Software Development Tips and Tricks


HiFive1 Rev B GPIO Pins

Let’s make use of the HiFive1 Rev B schematics to map out the GPIO controller device pins. Of particular interest is sheet 3, and the following components of the schematic in section D1:

We’re going to use this as our starting point. Three GPIO lines labeled 19, 21, and 22 with LEDs on the lines.

Editor’s Note: This write-up on the SiFive HiFive1 Rev B assumes you have one and are familiar with powering it up, connecting to the serial port, and uploading applications. See our last post if you need to orient yourself with the environment.

Using Metal GPIO

If you haven’t done so already, familiarize yourself with Freedom Metal and the API documentation because we’re going to make use of the GPIO functions.

Recall in the schematic above that the onboard LEDs are tied to GPIO 19 (green LED), GPIO 21 (blue LED), and GPIO 22 (red LED). While the Metal LED API could be used here, we’re going to work directly with the GPIO functions to prove to ourselves that the above GPIO pins are correct, and also check out the 8 color combinations the LED can produce.

Our basic loop is going to cycle from 0 to 7 and turning on the red LED if bit 1 is set, the green LED if bit 2 is set, and the blue LED if bit 3 is set. This should cause our HiFive1 on-board LED to cycle through the following color chart:

Okay, let’s get the basics down. The GPIO can be accessed by obtaining a struct metal_gpio* using the metal_gpio_get_device function, like this:

Now that we have the GPIO device itself, let’s enable GPIO 19 as an output pin. In our schematic take note that the other side of the LED is tied to 3.3V. In other words, there is no setting GPIO 19 high or low required here, if it is enabled you’re going to get the LED on.

To set GPIO 19 as an output:

To disable the output of the pin:

Now I thinkthis works for the onboard LED because of the way it is wired to the 3.3V supply. In fact, for Metal GPIO pin 19, if you try to write a 1 to it you actually turn off the onboard LED and turn on the header pin 3. Speaking of the header pins…

Arduino Header and Pins

Going back to the board schematic, this time on sheet 3 in sections A5, A6, B5, B6, and C5, C6.

Now this is interesting! Our Arduino headers as well have notes on which GPIOs are digital-only (high or low), those that are PWM-capable, and which pins are shared by SPI, I2C, and UART. All we need to do is put this into a nice table, and the following is (I think) a complete GPIO mapping for the SiFive HiFive1 Rev B board.

For example, if you wanted to start your project with blinking an LED on a breadboard, you might start with one of the “standard” digital input/output pins on the header, say pin 7. The chart above (which is derived from the schematic) shows that pin 7 on the header is tied to pin 23 of the GPIO controller. So to use header pin 7 we might write:

Or, we could set at the top of our file something along the lines of #define PIN7 23. Just a thought.

Notice once again that this mapping table shows that there are:

  • 20 digital pins
  • 6 PWM-capable pins
  • 1 SPI
  • 1 I2C
  • 1 UART

Unfortunately SiFive’s HiFive1 Rev B page indicates there are only 19 digital pins and there are 9 PWM-capable ones, so there’s something I’m missing. I think at least for PWM, header pins 17, 18, and 19 on the 6-pin header (to the left of the ESP32) are PWM-capable, though that isn’t clear from the schematic. I’ve asked about that here on the SiFive Forums and will reconcile this post once I learn more.

Getting Some Code

This code is a work in progress and is modeled around the Arduino digital IO library. In digitalio.c you’ll see my take on the functions pinMode and digitalWrite. There is also a delay function to write the ever-popular blinking lights demo. All of this code is MIT licensed with portions copied from the SiFive Freedom E SDK.

Let’s look at how we might use these functions to put the onboard LED through all of the possible colors.


Recall the GPIO pins that map to the on-board LEDs are:

  • Red – GPIO_22
  • Green – GPIO_19
  • Blue – GPIO_21

We iterate from 0 to 7 in a loop, test the bits, and set the pins as outputs (since we’re using the onboard LEDs) or disable them. The result is a nice rainbow light show from the HiFive1. If you connect to the serial port you’ll also see the colors being printed out as they are cycled through:


The code that’s actually uploaded to GitHub includes stepping through the binary representation of our LED rainbow and displaying that if you hook up a few LEDs to your breadboard. Check out the code and look closely here at header pins 2, 4, and 7.

