On the Pi Zero, we dont have pins PWM0 (pin #40) and PWM1 (pin #45)- those are not available on the PCB. That would normally be super :( :( but it turns out that while those pads are not exposed, we can re-route those signals to other pins that we can get to!
You can get to PWM0 on GPIO #18 (ALT5) and PWM1 on GPIO #13 (ALT0) or GPIO #19 (ALT5) - see the full list of pins and alternate functions here
Let's go!
Get started by burning and booting a Raspbian OS card in your Zero, we'll be using Raspbian Jessie. You don't need network connectivity to get this going but its handy so if you can configure WiFi or Ethernet thru a USB adapter, do that too.
Then log into a command line console.
Option 1. Use Device Tree Overlay
There's two options, the easy way is using a DTO which wil set up all your pins and PWMs. However, we have not tested it.
The other way is to manually set the pin GPIO alt functions. We've tested it but its more difficult!
Here's two DTO options that people have written in to suggest. The first uses a DTO that was recently added it seems:
-
There is a much simpler way to configure the Pi GPIO pins for PWM audio. Simply adding the following line to your /boot/config.txt will reconfigure the pins at boot without any external software or services:
dtoverlay=pwm-2chan,pin=18,func=2,pin2=13,func2=4
Or, you can craft your own DTO
- Hi, I got the pwm audio working on my pi-zero, with an dtoverlay: https://hackaday.io/project/9467-pigrrl-zero/log/35090-pi-zero-pwm-audio-device-tree-overlay
Either way, if you use a DTO you can skip down to the low pass wiring section
Option 2. Manually Assigning PWM pins
This is how to manually set the pin functionality - you can do this if you want to set up the individual pins and usages
Before we start its handy to use wiringpi's gpio utility to list all the GPIO pins and their current set functions/alternates.
If you have network access, run sudo apt-get update
Then install it (in the off chance it isn't installed/updated) with sudo apt-get install raspi-gpio
On our Pi Zero, it didn't have a Zero-compatible version, and said "unable to determine board type". If you have network access, and can run sudo apt-get update and then the install line, this will likely fix it by grabbing a new version.
However...I didn't have network access for my Pi and figured its worth documenting how to get around this. sudo shutdown -h now your Pi Zero, eject the SD card. Then on your desktop computer, download the latest snapshot from the wiringPi Git repo
In case its not available, you can download it by clicking this button:
Transfer the file to the root directory of the Jessie SD card
Re-boot your Pi Zero up, and get back to the command line. Then mv the file over and un-tar it
sudo mv /boot/wiringPi-78b5c32.tar.gz .
tar -zxvf wiringPi-78b5c32.tar.gz
then go into the uncompressed directory and run the compile/install script with
cd wiringPi-78b5c32.tar.gz
./build
OK now you can run the updated version of gpio to read the states of all the pins:
gpio -v
(to make sure it works with the Pi Zero now, it should properly detect the board type)
gpio readall
The table has a ton of information going on. What you want to look for is BCM pins #18 and #13
As you can see, these two pins have MODE type IN - that means they are just plain inputs. You can see that pins like TxD and RxD are 'ALT0', thats the built in serial console.
Anyways, time to change these pins!
Changing the GPIO ALTs
We can manually tweak the GPIO ALTs using a very handy tool by TimG in the Pi forums
Here's the code in entirety:
/* Utility to switch Raspberry-Pi GPIO pin functions Tim Giles 01/04/2013 Usage: $ gpio_alt -p PIN_NUMBER -f ALT_NUMBER Based on RPi code from Dom and Gert, 15-Feb-2013, <http://elinux.org/RPi_Low-level_peripherals#C_2> and Gnu getopt() example <http://www.gnu.org/software/libc/manual/html_node/Example-of-Getopt.html#Example-of-Getopt> */ #include <ctype.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <fcntl.h> #include <sys/mman.h> #define BCM2708_PERI_BASE 0x20000000 #define GPIO_BASE (BCM2708_PERI_BASE + 0x200000) /* GPIO controller */ #define PAGE_SIZE (4*1024) #define BLOCK_SIZE (4*1024) int mem_fd; void *gpio_map; volatile unsigned *gpio; void setup_io(); // GPIO setup macros. Always use INP_GPIO(x) before using OUT_GPIO(x) or SET_GPIO_ALT(x,y) #define INP_GPIO(g) *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3)) #define OUT_GPIO(g) *(gpio+((g)/10)) |= (1<<(((g)%10)*3)) #define SET_GPIO_ALT(g,a) *(gpio+(((g)/10))) |= (((a)<=3?