From the scenario that i mentioned above, the first transaction (read) can be implemented as mentioned in the spec. Once there is a repeat start on the bus for second transaction, master has to put (11110xx0) for second transaction i.e. master write. But there can be many devices on the bus with the same 2 MSB bits which acknowledges it. How can this scenario be handled ?

Also wanted to know how to perform a write followed by a read transaction with the repeated start for the same ten bit slave ?? (after repeated start (which comes after first transaction), are we supposed to send all 3 address bytes or only 11110xx1 address byte is enough).

Thanks in advance.

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Ten-bit addresses are sent if and only if the R/W bit is zero.

When the R/W bit is one, only the first address byte is sent. A set R/W bit cannot start an entirely new transaction, so all other ten-bit devices must ignore it.

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