Hi everyone! I’m currently diving into AXI4 as part of a design exercise, and I’m looking for some advice on how to build a better intuition for the protocol. While the spec is very detailed,.. I sometimes find it challenging to visualize the VALID/READY handshake in practice, especially on channels like Write Response (B channel), where the flow feels a bit less intuitive compared to the main data channels.
For those of you who work with AXI in real-world designs, I’d love to hear what helped you the most when you were starting out. did you find it more effective to sttictly follow timing diagrams, or was it more about building small test cases and observing the behavior in a simulator??? I’m trying to find a practical way to approach the protocol without getting completely overwhelmed by the full specification. my goal is to move past just 'making it work' and actually understand the underlying design logic. Any tips or learning strategies would be greatly appreciated. Thanks!
I’ve been reading through the AMBA AXI4 Specification (IHI0022) and the Introduction to AMBA AXI guide, specifically the sections on channel dependencies. However, seeing the handshake in a live simulation feels different from reading the tables
When I first looked at AXI I came to it knowing how the older AHB protocol worked, and compared to AHB the AXI transfer handshake was a lot simpler.
AXI handshakes are simple to follow, and the mechanism is the same on every channel. Ignore all other "payload" signals and just look at xVALID and xREADY, and when both are high the transfer completes on the next ACLK rising edge. So viewing AXI simulations isn't too difficult.
Thinking of when an xVALID or xREADY signal is asserted on any channel just depends on the ability of the manager or subordinate driving that handshake control. If the channel source has data to send it will assert xVALID, and if the channel destination has space to accept data it will assert xREADY.
There are a few handshake dependency rules to follow, but common sense rather than confusing (i.e. not signaling valid read data until a read address has been transferred, not supplying a write response until all the write data has been received).
Once you get used to the handshakes, then you can get into more complexity, looking at IDs and so on, but initially I'd just focus on the xVALID/xREADY signals controlling transfer flow.