This used to be relatively straightforward, but with move from Raspbian Wheezy to Raspbian Jessie, things changed. Add to this, the new Raspberry Pi 3 with new hardware and the whole thing became a bit of a Dog’s Breakfast and issues with the Bluetooth to boot.
It suddenly got very confusing.
Following lots of wasted time, I’ve noted down what I think I know so far in this post. With luck you can have your cake and eat it: use the serial port on a Raspberry Pi 3 and use the Bluetooth AND have the same code work on other Raspberry Pi’s (non RPi3).
NOTE: You will need the latest firmware May 2016 (2016-05-10) or later for this to work. Should this change again, I’ll update this post.
Before I dive into the configuration, it’s worth taking a moment for a little history and orientation about the serial port on the Raspberry Pi.
If you’re a bit old school like me, you’d be expecting to find something called COM1 or similar on a header. In Raspberry Pi / Linux land this COM1 equivalent is found on pins 14 and 15 of the GPIO header and is called /dev/ttyAMA0 (obvious, right?).
Also in Raspberry Pi land, you can use the serial port as a terminal to log in, which is useful if you don’t have a network connection to hand. You can connect to another computer via their serial ports and run a terminal emulator on the other computer and you’ll get a login prompt.
By default the Raspberry Pi uses the serial port for this “console” login and via a software service called “getty”.
Using the serial port with other hardware
So that’s the ‘normal” configuration of the serial port, but serial ports are very useful things. What if we want to use the serial port to get data from a GPS card or program an arduino? In this case we need to disable the console login so that we alone get control of the port. Easy right? Yes and no. There is a big elephant in the room and he’s called Raspberry Pi 3.
Before we can describe using the serial port, we have to talk about Raspberry Pi 3, which throws a great big spanner in the works as far as serial ports are concerned.
Raspberry Pi 3
Raspberry Pi 3’s are great little beasts, and add Bluetooth, yay! However, in order to use the Bluetooth correctly the /dev/ttyAMA0 has been “stolen” from the GPIO header and an inferior second one has been substituted in it’s place. No-one will ever know! Unfortunately /dev/ttyAMA0 was a hardware serial port (uart) and high performance (hence it was nabbed for the Bluetooth) and the second port is partly software and a bit flaky. Many people’s applications got broken.
The second serial port you will see referred to as the “mini uart” and lives at /dev/ttyS0. It also calculates it’s bit timing’s from the CPU cores frequency and if the CPU is under heavy load it can corrupt the serial communications. Not good.
In order to work around this, many people “fix” the CPU core frequency so that the serial port is stable. This comes at a slight loss in performance (though normally not noticeable). I’ll describe how you do this in the next section.
By the way, it’s not all bad for the change of serial port on the Raspberry Pi 3. The Arduino IDE expects the serial communications to be on /dev/ttyS0 so you have no work to do to map the serial ports across. Yay!
To summarise the ports on a Raspberry Pi 3 and be crystal clear:
/dev/ttyAMA0 -> Bluetooth
/dev/ttyS0 -> GPIO serial port.
If you stick with these as is, your Bluetooth will work as nature intended AND you can use a serial port over the GPIO (there is a way of swapping the serial ports around if you don’t want to use the Bluetooth and I’ll cover that at the end of this post).
There is yet another wrinkle in that in the latest Jessie releases (as of May 2016) the GPIO serial port is disabled by default. In order to enable it, edit config.txt:
$ sudo nano /boot/config.txt
and add the line (at the bottom):
As of May 2016 this will also lock the cpu core frequency for you so there’s nothing else you need to do (If you aren’t convinced and you really like to belt and braces it the command is: core_freq=250 which you add to the same file aswell).
Reboot for the changes to take effect.
This should get you good serial communications for most uses.
On the Raspberry Pi 3 the second serial port is called /dev/ttyS0 and is by default mapped to the GPIO pins 14 and 15. So immediately, if you have code that references /dev/ttyAMA0 you’re going to have problems and things aren’t going to work.
You could go through your code and replace ttyAMA0 with ttyS0 and that should work. However, if you find yourself use the same SD card on a Raspberry Pi other than a rpi3 your code won’t work again.
In order to try and get around this the Foundation have introduced a serial port alias (as of May 2016 – 2016-05-10). Thus you have serial ports: serial0 and serial1 (rpi3). The Raspberry Pi kernel sorts out where these point to depending on which Raspberry Pi you are on. Thus on a Raspberry Pi 3 serial0 will point to GPIO pins 14 and 15 and use the “mini-uart” aka /dev/ttyS0. On other Raspberry Pi’s it will point to the hardware UART and /dev/ttyAMA0.
To find out where it is pointing you can use the command:
$ ls -l /dev
So where possible refer to the serial port via it’s alias of “serial0” and your code should work on both Raspberry Pi 3 and other Raspberry Pi’s.
Disabling the Console
If you are using the serial port for anything other than the console you need to disable it. This will be slightly different depending on whether you are running a Raspberry Pi 3 or not.
For non Raspberry Pi 3 machines, remember it’s /dev/ttyAMA0 that is linked to the getty (console) service. So you need to perform this command from a terminal window:
$ sudo systemctl stop serial-getty@ttyAMA0.service
$ sudo systemctl disable serial-getty@ttyAMA0.service
The “disable” will stop it loading in the future.
For Raspberry Pi 3’s the command is similar but referencing /dev/ttyS0:
$ sudo systemctl stop serial-getty@ttyS0.service
$ sudo systemctl disable serial-getty@ttyS0.service
You also need to remove the console from the cmdline.txt. If you edit this with:
$ sudo nano /boot/cmdline.txt
you will see something like:
dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes root wait
remove the line: console=serial0,115200 and save and reboot for changes to take effect.
Swapping the Serial Ports on Raspberry Pi 3
What if you don’t want to use the Bluetooth and you want that high performance /dev/ttyAMA0 back on the GPIO? Well you can do this and the way you do this is via a device overlay called “pi3-miniuart-bt” i.e. use the mini-uart (/dev/ttyS0) for Bluetooth (you may get some loss of performance on your Bluetooth though).
You can also just disable the Bluetooth all together by using another overlay “pi3-disable-bt”. In both cases if you can find out more of what they do here: /boot/overlays/README
To use add the following line to the /boot/config.txt
$ sudo nano /boot/config.txt
Save and reboot for changes to take effect.
You can check that it has worked by:
$ ls -l /dev
and you’ll see something like this: