In the context of the AXI protocol, the term "strobe signal" most commonly refers to the wstrb (write strobe) signal. This signal is crucial for indicating exactly which bytes within the write data bus are considered valid during a write transaction.
Understanding the AXI Write Strobe (wstrb)
The wstrb signal is specifically associated with the AXI write data channel and works in conjunction with the wdata (write data) signal. Its primary role, as highlighted by the reference, is to:
...indicate which bytes of the wdata signal (write data) are valid during a write transaction. The wstrb signal is a bitmask where each bit corresponds to a byte in the wdata signal, indicating whether the byte is valid (1) or should be ignored (0).
This means that during a single write transfer, the master can choose to update only specific bytes within a larger data word being sent.
How wstrb Works: The Bitmask
The wstrb signal is a vector with a width equal to the number of bytes in the wdata bus. For example, if the wdata bus is 64 bits wide (8 bytes), the wstrb signal will be 8 bits wide (wstrb[7:0]).
Each bit in wstrb corresponds directly to a byte in wdata:
- wstrb[i] = 1: Indicates that the corresponding byte
wdata[(i*8)+7 : i*8]is valid and should be written by the slave. - wstrb[i] = 0: Indicates that the corresponding byte
wdata[(i*8)+7 : i*8]is not valid and should be ignored by the slave (i.e., the slave should not overwrite the byte at that position in memory).
This mechanism allows for granular, byte-level control over write operations.
Example wstrb Mapping
Let's consider a common AXI data bus width of 32 bits (wdata[31:0]), which is 4 bytes wide. The corresponding wstrb signal would be 4 bits wide (wstrb[3:0]).
| wstrb Bit | Corresponds to wdata Bytes |
|---|---|
| wstrb[0] | wdata[7:0] (Byte 0) |
| wstrb[1] | wdata[15:8] (Byte 1) |
| wstrb[2] | wdata[23:16] (Byte 2) |
| wstrb[3] | wdata[31:24] (Byte 3) |
Purpose and Practical Use
The wstrb signal is essential for enabling partial write operations. Instead of requiring the master to read, modify, and then write back an entire data word just to change a few bytes, wstrb allows the master to write only the bytes that need updating.
Key advantages of using wstrb:
- Efficiency: Reduces unnecessary read operations for modifying partial data.
- Flexibility: Allows writes to be performed at byte granularity, simplifying memory or register access where only specific bytes need modification.
- Bandwidth Saving: Potentially reduces the amount of data transferred by only sending valid bytes (though the entire
wdatabus is often still driven).
In summary, the strobe signal, or wstrb, in AXI is a byte-enable mechanism used during write transactions to specify which parts of the write data bus carry valid information to be written.
