Simple Read With Full Automation - Texas Instruments UCD3138 Technical Reference Manual

PMBus Slave Mode Command Examples
DATA_REQUEST
Write to TXBUF
EOM
Write to ACK
1. DATA_REQUEST is set t
address.
2. The firmware reads from PMBST, clearing the DATA_REQUEST bit. Since one byte is being sent out,
the firmware needs to make sure that TX_COUNT is set to 1.
3. Next the firmware needs to write the byte to TXBUF. It takes ttxbwrite ns after the write for any clock
stretching of the ACK to be ended. If the firmware is fast enough, no clock stretching will occur.
4. As soon as the data starts being transmitted, the TXBUF is transferred to the shift register
5. The EOM bit will be set t
6. The firmware needs to read the PMBST register, which will clear the EOM bit.
7. Then the firmware needs to write to the ACK register. This is just an internal ACK to tell the interface
that the EOM has been detected. The ACK bit is cleared immediately, since there is no wait for an
external ACK to complete.
8. Like the write message, there is no clock stretch until a valid address is detected. If the ACK is written
to before this, there is no clock stretch at all.
Note that the A with a 1 below and with no grey background means a NACK from the master, which is
appropriate for the last byte of a read message.
10.3.9 Simple Read of 4 Bytes with Full Automation
Up to 4 bytes can be read with the same number of firmware steps as 1 byte, taking advantage of the 4
byte TXBUF. Here is a sequence diagram for 4 bytes:
362
PMBus Interface/I2C Interface
1 7 1 1
S Address Rd A
1
2
Figure 10-10. Simple Read with Full Automation
nanoseconds after the falling edge of the clock for the last bit of the
DREQ1
ns after the falling edge of the data line indicating the stop signal.
EOM
Copyright © 2016, Texas Instruments Incorporated
First Message Second Message
8 1 1
A P
Data Byte
1
3
4
SNIU028A – February 2016 – Revised April 2016
www.ti.com
1 7 1
Address
S A
5 7 8
6
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