Closing Thoughts and What’s Next

Nothing is more fun than working with microcontrollers and embedded systems, and the HiFive1 is no exception. If you take a look at gpios.h in GitHub you can see that I’m looking as to how to best add additional #defines for the various pins. We will see.


An Introduction to the HiFive1 Rev B and RISC-V

Today I’d like to introduce you to a new development board, the HiFive1 Rev B. Equipped with a RISC-V Freedom E310 microcontroller and designed with an Arduino Uno “form factor”, the HiFive1 Rev B is a neat little board that I hope to learn and develop for.

My HiFive1

There is a lot of material out there about RISC-V and how it is going to change the future of CPUs, but what attracted me to it was the notion of a completely open standard Instruction Set Architecture (ISA). That and I think working with new hardware and development environments is cool.

Getting Started

The Getting Started Guide is crucial to read. If you’re anything like me you want to dig in as quickly as possible with as little reading as possible, but trust me, reading the guide first is very useful.

You don’t get anything but the HiFive1 Rev B board if you’ve ordered it from Crowd Supply and will need a trusty USB-A male to USB-micro-B male cable. This connection can be used for both serial communication and power. Of course, if you have only a system with USB-C you’ll need some set of adapters to get to USB micro-B.

For the host platform we will be using a MacBook Pro (Retina, 13-inch, Early 2015) running macOS 10.15 (Catalina). Hopefully if you’re reading this with the intention of working on the HiFive1 with your Macbook Pro you’ll already have the best terminal program ever installed, but if you don’t regular Terminal.app works.

Let’s see our boot screen first:

To see this boot screen you’ll need to use a serial terminal program. macOS is going to present the HiFive as the two USB modem devices in the /dev directory.

The first cu.usbmodem device presented will be the HiFive1, and my suggestion is to open an iTerm and use screen to connect to it. 115200 bps is your speed and the default 8N1 settings should work, so in our case screen /dev/cu.usbmodem0009790151821 115200 is all we had to type in the terminal.

Time to Develop

There are several key pieces of software you’ll need to install on your Mac to begin developing on the HiFive1.

  • a toolchain (i.e., the compiler, assembler, linker, and associated libraries)
  • OpenOCD On-Chip Debugger
  • the Freedom E SDK
  • Segger J-Link OB debugger

We’ll take each in turn.

Installing the Toolchain and OpenOCD

The reference toolchain is a GNU Embedded Toolchain — v2019.08.0 and can be downloaded directly from SiFive. Go to the Boards page, scroll down until you see the section labeled Prebuilt RISC-V GCC Toolchain and Emulator.

Download both the GNU toolchain and OpenOCD packages and untar both into a suitable installation location. I prefer to keep things like this in the /opt directory, so we’ll do the same here in /opt/riscv.

Before going any further ensure that the compiler can run:

You may have received the error message "riscv64-unknown-elf-gcc" cannot be opened because the developer cannot be verified.. Now, this may be controversial, but I don’t hold to my OS telling me what software I can and can’t run, so let’s get rid of that silliness with spctl:

Let’s try again:

Much better.

Get the SDK

We have a HiFive1 board which uses the Freedom E310 chip, thus will want to get the Freedom E SDK. For this I like to keep the SDK in my projects directory.

Note: You must use --recursive here to check out all of the required packages.

Now, let’s compile that first program!

In the top-level directory freedom-e-sdk is a set of Makefiles that make it easy to compile and generate an image suitable for uploading to the HiFive1 board. In this case, PROGRAM is the name of a subdirectory in freedom-e-sdk/software and TARGET is the board you have.

If this is the first time through you’re going to see a bunch of gibbersh about Python, pip3, virtualenv, etc:

And, if you receive an error regarding the C compiler cannot create executables, you need to set your RISCV_PATH environment variable to point to the correct toolchain like export RISCV_PATH=/opt/riscv/riscv64-unknown-elf-gcc-8.3.0-2019.08.0-x86_64-apple-darwin'.

