33.3.3.4 ST - Store Data from UPDI to Data Space Using Indirect Addressing

The ST instruction is used to store data from the UPDI PHY shift register to the data space. The ST instruction is used to store data that are shifted serially into the PHY layer. The ST instruction is based on indirect addressing, which means that the Address Pointer in the UPDI needs to be written before the data space. The automatic pointer post-increment operation is supported and is useful when the ST instruction is utilized with the REPEAT instruction. The ST instruction is also used to store the UPDI Address Pointer into the Pointer register. The maximum supported size for storing address and data is 32 bits.

Figure 33-11. ST Instruction Operation

The figure above gives an example of an ST instruction to the UPDI Pointer register and the storage of regular data. A SYNCH character is sent before each instruction. In both cases, an Acknowledge (ACK) is sent back by the UPDI if the ST instruction was successful.

To write the UPDI Pointer register, the following procedure has to be followed:
  1. Set the PTR field in the ST instruction to signature 0x2.
  2. Set the address size (Size A) field to the desired address size.
  3. After issuing the ST instruction, send Size A bytes of address data.
  4. 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 similarly:
  1. Set the PTR field in the ST instruction to 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 Size B field of the instruction after the write is executed.
  2. Set the Size B field in the instruction to the desired data size.
  3. After sending the ST instruction, send Size B bytes of data.
  4. Wait for the ACK character, which signifies a successful write to the bus matrix.

When used with the REPEAT instruction, it is recommended to set up the Address register with the start address for the block to be written and use the Pointer Post Increment register to automatically increase the address for each repeat cycle. When using the REPEAT instruction, the data frame of Size B data bytes can be sent after each received ACK.