Fragment

Specification

The Fragment bundle is the concept of transmitting a “big” thing by using multiple “small” fragments. For examples :

A picture transmitted with width*height transaction on a Stream[Fragment[Pixel]]
An UART packet received from an controller without flow control could be transmitted on a Flow[Fragment[Bits]]
An AXI read burst could be carried by an Stream[Fragment[AxiReadResponse]]

Signals defined by the Fragment bundle are :

Signal	Type	Driver	Description
fragment	T	Master	The “payload” of the current transaction
last	Bool	Master	High when the fragment is the last of the current packet

As you can see with this specification and precedent example, the Fragment concept doesn’t specify how transaction are transmitted (You can use Stream,Flow or any other communication protocol). It only add enough information (last) to know if the current transaction is the first one, the last one or one in the middle of a given packet.

Note

The protocol didn’t carry a 'first' bit because it can be generated at any place by doing 'RegNextWhen(bus.last, bus.fire) init(True)'

Functions

For Stream[Fragment[T]] and Flow[Fragment[T]], following function are presents :

Syntax	Return	Description
x.first	Bool	Return True when the next or the current transaction is/would be the first of a packet
x.tail	Bool	Return True when the next or the current transaction is/would be not the first of a packet
x.isFirst	Bool	Return True when an transaction is present and is the first of a packet
x.isTail	Bool	Return True when an transaction is present and is the not the first/last of a packet
x.isLast	Bool	Return True when an transaction is present and is the last of a packet

For Stream[Fragment[T]], following function are also accessible :

Syntax	Return	Description
x.insertHeader(header : T)	Stream[Fragment[T]]	Add the `header` to each packet on `x` and return the resulting bus