It’s often a good idea to put the various environment variables in a file such as sifive.env and this source the script into your environment.


If everything worked properly you’ll see at the end something like after compiling.

Installing J-Link

Now, we’re going to upload this to our board, but we will need the Segger J-Link OB debugger. If it isn’t installed you’ll see something like this when trying to use the upload target.

To get J-Link head over to the download page and grab the J-Link Software and Documentation pack for macOS.

Install per the instructions, and once that is done you can go back to your SDK directory and type:

If everything is installed correctly and the board is attached you’ll see something like:

And that’s it! If you have a terminal window with a connection to the serial output of the board you’ll see Hello, World!.

Your Own C Program

In the directory freedom-e-sdk there is a software folder that has an example template. We’ll use that to create our own ASCII art banner.

Copy the template with something like cp -R software/empty software/leo-welcome and then edit the software/leo-welcome/Makefile to change the PROGRAM name to leo-welcome. Open main.c in the same directory and replace the contents with:

Compile and upload with make PROGRAM=leo-welcome TARGET=sifive-hifive1-revb upload and behold.

Some Assembly

I’ll be honest, I struggled with this part, but that’s primarily due to the school of hard knocks with piecing together the calling convention for RISC-V. Let’s review a few examples.

Hello World in RISC-V Assembly

Again, start off with the example folder inside freedom-e-sdk with something like

In this case we’re going to write Hello World in assembly so delete software/helloasm/main.c and instead create a file called main.S with the following content.


In RISC-V the a-registers will contain our procedure arguments, the procedure here being C printf. We use la (load address) to bring the address of the hellomsg label into a0 and then call the printf function. All of these, in reality, are pseudoinstructions.

The Makefile in the template will pick up files with a .S extension, so make PROGRAM=helloasm upload will assemble, link, and upload our file to the HiFive1.

Counting Down

Finally, let’s look at a countdown routine with a max and min. Here things are a bit more complicated as we are going to make use of a prologue and epilogue in our routine. The prologue saves the return address on the stack, and the epilogue loads the address back into the ra register such that the instruction ret will branch back to our caller.


We can call this function with C like this:


Compile and upload!

Good References

There are a lot of great references on RISC-V, its instruction set architecture, and so on. I’ve compiled a few of my favorites here.

Closing Thoughts and What’s Next

That wraps it up for this first look at the SiFive HiFive1 Rev B board. Of course, we haven’t even talked about the ESP32-based wireless capabilities of the board, haven’t talked about the Freedom Metal library or any of the stuff that will accelerate our development. Perhaps that’s next!


Working with ARM Assembly

Don’t ask me why I started looking at writing basic ARM assembly routines. Perhaps it’s for the thrill of it, or taking a walk down memory lane. My first assembly language program was for an IBM System/360 using WYLBUR in college.

This post is not a tutorial on assembly language itself, or the ARM processor for that matter. If the phrases mnemonic, register, or branch on not equal are foreign to you, have a look here. I just wanted to write some easy routines and pick up some basics.

Editor’s note: All of the code below is available on GitHub.

We’ll be using a Raspberry Pi 4. You will (obviously) need the GCC toolchain installed, which can be accomplished with sudo apt-get install build-essential.

Let’s save the following in a file named helloworld.s:


Assemble and link the application together with gcc helloworld.s -o helloworld and run it.

It doesn’t get much more straightforward than this, and you’ve learned three new ARM assembly instructions: ldr, mov, and bl. The remaining text are directives to the GNU assembler which we’ll cover in a minute.

The ldr instruction loads some value from memory into a register. This is key with ARM: load instructions load from memory. In the example above we’re loading the address of the beginning of the string into register r0. A technical note: ldr is actually a psuedoinstruction, but let’s gloss over that.

bl branches to the label indicated (and updates the link register), and in our case, there is this magical printf we’re branching to. More on that later.

Finally, mov r0,#0 is positioning our program’s return code (zero) into r0. Check it:

What if we change the mov r0,#0 to mov r0,#0xff? Try it:

Okay, now for something interesting. Let’s count down from 10 to 1 and then print Hello, world!.