(a)+4:(a)==4?3:2)<<(((g)%10)*3)) #define GPIO_SET *(gpio+7) // sets bits which are 1 ignores bits which are 0 #define GPIO_CLR *(gpio+10) // clears bits which are 1 ignores bits which are 0 int main (int argc, char **argv) { int opt, flag, n_pin, n_alt; flag=0; while ((opt = getopt (argc, argv, "hp:f:")) != -1) { switch (opt) { case 'h': break; case 'p': n_pin = atoi(optarg); flag |= 0b0001; break; case 'f': n_alt = atoi(optarg); flag |= 0b0010; break; case '?': // getopt() prints error messages, so don't need to repeat them here return 1; default: abort (); } } if (flag != 0b0011) { fprintf (stderr, "Usage:\n$ gpio_alt -p PIN_NUM -f FUNC_NUM\n"); return 1; } setup_io(); // Set up gpi pointer for direct register access INP_GPIO(n_pin); // Always use INP_GPIO(x) before using SET_GPIO_ALT(x,y) SET_GPIO_ALT(n_pin, n_alt); printf("Set pin %i to alternative-function %i\n", n_pin, n_alt); return 0; } void setup_io() { /* open /dev/mem */ if ((mem_fd = open("/dev/mem", O_RDWR|O_SYNC) ) < 0) { printf("can't open /dev/mem \n"); exit(-1); } /* mmap GPIO */ gpio_map = mmap( NULL, //Any adddress in our space will do BLOCK_SIZE, //Map length PROT_READ|PROT_WRITE,// Enable reading & writting to mapped memory MAP_SHARED, //Shared with other processes mem_fd, //File to map GPIO_BASE //Offset to GPIO peripheral ); close(mem_fd); //No need to keep mem_fd open after mmap if (gpio_map == MAP_FAILED) { printf("mmap error %d\n", (int)gpio_map);//errno also set! exit(-1); } // Always use volatile pointer! gpio = (volatile unsigned *)gpio_map; }
On your Pi Zero, create a new file and edit it with
nano gpio_alt.c
(it doesnt matter what directory you are in)
Then paste in the entire code above
Save it...
Compile it & install with
gcc -o gpio_alt gpio_alt.c
sudo chown root:root gpio_alt
sudo chmod u+s gpio_alt
sudo mv gpio_alt /usr/local/bin/
Now you can set the ALT functions of the two GPIO!
gpio_alt -p 13 -f 0
gpio_alt -p 18 -f 5
Sweet. Now go back to wiringPi to check that we did it!
Yep! You can see the new ALT settings.
Low Pass Filter Wiring
Now wire up the schematic to GPIO #13 as PWM1 and #18 PWM0 on a breadboard. You can skip the diodes. If you don't have the exact values it's ok. I built it with a 10nF capacitor (0.01uF) rather than 33nF and it worked just fine (the cutoff frequency is higher but the speakers dont hear those high frequencies anyways)
I'm using a 3.5mm audio jack terminal block to wire up the left & right channels + ground.
Set Audio Output
You'll also want to 'fix' the Pi so the audio is definitely getting piped out the 'headphone jack' (PWM output) rather than HDMI. From the console run sudo raspi-config
Go to Advanced
Then Audio
Finally Force 3.5mm (Headphone)
You only have to do this once, you can hit return and then Finish to exit
First Test
You can now play audio! Plug in powered speakers or headphones. We will use the built in aplay audio player. Run
aplay /usr/share/sounds/alsa/Front_Center.wav
You should hear audio!
Adjusting Volume
You may notice a bit of hum, or maybe its just not very loud. To get the best quality audio, you'll want to have the audio level out of the Pi be as high as possible. You can do this with alsamixer which is easier in my opinion than amixer although both work.
On my terminal the ascii art is a bit tough to read, just press the up arrow until the volume is at 100% Then you can hit Esc to save & quit
You only have to do this once, alsamixer saves the settings between reboots.
Automate it!
OK now you have that working, you probably want it to happen at boot, and not need to run gpio_alt! No biggies, we have a fine tutorial on how to automate things at boot on Jessie with systemd
Here's how to do it.
Start by creating a shell script with sudo nano /root/pwmaudio.sh
and inside put:
#!/bin/bash /usr/local/bin/gpio_alt -p 13 -f 0 /usr/local/bin/gpio_alt -p 18 -f 5
run sudo chmod +x /root/pwmaudio.sh and then create another script with sudo nano /lib/systemd/system/pwmaudio.service
and in there stick:
[Unit] Description=PWM Audio Service [Service] ExecStart=/root/pwmaudio.sh StandardOutput=null [Install] WantedBy=multi-user.target Alias=pwmaudio.service
save the file. Now enable the service with
sudo systemctl enable pwmaudio.service
and test-start the service with
sudo systemctl start pwmaudio.service
Reboot the pi (sudo reboot) and re-log in. You can run the gpio readall commands to verify that the alts are set
That's it! You're done, enjoy :)
Page last edited December 03, 2015
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