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 |