lab7

Lab 7: Using an External Component

In this lab you will use an external component from a Chisel library hosted at Maven central. This shows you how open-source HW components can be shared easily with Chisel.

In this lab you will add a UART component to your design. The UART is a serial interface where you can send and receive characters between the FPGA board and a terminal program on your computer. The UART is a standard component in many FPGA boards and is used for debugging and for communication.

Preparation

Do have some feedback that your setup in the FPGA is working add a blinking LED to your design. You can use the code from the previous lab1.

Using an External Component

The UART component is available as a Chisel library ip-contributions from Maven central. You can add it to your project with the following lines in your build.sbt file:

libraryDependencies += "edu.berkeley.cs" % "ip-contributions" % "0.5.1"

The UART component is available in the package package chisel.lib.uart. Therefore, include the following line in your source file:

import chisel.lib.uart._

The UART component is a Chisel module. You can instantiate it with the following code:

val uart = Module(new BufferedTx(100000000, 115200))

The first parameter is the baud rate, the second parameter is the clock frequency. The BufferedTx component has a read/valid interface to send characters to the UART.

To see the output of the UART you need to connect it to a terminal program. Under Winows PuTTY is a good choice. Under Linux you can use the gtkterm. Under Mac OS X you can use the screen command.

Set the baud rate of the terminal program to 115200 and one stop bit, no parity, no flow control. The UART sends the characters as ASCII characters. Therefore, you can see the characters in the terminal program.

Sending Characters without Handshaking

As the initial exercise send alternating '0' and '1' characters to the UART at the blink frequency of your LED. As this is way slower than the UART baud rate, you can ignore handshaking for now.

The following code sends the character '0' to the UART:

uart.io.channel.valid := true.B
uart.io.channel.bits := '0'.U

Note that the data is not the binary zero, but needs ot be encoded in ASCII. The character '0' is encoded as 48 in ASCII. However, in Chisel you can simply write '0' as character literal. The character literal is automatically converted to the corresponding ASCII code.

However, if you keep valid true.B for more than one clock cycle, the UART will send the same character multiple times. Therefore, you shall have the valid signal set only for a single clock cycle.

Send a String

Now, send the string "Hello World!" to the UART. You can use a Vec set to a default value with a VecInit to store the characters of the string. The following code creates a Vec with the characters of the string "Hello World!".

val hello = VecInit('H'.U, 'e'.U, 'l'.U, ...)

Use a counter to send the characters of the string one by one. Note that your components needs to consider now handshaking. Only when the when ready is true.B you can send the next character.