ST - Store Data from Data Space Using Indirect Addressing

The ST instruction is used to store data that is shifted serially into the PHY layer to the bus matrix address space. The ST instruction is based on indirect addressing, which means that the address pointer in the UPDI needs to be written prior to bus matrix access. Automatic pointer post increment operation is supported, and is useful when the ST instruction is used with REPEAT. ST is also used to store the UPDI address pointer into the pointer register. Maximum supported size for storing address and data is 16 bit.

Figure 1. ST Instruction Operation

The Figure above gives an example of ST to the UPDI pointer register, and store of regular data. In both cases an Acknowledge (ACK) is sent back by the UPDI if the store was successful, and a SYNCH character is sent before each instruction. To write the UPDI pointer register, the following procedure should be followed.

Set the PTR field in the ST instruction to the signature 0x2
Set the address size field SizeA to the desired address size
After issuing the ST instruction, send SizeA bytes of address data
Wait for the ACK character which signifies a successful write to the address register

After the address register is written, sending data is done in a similar fashion

Set the PTR field in the ST instruction to the signature 0x0 to write to the address specified by the UPDI pointer register. If the PTR field is set to 0x1, the UPDI pointer is automatically updated to the next address according to the data size SizeD field of the instruction after the write is executed.
Set the SizeD field in the instruction to the desired data size.
After sending the ST instruction, send SizeD bytes of address data
Wait for the ACK character which signifies a successful write to the bus matrix

When used with the REPEAT, it is recommended to set up the address register with the start address for the block to be written, and use pointer post increment register to automatically increase the address for each repeat cycle. When using repeat, the data frame of SizeD data bytes can be sent after each received ACK.