First-in first-out queues

The nmigen.lib.fifo module provides building blocks for first-in, first-out queues.

class nmigen.lib.fifo.FIFOInterface(*, width, depth, fwft)

Data written to the input interface (w_data, w_rdy, w_en) is buffered and can be read at the output interface (r_data, r_rdy, r_en). The data entry written first to the input also appears first on the output.

Parameters:

  • width (int) – Bit width of data entries.

  • depth (int) – Depth of the queue. If zero, the FIFO cannot be read from or written to.

  • fwft (bool) – First-word fallthrough. If set, when r_rdy rises, the first entry is already available, i.e. r_data is valid. Otherwise, after r_rdy rises, it is necessary to strobe r_en for r_data to become valid.

Variables:

  • w_data (Signal(width), in) – Input data.

  • w_rdy (Signal(1), out) – Asserted if there is space in the queue, i.e. w_en can be asserted to write a new entry.

  • w_en (Signal(1), in) – Write strobe. Latches w_data into the queue. Does nothing if w_rdy is not asserted.

  • w_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_data (Signal(width), out) – Output data. The conditions in which r_data is valid depends on the type of the queue.

  • r_rdy (Signal(1), out) – Asserted if there is an entry in the queue, i.e. r_en can be asserted to read an existing entry.

  • r_en (Signal(1), in) – Read strobe. Makes the next entry (if any) available on r_data at the next cycle. Does nothing if r_rdy is not asserted.

  • r_level (Signal(range(depth + 1)), out) – Number of unread entries.

Note

The FIFOInterface class can be used directly to substitute a FIFO in tests, or inherited from in a custom FIFO implementation.

class nmigen.lib.fifo.SyncFIFO(*, width, depth, fwft=True)

Synchronous first in, first out queue.

Read and write interfaces are accessed from the same clock domain. If different clock domains are needed, use AsyncFIFO.

Parameters:

  • width (int) – Bit width of data entries.

  • depth (int) – Depth of the queue. If zero, the FIFO cannot be read from or written to.

  • fwft (bool) – First-word fallthrough. If set, when the queue is empty and an entry is written into it, that entry becomes available on the output on the same clock cycle. Otherwise, it is necessary to assert r_en for r_data to become valid.

  • mem_attrs (dictionary) – Dictionary of synthesis attributes passed through to the Memory used in the FIFO

Variables:

  • level (Signal(range(depth + 1)), out) – Number of unread entries. This level is the same between read and write for synchronous FIFOs.

  • w_data (Signal(width), in) – Input data.

  • w_rdy (Signal(1), out) – Asserted if there is space in the queue, i.e. w_en can be asserted to write a new entry.

  • w_en (Signal(1), in) – Write strobe. Latches w_data into the queue. Does nothing if w_rdy is not asserted.

  • w_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_data (Signal(width), out) – Output data. For FWFT queues, valid if r_rdy is asserted. For non-FWFT queues, valid on the next cycle after r_rdy and r_en have been asserted.

  • r_rdy (Signal(1), out) – Asserted if there is an entry in the queue, i.e. r_en can be asserted to read an existing entry.

  • r_en (Signal(1), in) – Read strobe. Makes the next entry (if any) available on r_data at the next cycle. Does nothing if r_rdy is not asserted.

  • r_level (Signal(range(depth + 1)), out) – Number of unread entries.

class nmigen.lib.fifo.SyncFIFOBuffered(*, width, depth)

Buffered synchronous first in, first out queue.

This queue’s interface is identical to SyncFIFO configured as fwft=True, but it does not use asynchronous memory reads, which are incompatible with FPGA block RAMs.

In exchange, the latency between an entry being written to an empty queue and that entry becoming available on the output is increased by one cycle compared to SyncFIFO.

Parameters:

  • width (int) – Bit width of data entries.

  • depth (int) – Depth of the queue. If zero, the FIFO cannot be read from or written to.

  • fwft (bool) – Always set.

Variables:

  • level (Signal(range(depth + 1)), out) – Number of unread entries. This level is the same between read and write for synchronous FIFOs.

  • w_data (Signal(width), in) – Input data.

  • w_rdy (Signal(1), out) – Asserted if there is space in the queue, i.e. w_en can be asserted to write a new entry.

  • w_en (Signal(1), in) – Write strobe. Latches w_data into the queue. Does nothing if w_rdy is not asserted.

  • w_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_data (Signal(width), out) – Output data. Valid if r_rdy is asserted.

  • r_rdy (Signal(1), out) – Asserted if there is an entry in the queue, i.e. r_en can be asserted to read an existing entry.

  • r_en (Signal(1), in) – Read strobe. Makes the next entry (if any) available on r_data at the next cycle. Does nothing if r_rdy is not asserted.

  • r_level (Signal(range(depth + 1)), out) – Number of unread entries.

class nmigen.lib.fifo.AsyncFIFO(*, width, depth, r_domain='read', w_domain='write', exact_depth=False)

Asynchronous first in, first out queue.

Read and write interfaces are accessed from different clock domains, which can be set when constructing the FIFO.

AsyncFIFO can be reset from the write clock domain. When the write domain reset is asserted, the FIFO becomes empty. When the read domain is reset, data remains in the FIFO - the read domain logic should correctly handle this case.

AsyncFIFO only supports power of 2 depths. Unless exact_depth is specified, the depth parameter is rounded up to the next power of 2.

Parameters:

  • width (int) – Bit width of data entries.

  • depth (int) – Depth of the queue. If zero, the FIFO cannot be read from or written to.

  • r_domain (str) – Read clock domain.

  • w_domain (str) – Write clock domain.

  • fwft (bool) – Always set.

Variables:

  • w_data (Signal(width), in) – Input data.

  • w_rdy (Signal(1), out) – Asserted if there is space in the queue, i.e. w_en can be asserted to write a new entry.

  • w_en (Signal(1), in) – Write strobe. Latches w_data into the queue. Does nothing if w_rdy is not asserted.

  • w_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_data (Signal(width), out) – Output data. Valid if r_rdy is asserted.

  • r_rdy (Signal(1), out) – Asserted if there is an entry in the queue, i.e. r_en can be asserted to read an existing entry.

  • r_en (Signal(1), in) – Read strobe. Makes the next entry (if any) available on r_data at the next cycle. Does nothing if r_rdy is not asserted.

  • r_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_rst (Signal(1), out) – Asserted, for at least one read-domain clock cycle, after the FIFO has been reset by the write-domain reset.

class nmigen.lib.fifo.AsyncFIFOBuffered(*, width, depth, r_domain='read', w_domain='write', exact_depth=False)

Buffered asynchronous first in, first out queue.

Read and write interfaces are accessed from different clock domains, which can be set when constructing the FIFO.

AsyncFIFOBuffered only supports power of 2 plus one depths. Unless exact_depth is specified, the depth parameter is rounded up to the next power of 2 plus one. (The output buffer acts as an additional queue element.)

This queue’s interface is identical to AsyncFIFO, but it has an additional register on the output, improving timing in case of block RAM that has large clock-to-output delay.

In exchange, the latency between an entry being written to an empty queue and that entry becoming available on the output is increased by one cycle compared to AsyncFIFO.

Parameters:

  • width (int) – Bit width of data entries.

  • depth (int) – Depth of the queue. If zero, the FIFO cannot be read from or written to.

  • r_domain (str) – Read clock domain.

  • w_domain (str) – Write clock domain.

  • fwft (bool) – Always set.

Variables:

  • w_data (Signal(width), in) – Input data.

  • w_rdy (Signal(1), out) – Asserted if there is space in the queue, i.e. w_en can be asserted to write a new entry.

  • w_en (Signal(1), in) – Write strobe. Latches w_data into the queue. Does nothing if w_rdy is not asserted.

  • w_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_data (Signal(width), out) – Output data. Valid if r_rdy is asserted.

  • r_rdy (Signal(1), out) – Asserted if there is an entry in the queue, i.e. r_en can be asserted to read an existing entry.

  • r_en (Signal(1), in) – Read strobe. Makes the next entry (if any) available on r_data at the next cycle. Does nothing if r_rdy is not asserted.

  • r_level (Signal(range(depth + 1)), out) – Number of unread entries.

  • r_rst (Signal(1), out) – Asserted, for at least one read-domain clock cycle, after the FIFO has been reset by the write-domain reset.