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I am reading PCI express documentation and have a question connected with “TLP Flow control Credits”.

This is what I understood: as PCI express does not have sideband signal e.g. IRDY, TRDT, RBF, is uses flow control credit model. There are different type of TPs PH, PD, NPH, .. , and eash of them has their own credit.

Is my understanding right? And I don’t clearly understand what are the next steps with these credits. Can someone explain please?

Thanks

haykp
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1 Answers1

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In system using credit based flow control, the receiver doesn't need to provide huge buffers for the flow control, because the sender knows how many space is left in the receiver's buffers. Especially the reserve buffer space for the wire delay can be spared.

If no credit (space) is guaranteed at the receiver side, the sender won't send it's data because it knows the data would be dropped/lost. If the receiver side has more buffer space, it will send credit updates to the sender notifying the sender that more space is available. When the sender sends data, it will decrement the credits based on the amount of send data.

Paebbels
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