What is Burst Length and Burst Size in AXI Protocol

Burst length

Burst length in AXI is the number of transfers for the Read/Write. AXI3 supports burst lengths of 1 to 16 transfers for all burst types (FIXED, INCR and WRAP). AXI4 extends burst length support for INCR burst type to 1 to 256 transfers and remains burst length to 1 to 16 transfers for all other burst types(FIXED and WRAP).

Signal for read transfers -> ARLEN[7:0]
Signal for write transfers -> AWLEN[7:0]

Burst length for AXI3 and AXI4 Write/Read transfer defined as,
Burst length = AxLEN[3:0] + 1
Burst length for INCR burst type of extended AXI4 Write/Read transfer defined as,
Burst length = AxLEN[7:0] + 1

Above definition explains that, AxLEN[3:0] = 0 means burst length is 1, same way AxLEN[7:0] = 255 means burst length is 256.

One rule about burst lengths of wrapping bursts is, its burst length must be 2, 4, 8 or 16.

Burst size

 Burst size is the maximum number of bytes can be transfer in a burst or transfer or a beat. Burst size is nothing but data bus width.

Signal for read transfers -> ARSIZE[2:0]
Signal for write transfers -> AWSIZE[2:0]

Encoding of AxSIZE[2:0] is pasted below.

Total number of transferred bytes can be calculated as, (Bytes in transfer) * (Number of transfers).