u-blox 6

4.3.1.1 Random Read Access
Random read operations allow the master to access any register in a random manner. To perform this type of
read operation, first the register address to read from must be written to the receiver (see Figure DDC Random
Read Access). Following the start condition from the master, the 7-bit device address and the RW bit (which is a
logic low for write access) are clocked onto the bus by the master transmitter. The receiver answers with an
acknowledge (logic low) to indicate that it is responsible for the given address. Next, the 8-bit address of the
register to be read must be written to the bus. Following the receiver’s acknowledge, the master again triggers
a start condition and writes the device address, but this time the RW bit is a logic high to initiate the read
access. Now, the master can read 1 to N bytes from the receiver, generating a not-acknowledge and a stop
condition after the last byte being read. After every byte being read, the internal address counter is
incremented by one, saturating at 0xFF. This saturation means, that, after having read all registers coming after
the initially set register address, the raw message stream can be read.
DDC Random Read Access
4.3.1.2 Current Address Read
The receiver contains an address counter that maintains the address of the last register accessed, internally
incremented by one. Therefore, if the previous read access was to address n (where n is any legal address), the
next current address read operation would access data from address n+1 (see Figure DDC Current Address
Read Access). Upon receipt of the device address with the RW bit set to one, the receiver issues an acknowledge
and the master can read 1 to N bytes from the receiver, generating a not-acknowledge and a stop condition
after the last byte being read.
To allow direct access to streaming data, the internal address counter is initialized to 0xFF, meaning that
current address reads without a preceding random read access return the raw message stream. The address
counter can be set to another address at any point using a random read access.
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