As required by PMBus® specification, TPS25990 supports the Group Command Protocol. The Group Command Protocol is used to send commands to more than one PMBus® target device. The commands are sent in one continuous transmission. When the target devices detect the STOP condition that ends the sending of commands, they all begin executing the command they received.

It is not necessary that all target devices receive the same command.

No more than one command can be sent to any one device in one Group Command packet.

The Group Command Protocol must not be used with commands that require the receiving device to respond with data, such as the STATUS_BYTE command.

The Group Command Protocol uses REPEATED START conditions to separate commands for each device. The Group Command Protocol begins with the START condition, followed by the seven bit address of the first target device to receive a command and then by the write bit zero (0). The secondary device ACKs and the host controller sends a command with the associated data byte or bytes.

After the last data byte is sent to the first device, the host controller does NOT send a STOP condition. Instead, it sends a REPEATED START condition, followed by the seven bit address of the second device to receive a command, a write bit and the command code and the associated data bytes.

If, and only if, this is the last target device to receive a command, the host controller sends a STOP condition. Otherwise, the host controller sends a REPEATED START condition and starts transmitting the address of the third device to receive a command.

This process continues until all target devices have received their command codes, data bytes, and if used and supported, PEC byte. Then when all target devices have received their information, the host controller sends a STOP condition.

If PEC is used, then each target device’s sub-packet has its own PEC byte, computed only for that device’s sub-packet, including that target device’s address.

When the target devices who have received a command through this protocol detect the STOP condition, they are to begin execution immediately of the received command.

When using Packet Error Checking with the Group Command Protocol, the PEC byte is calculated using only the address, command and data bytes for each target device. For example, PEC 1 is calculated using Device Address 1 including the Write bit, Command Code 1, and the data associated with Command Code 1. PEC 1 need only be calculated by the device at Device Address 1.

Similarly, PEC Byte 2 is calculated using Device Address 2 including the Write bit, Command Code 2, and the data associated with Command Code 2. Device 1 must not continue calculating PEC 1 after it sees the Repeated Start.