FPGA to 74HC595 Shift Register Module in Verilog

I have had this 74HC595 IC sitting around for quite some time. And earlier this week I decided it was time for me to actually start using it...or at least start learning how I could use it. This chip is a Serial-In Parallel-Out 8-bit shift register that can be used in a variety of ways and it can be cascaded with more of these ICs to create a larger shift register if needed. My plan is to use this newly developed module to act as a low pin count interface between the FPGA and my 16x2 Character LCD module. This will allow me to utilize 4-wires to control the 8-bit parallel interface on the LCD (not including the LCD control lines). This reduced the FPGA pin utilization from 11 pins down to 7 pins...not to shabby. Anyways for those that are interested, the code is linked below. Please note: this module is very much application specific and that it does not enable all practical uses of the 74HC595 IC. For instance, the SRCLR signal is not used and is permanently tied to VCC so that it never clears the register; this of course, may not suit your needs, however the code is heavily commented and adaptation of the code should be fairly simple.

Simulation:
The functional operation of the module can be seen in this timing diagram simulation:

The yellow markers are showing the beginning and end of one complete 8-bit shift operation. This simulation is using a 50ns clock period which is slightly slower than the 24MHz clock that my actual FPGA is using. This simulation shows that it takes about 89 clocks to process a request. 89 clocks at 24MHz is about 3.7us.

Signal Descriptions:

• RDY: A bit that indicates that the FPGA_2_ShiftReg is idle and is ready to process a request
• RCLK: A signal sent to the shift register which instructs the output registers to read from the shift register taps
• SRCLK: A signal sent to the shift register which instructs the 8-bit shift register to shift the register bits and read from the serial input and push it into the LSB.
• OE: A signal sent to the shift register which sets the output in either Hi-Z (output disabled) or Lo-Z (output enabled). This is an active low signal.
• SER_OUT: The Serial signal sent to the shift register.
• CLK: The FPGA's local oscillator which drives the rest of the logic.
• BYTE_IN: An 8-bit value that is fed into the FPGA_2_ShiftReg module
• PB: A signal created for simulation purposes to instruct the module to read the 8-bit value from BYTE_IN. Every time this pushbutton is pressed the FPGA_2_ShiftReg module is activated and instructed to read the BYTE_IN. Also the driving module (the module that instantiates and uses the FPGA_2_ShiftReg module) increments the BYTE_IN value by one.

DOWNLOAD: FPGA_2_ShiftReg GitHub Repo

GPS to 16x2 Character LCD Using an FPGA

This is a project that uses a Spartan-3E FPGA Board (Xylo-LM) to grab the serial stream transmitted by a Locosys LS20031 GPS Module and parse the messages for the latitude and longitude. The latitude and longitude values are then feed into a 16x2 Character LCD display on a push of a button.

Note: The GPS coordinates shown will not lead you to me. The GPS module at the time of recording this, was not locked onto my coordinates...so don't try hunting me down..else you will find yourself in the Atlantic =P

Below is a basic block diagram that helps to illustrate what the FPGA does in the system. The FPGA has 3 modules: A Serial Module, a Parser Module, and an LCD Controller Module. The serial module is only responsible for acquiring the serial data at the specified baud rate. Over sampling is used in the FPGA to help minimize bit errors.  The serial module passes the raw data to the parser module. At this point in time the raw data is the NMEA messages (with the overhead information removed, i.e. stopbit/startbit). The parser module waits for the occurrence of a '$' which indicates the start of an MTK/NMEA message. Once the '$' is received, the parser extracts the message type and checks for a "GGA" message. If the current message is a GGA message, then we extract the Latitude and Longitude based on the standard format of the GGA message. This is currently being done by counting the characters; however, counting the comma delimiters work just as well. The parser module passes the latitude and longitude data to the LCD module as it is received on the serial line. The LCD controller module handles the initialization process required to set the LCD module into an operative mode, and it handles all the timing and writing data to the display.

This is an on-going project and I plan to add many more features to it. Below are some changes that I plan on making in the future.

Future Improvements/Additions:

• Write the serial stream from the serial module to a Block Memory or RAM
• Read the data in a Block Memory or RAM and feed the data into the Parser Module.
• Perform mathematical operations on the acquired data, such as computing the displacement from the current location to the last saved location.

For those interested in knowing more about this project, or want more information about certain aspects of the HDL code that I wrote, please don't hesitate to post a comment.

For those interested in the LCD controller module's code, go here: LCD Controller.
Or to download all of the related HDL files, go here: GPS2LCD_Files.zip

My project was featured on Sparkfun's Main Page! Pretty cool and I got some good constructive criticism.