Okay, that escalated quickly! One of the reasons assembly language is so much fun. Let’s take a look at what is going on here and add some comments to our code.

There’s a few things to note here. First, let’s talk about the use of r5 and why that register was deliberately chosen. It turns out that when calling routines in assembly you better not use registers that will get trashed by whatever subroutine your calling (r0-r3). printf can use these registers, so we’ll use r5 in our routine.

Now, I will confess, I am not an assembly language expert much less an ARM assembly language expert. Someone may look at the above code and ask why I didn’t use the subtract-and-compare-to-zero instruction (if there is one) or some other technique. If there is a better way to write the above, please let me know!

Counting Up

In the above example we counted down, now let’s count up, and instead of counting from zero to some max, let’s count up from some minimum value to some maximum value. In other words, we’ll step through a sequence of values using an increment of one.


There’s some new syntax here, in particular the ldr rx,[rx]. This syntax is “load the value that is pointed to by the address in the register.” It makes sense in that there is an instruction immediately before it ldr,=min which is load the address identified by the label min. To be clear, the actual value of that label is going to be dependent on the assembler, your application size, and where it gets loaded into memory. Let’s look at an example of that:


Compile and execute this code to see something like Address of x is 0x21028. Then move x to after fmtstr and you will see the address change. What it will change to, again, is highly dependent on a number of factors. Suffice it to say, using ldr with memory addresses loads the address into a register, not the value at the address. That is what we use ldr rx,[rx] for.

Running our countup code indeed counts up from 14 to 28 and if we look at the return code (echo $?) we get 29, the last value that was in r5.

Writing a Procedure

Here is a basic ARM assembly procedure that computes and returns fib(n), the nth element of the Fibonacci sequence. We chose this specifically to demonstrate the use of the stack with push and pop.


What should be noticed here is the use of push and the list of register values we’re going to save onto the stack. In ARM assembly the Procedure Call Standard convention is to save registers r4-r8 if you’re going to work with them in your subroutine. In the above example we use r4 and r5 to compute fib(n) so we first push r4 and r5 along with the link register. Before returning we pop the previous values off the stack back into the registers.

To use this routine in C we can write:


Then, compile, assemble, and link with gcc fibmain.c fib.s -o fibmain. Recall the procedure call standard convention that the arguments to the procedure will be in r0-r3, hence why our first instruction mov r4,r0 to capture what n we’re calculating the Fibonacci number of.

Taking the Average

Okay, one last routine. We want to take the average of an array of integers. In C that would look something like this:

Here’s a go at it in ARM assembly.


There are some new instructions, an interesting form of the ldr instruction, and a new type of register.

First, the vmov instruction and register s1. vmov moves values into registers of the Vector Floating-Point Coprocessor, assuming your processor has one (if it is a Pi it will). s1 is one of the single-precision floating-point registers. Note that this is a 32-bit wide register that can store a C float.

Next up is the ldr r5,[r0],#4 instruction. Recall that ldr ra, loads the value stored at the address in rb into ra. The #4 at the end instructs the processor to then increment the value in rb by 4. In effect we are walking the array of integers whose starting address is in r0.

Finally, once we add all of the values in the array we have the sum in the register r4. To divide that sum by the length of the array (which was saved off in the floating-point register s1) we load r4 into s0 and perform one last thing: vcvt. vcvt converts between integers and floating-point numbers (which are, after all, an encoding). So s0 gets converted to a floating-point value, as does s1, and then we perform our division with vdiv.

As with r0 being the standard for returning an int from a procedure call, s0 will hold our float value.

We can use this function in our main routine.


Compile with gcc averagemain.c average.s -o averagemain and run.

Closing Thoughts

This post has been a lot of fun to write because assembly is actually fun to write and serves as a reminder that even the highest-level languages get compiled down to instructions that the underlying CPU can execute. One instruction set we didn’t touch on is the store instructions. These are used to save the contents (store) of registers to memory. Perhaps next time.

Once again, all of the code in this post can be found in the armassembly repository on GitHub.