Microprogrammed Receive/Transmit Block RAM FIFO for UARTs or similar devices
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README.md

Microprogrammed Rx/Tx Block RAM FIFO

Copyright (C) 2012, 2015 Michael A. Morris morrisma@mchsi.com. All Rights Reserved.

General Description

This project provides a microprogrammed implementation of dual receive/transmit FIFO which uses Block RAM for the storage element. The present implementation is based on a Foundation 2.1i SP6 macro designed and implemented 8 years ago. That design and implementation has been successfully employed in several projects as the FIFO component of 16C550-compatible UARTs.

This project is an effort to reimplement that Rx/Tx FIFO implementation in Verilog HDL like the UARTs.

Implementation

The implementation of the core provided consists of a single Verilog source file and several memory initialization files:

RTFIFO.v                - Top level module

RTFIFO_uPgm.coe         - RTFIFO Microprogram ROM Memory Configuration File
RTFIFO_Init.coe         - RTFIFO FIFO Control Register (LUT) Initialization
RTFIFO_BRAM.coe         - RTFIFO Block RAM Initialization

RTFIFO.ucf              - User Constraints File: period and pin LOCs
RTFIFO.tcl              - Project settings file

tb_RTFIFO.v             - Completed core testbench with test RAM

RTFIFO_uPgm.txt         - Microprogram Source File 

Synthesis

The objective for the core is to synthesize such that the FF-FF speed is 100 MHz or higher in a Xilinx XC2S150-6PQ208 FPGA using Xilinx ISE 10.1i SP3. In that regard, the core provided meets and exceeds that objective. Using the settings provided in the RTFIFO.tcl file, the ISE 10.1i tool implements the design and reports that the 9.761 ns period (102 MHz) constraint is satisfied.

The ISE 10.1i SP3 implementation results are as follows:

Number of Slice FFs:            24
Number of 4-input LUTs:         97  (14 : LUT-based uPgm ROM)
Number of Occupied Slices:      66
Total Number of 4-input LUTs:   105 ( 8 : SP LUT-based RAM  )

Number of BUFGMUXs:             1
Number of BlockRAMs             1   (RTFIFO_BRAM.coe)

Best Case Achievable:           9.704 ns (0.296 ns SETUP; 2.060 ns HOLD )

Status

Design is complete, and testbench development and verification is currently underway.

Release Notes

In addition to providing an example of a microprogrammed Rx/Tx FIFO controller, the repository includes an example of a non-microprogrammed, standard controller for a Rx/Tx FIFO. The example of a standard controller demonstrates how resource effecient a microprogrammed Rx/Tx FIFO controller can be in a RAM-based FPGA.

The standard Rx/Tx FIFO controller provided is not a contrived example. With the the exception that it uses the same number of registers as the microprogrammed implementation, the standard state machine approach uses an RTL implementation for the registers (word counter, read pointer, and write pointer) and for the state machine. Although specific encoding of the states of the controller has been used, the synthesizer has not been constrained in any manner. Thus, after analysis of the RTL implementation, the synthesizer has opted to convert the sequential state assignments provided in the code into a one-hot implementation of the state machine. Furthermore, two of the encoded control fields have also been synthesized as one-hot fields.

These two optimizations, automatically applied by the synthesizer, will result in state transition and control logic equations which are more efficient than a similar implementation which uses encoded states and control fields. Thus, no special effort was expended to keep the synthesizer from optimizing the standard state machine implementation as much as possible.

The following table summarizes the resource utilization for the optimized, standard state machine approach:

The ISE 10.1i SP3 implementation results are as follows:

Number of Slice FFs:            105
Number of 4-input LUTs:         178
Number of Occupied Slices:      146
Total Number of 4-input LUTs:   179 (1 used for route-through)

Number of BUFGMUXs:             1
Number of BlockRAMs             1   (RTFIFO_BRAM.coe)

Best Case Achievable:           11.316 ns (N/A ns SETUP; 2.939 ns HOLD)

Comparing the two results shows that the microprogrammed implementation has some distinct advantages over the RTL-only approach. It is possible to take advantage of some of the benefits of the microprogrammed state machine approach without resorting to a microprogrammed state machine. For example, it is possible to use a LUT-based, distributed RAM to implement the the registers. This optimization alone will reduce the number of registers/FFs from 105 to approximately 57. It will not, however, come close to matching the better than 4:1 reduction in registers that the microprogrammed approach provides. Neither will it achieve the better than 2:1 in the number of occupied slices that the microprogrammed approach achieves.

The microprogrammed Rx/Tx FIFO provided in this repo is a design approach that should be carefully considered when using RAM-based FPGAs. In many situtations, a standard, RTL-based approach fails to take full advantage of the features of RAM-based FPGAs, and will result in inefficient use of the FPGA. Given the cost of implementing any design in an FPGA and the cost of the device itself, designers should seek to maximize the amount of circuitry/logic that can be incorporated into an FPGA. In many cases, a microprogrammed approach will result in a faster, more resource efficient implementation. This alone may be sufficient to reduce the size of the device shipped in a product, or it may allow an existing product to include additional features prior to or after shipment.

As a final comment. Given the dual port nature of most FPGA block RAMs, or the ease with which a dual-port capability can be incorporated into distributed, LUT-based RAM, it is possible to incorporate some form of writable control store in virtually any microprogrammed state machine implementation. With some forethought and planning, FPGAs incorporating complex state machines can easily be configured to be in-circuit re-writable. The potential flexibility that a writable microprogram control store provides can not be easily quantified. In other words, it is potentially "priceless".

Corrections

###Update 1 - 14 Mar 2015 While building a simulation, found an error in the definition of the ROM that holds the microprogram. In the original release, ROM was defined as a wire. Synthesis of the module did not report any unexpected errors or warnings regardin ROM. However, ISim would terminate with an access violation exception. After a bit of work to isolate the issue, the definition of ROM was changed from a wire to a reg. This change cleared up the ISim access violation exception. Resynthesizing to module yielded the same results. Apparently synthesis is more tolerant than ISim.

###Update 2 - 15 Mar 2015 Completed the self-checking testbench. Found two errors in the microprogrammed SM implementation: (1) width of the summer set to match the width of the BRAM address instead of the required width of the FIFO address; (2) used to wrong signal to create the delayed output register write strobe. Also found one error in the standard SM version, after synchronization with the microprogrammed version: the FIFO select signal had been left out of the address for the FIFO RAM. Both versions now pass the testbench, and have a state diagram that matches the original implementations.