Write/Read With Repeated Start - Texas Instruments UCD3138 Technical Reference Manual

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1. The SLAVE_ADDR_RDY BIT is set t
address.
2. The firmware reads from the PMBST register, clearing the SLAVE_ADDR_RDY bit. Next the firmware
reads the address from the RXBUF, using only the low 7 bits – the most significant bit is random and
must be masked out.
3. The firmware writes to the ACK bit. If a 1 is written, the DATA_REQUEST bit will be set t
the ACK bit is set.
4. The firmware reads the PMBST register, clearing the DATA_REQUEST bit.
5. Then the firmware writes the appropriate outgoing data into the TXBUF register, after first making sure
that the TX_COUNT bits reflect the correct number of bytes. Any clock stretch will be cleared t
ns after the write to TXBUF. If the firmware is fast enough, no clock stretch will occur.
6. Once the ACK is over, the ACK bit will be cleared, and the TXBUF will be moved into the shift register
for transmission.
After the manual slave ACK, the rest of the read will continue as described in the sequences above.

10.3.13 Write/Read with Repeated Start

Both PMBus and I2C provide for a read message which starts with a write of address and command,
followed by a repeated start and a simple read command as above. Here is a sequence diagram with full
automation:
RPT_START
DATA_RDY
RD_BYTE_COUNT
Write to ACK
1. RPT_START and DATA_RDY are set t
the repeated start. RD_BYTE_CNT is set to a 1.
2. The firmware reads the PMBST, clearing the status bits. The firmware then reads RXBUF to get the
command.
3. The firmware then writes a 1 to the ACK bit. This is an internal ACK to tell the interface that the
RXBUF has been read.
4. If the ACK is not written by this time, the clock will be stretched until the ACK is written
All byte sequences after the repeated start will be the same as described above in the simple read
sequences after the start.
SNIU028A – February 2016 – Revised April 2016
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ns after the falling edge of the clock for the R/W bit of the
SAR
1 7 1 1
S Address Wr A
Figure 10-14. Write/Read with Repeated Start
RPTSTART
Copyright © 2016, Texas Instruments Incorporated
PMBus Slave Mode Command Examples
8 1 1
A S
Command
ns after the falling edge on the data line which indicates
ns after
DREQ2
TXWRITE
7 1 1
Address Rd A
2 3
1 4
PMBus Interface/I2C Interface
365