FIXED Burst Transactions with AxLEN > 0: Addressing and WSTRB Behavior
The FIXED burst type in ARM’s AXI protocol is a specialized transaction mode designed for scenarios where the address does not increment between transfers. This is particularly useful for accessing hardware components like FIFOs, where the data width is fixed, and the same memory location or register is accessed repeatedly. However, the behavior of FIXED burst transactions becomes nuanced when the AxLEN signal is greater than 0, especially in the context of unaligned start addresses and the corresponding WSTRB (write strobe) signals.
In a FIXED burst transaction, the address remains constant across all beats of the burst. This is in contrast to INCR (incrementing) and WRAP (wrapping) burst types, where the address increments or wraps around after each beat. The AxLEN signal, which defines the number of transfers in a burst, can indeed be greater than 0 for FIXED bursts, allowing for multiple transfers to the same address. This is particularly relevant in scenarios where a peripheral or memory location requires multiple data beats to be written or read without changing the address.
The WSTRB signals, which indicate which byte lanes are valid during a write transfer, must be carefully managed in FIXED burst transactions, especially when the start address is unaligned. For example, if the AxSIZE is set to 32 bits (0x2) and the start address (AWADDR) is 0x1, the WSTRB signals for each beat will be constrained by the unaligned address. Specifically, WSTRB[0] will always be 0 because the address is unaligned, while WSTRB[3:1] can be used to indicate valid byte lanes for each transfer.
Memory Alignment Constraints and WSTRB Signal Implications
The primary challenge with FIXED burst transactions lies in the handling of unaligned start addresses and the corresponding WSTRB signals. In an INCR burst, only the first transfer can be unaligned, and subsequent transfers will align to the AxSIZE boundary. However, in a FIXED burst, the address remains constant, meaning that if the start address is unaligned, every transfer in the burst will also be unaligned. This imposes specific constraints on the WSTRB signals, as certain byte lanes will always be invalid due to the unaligned address.
For instance, consider a scenario where AxSIZE is set to 32 bits (0x2), and the start address (AWADDR) is 0x1. In this case, the WSTRB signals for each transfer will be as follows: WSTRB[0] will always be 0 because the address is unaligned, while WSTRB[3:1] can be used to indicate valid byte lanes. This behavior is consistent across all beats of the FIXED burst, as the address does not increment.
The implications of this behavior are significant for system designers, particularly when interfacing with peripherals that require precise control over data transfers. For example, when writing to a FIFO, the data must be written to the same address repeatedly, and the WSTRB signals must accurately reflect the valid byte lanes for each transfer. Failure to properly manage the WSTRB signals in a FIXED burst transaction can result in data corruption or incomplete transfers.
Implementing Correct WSTRB Handling and Address Management in FIXED Bursts
To ensure correct operation of FIXED burst transactions, system designers must carefully manage the WSTRB signals and address alignment. The following steps outline the key considerations and best practices for implementing FIXED burst transactions in ARM AXI systems:
First, determine the AxSIZE and AxLEN values for the burst. The AxSIZE defines the size of each transfer (e.g., 8 bits, 16 bits, 32 bits), while AxLEN defines the number of transfers in the burst. For FIXED bursts, AxLEN can be greater than 0, allowing for multiple transfers to the same address.
Next, evaluate the start address (AWADDR) to determine if it is aligned with the AxSIZE boundary. If the start address is unaligned, the WSTRB signals must be configured to reflect the valid byte lanes for each transfer. For example, if AxSIZE is 32 bits (0x2) and the start address is 0x1, WSTRB[0] will always be 0, while WSTRB[3:1] can be used to indicate valid byte lanes.
When generating the WSTRB signals, ensure that they are consistent across all beats of the burst. Since the address does not increment in a FIXED burst, the WSTRB signals will remain the same for each transfer. This is in contrast to INCR bursts, where the WSTRB signals may change as the address increments.
Finally, verify the behavior of the FIXED burst transaction in the context of the specific peripheral or memory location being accessed. For example, when writing to a FIFO, ensure that the data is correctly written to the same address repeatedly, and that the WSTRB signals accurately reflect the valid byte lanes for each transfer.
By following these steps, system designers can ensure correct operation of FIXED burst transactions in ARM AXI systems, avoiding common pitfalls such as data corruption or incomplete transfers. Proper management of the WSTRB signals and address alignment is critical for achieving reliable and efficient data transfers in FIXED burst scenarios.
Conclusion
FIXED burst transactions in ARM’s AXI protocol offer a powerful mechanism for accessing hardware components like FIFOs, where the data width is fixed and the same memory location or register is accessed repeatedly. However, the behavior of FIXED bursts becomes complex when AxLEN is greater than 0, particularly in the context of unaligned start addresses and the corresponding WSTRB signals. By understanding the constraints and implications of FIXED burst transactions, and by following best practices for managing WSTRB signals and address alignment, system designers can ensure reliable and efficient data transfers in their ARM AXI systems.
