Why my program won't communicate through ttyAMA0 on Raspbian (Jessie)?

Question

I am running two different Raspberry PI and they are running Raspbian Wheezy and Jessie. On Wheezy, I simply commented out a line T0:23: respawn:/sbin/getty -L ttyAMA0 115200 Vt00 in /etc/inittab file. That worked and my program was able to get access to ttyAMA0. However, when I try to do the same on Jessie, I can't even find the file /etc/inittab. Then, I learned it is completely based on Debian. So, ttyAMA0 is treated like a service.

I searched online for solution about this and found a terminal command to stop or disable getty ttyAMA0 service as follow. So, my program could get access to ttyAMA0.

sudo systemctl stop [email protected]

or

sudo systemctl disable [email protected]

These commands did not help or make any changes. My program complains it can't connect. How do I make my program get access to ttyAMA0 on Jessie?

Answer 1

1. Ensure terminal over serial is disabled in raspi-config

Run

sudo raspi-config

and in "Advanced" choose "Serial" (Enable/Disable shell and kernel messages on the serial connection) and disable it.

Steps 2 and 3 should not be necessary if you do this step first, but in case it didn't work - check them also.

2. Ensure /boot/cmdline.txt has no ttyAMA0

The previous step should have already done that, but just in case do

cat /boot/cmdline.txt

and ensure its like this:

dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

and not like this:

dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

See also: http://elinux.org/RPi_Serial_Connection#S.2FW:_Preventing_Linux_from_using_the_serial_port

3. Disable serial-getty properly if it's still running

If ps aux | grep tty still shows getty using ttyAMA0 - disable it, but don't do

sudo systemctl disable [email protected]

As explained here, the service may still run. Use this instead:

sudo systemctl mask [email protected]

4. Ensure UART pins (15 and 16 wPi) are in state ALT0

Use command-line gpio util to check state of all pins

gpio readall

It should produce result like this

 +-----+-----+---------+------+---+-Model B2-+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 |     |     |    3.3v |      |   |  1 || 2  |   |      | 5v      |     |     |
 |   2 |   8 |   SDA.1 |   IN | 1 |  3 || 4  |   |      | 5V      |     |     |
 |   3 |   9 |   SCL.1 |   IN | 1 |  5 || 6  |   |      | 0v      |     |     |
 |   4 |   7 | GPIO. 7 |   IN | 1 |  7 || 8  | 1 | ALT0 | TxD     | 15  | 14  |
 |     |     |      0v |      |   |  9 || 10 | 1 | ALT0 | RxD     | 16  | 15  |
 |  17 |   0 | GPIO. 0 |   IN | 0 | 11 || 12 | 0 | IN   | GPIO. 1 | 1   | 18  |
 |  27 |   2 | GPIO. 2 |   IN | 0 | 13 || 14 |   |      | 0v      |     |     |
 |  22 |   3 | GPIO. 3 |   IN | 0 | 15 || 16 | 0 | IN   | GPIO. 4 | 4   | 23  |
 |     |     |    3.3v |      |   | 17 || 18 | 0 | IN   | GPIO. 5 | 5   | 24  |
 |  10 |  12 |    MOSI |   IN | 0 | 19 || 20 |   |      | 0v      |     |     |
 |   9 |  13 |    MISO |   IN | 0 | 21 || 22 | 0 | IN   | GPIO. 6 | 6   | 25  |
 |  11 |  14 |    SCLK |   IN | 0 | 23 || 24 | 1 | IN   | CE0     | 10  | 8   |
 |     |     |      0v |      |   | 25 || 26 | 1 | IN   | CE1     | 11  | 7   |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 |  28 |  17 | GPIO.17 |   IN | 0 | 51 || 52 | 0 | IN   | GPIO.18 | 18  | 29  |
 |  30 |  19 | GPIO.19 |   IN | 0 | 53 || 54 | 0 | IN   | GPIO.20 | 20  | 31  |
 +-----+-----+---------+------+---+----++----+---+------+---------+-----+-----+
 | BCM | wPi |   Name  | Mode | V | Physical | V | Mode | Name    | wPi | BCM |
 +-----+-----+---------+------+---+-Model B2-+---+------+---------+-----+-----+

Note that Tx and Rx pins are in state ALT0.

5. If in your case Tx and Rx pins are not in state ALT0, you can use same utility to manually change their state to ALT0. This should fix your problem until next reboot.

gpio mode 15 ALT0; gpio mode 16 ALT0

UPDATE:

6. Enable uart in /boot/config.txt

In order to avoid manual step 5 and some issues that might occur with port ttyAMA0 not being found (/dev/ttyAMA0 being absent) you can set enable_uart=1 in /boot/config.txt, which seems to bring back /dev/ttyAMA0 and ports 15+16 are now in ALT0 state after reboot.