TKEEP and TSTRB in AXI Stream

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Hi,

I am trying to understand the concepts TKEEP and TSTRB in AXI Stream. Both are byte qualifiers but in documents they are described in terms of data byte and position byte which I don't understand. Can someone please explain them in simple words with example ?
 

Most interfaces dont bother with TSTRB. So they only have TKEEP which is essentially a byte enable.
When used together, TKEEP indicates that a byte must be transmitted, while TSRB indicates a valid byte. So here, TKEEP can be high while TSTRB is low, meaning a null byte must be sent.

In reality, I have only ever seen TKEEP used. With packet based streams, TKEEP is used to indicate the start/end position of a packet in a stream (with all words in between set with TKEEP = all 1)
 

If all the bytes are valid and there is no need of byte positioning and alignment etc, then is it ok to connect TKEEP to all 1's ?
 

joniengr said:
If all the bytes are valid and there is no need of byte positioning and alignment etc, then is it ok to connect TKEEP to all 1's ?
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For most words, tkeep will be all 1s. Are you using some packet based interface?
 

Yes, I am sending data packets from custom AXI streaming block. Each data is 32 bits wide. There is a TLAST signal on every 8 data packet.
 

In that situation binding TKEEP to all 1's will work.

I would suggest to check your vendor specification about the IP and AXI-Stream protocol, because e.g. Xilinx in UG1037 at page 100 gives clear information how they use it e.g. in IP using AXI4-Stream Video Protocol:
IMPORTANT: TKEEP and TSROBE are not used in IP using AXI4-Stream Video Protocol. When connecting to IP requiring TKEEP or TSTRB assignments, use the default values of TKEEP=1 and TSTRB=1.
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