prim_packer_fifo.md

Primitive Component: Packer FIFO

Overview

prim_packer_fifo is a module that supports three modes of operation: packing, unpacking, and single depth FIFO modes. Packing mode is where the input data width is less than the output data width. Unpacking mode is where the input data width is greater than the output data width. Single depth FIFO is where the input and output data widths are the same. Because masking options are not supported, the larger data size must be an even multiple of the smaller size. The controls for this module are modeled after the prim_fifo_sync module, both in name and functional behavior. It is one of a set of shared primitive modules available for use within OpenTitan as referred to in the Comportability Specification section on shared primitives.

Parameters

Name	type	Description
InW	int	Input data width
OutW	int	Output data width

Signal Interfaces

Name	In/Out	Description
clk_i	input	Input clock
rst_ni	input	Input reset, negative active
clr_i	input	Input clear, clears all internal flops.
wvalid_i	input	Writes data into the first available position.
wdata_i[InW]	input	Input data.
wready_o	output	Indicates if prim_packer_fifo is able to accept data.
rvalid_o	output	Indicates if output data is valid.
rdata_o[OutW]	output	Output data.
rready_i	input	Output data is popped from the FIFO.
depth_o	output	Indicates the fullness of the FIFO.

Theory of Operations

           /----------\
wvalid_i   |          |      rvalid_o
---------->|          |--------------->
wdata_i    |   Flop   |      rdata_o
=====/====>|   FIFO   |=======/=======>
  [InW]    |          |      [OutW]
           |          |      depth_o
           |          |--------------->
wready_o   |          |      rready_i
<----------|          |<---------------
           |          |
           \----------/

In pack mode, prim_packer_fifo accepts InW bits of data. On a wvalid_i/ wready_o handshake, wdata_i is stored to internal registers and accumulated until OutW data has been gathered. Once the FIFO is full, a single pop (when rvalid_o and rready_i are coincident), will clear the data and depth values on the next clock cycle. The complimentary flow occurs when theprim_packer_fifo module is in unpack mode.

The internal register size is the greater of InW and OutW bits. Timing diagrams are shown in the header of the prim_packer_fifo module.