34.3.3.7 REPEAT - Set Instruction Repeat Counter

The REPEAT instruction is used to store the repeat count value into the UPDI Repeat Counter register on the DL layer. When instructions are used with REPEAT, the protocol overhead for SYNCH and instruction frame can be omitted on all instructions except the first instruction after the REPEAT is issued. REPEAT is most useful for memory instructions (LD, ST, LDS, STS), but all instructions can be repeated, except for the REPEAT instruction itself.

The DATA_SIZE operand field refers to the size of the repeat value. Only up to 255 repeats are supported. The instruction loaded directly after the REPEAT instruction will be issued for RPT_0 + 1 times. If the Repeat Counter register is ‘0’, the instruction will run just once. An ongoing repeat can be aborted only by sending a BREAK character.

The figure above gives an example of repeat operation with an ST instruction using pointer post-increment operation. After the REPEAT instruction is sent with RPT_0 = n, the first ST instruction is issued with SYNCH and instruction frame, while the next n ST instructions are executed by only sending data bytes according to the ST operand DATA_SIZE, and maintaining the Acknowledge (ACK) handshake protocol.

For LD, data will come out continuously after the LD instruction. Note the guard time on the first data block.

If using indirect addressing instructions (LD/ST), it is recommended to always use the pointer post-increment option when combined with REPEAT. The ST/LD instruction is necessary only before the first data block (number of data bytes determined by DATA_SIZE). Otherwise, the same address will be accessed in all repeated access operations. For direct addressing instructions (LDS/STS), the address must always be transmitted as specified in the instruction protocol, before data can be received (LDS) or sent (STS).