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Raspberry Pi 4 with Multiple UART Interfaces

Environment

## Test Command
$ cat /proc/cpuinfo
$ cat /etc/os-release
$ uname -a
$ uname -m

1. Model : Raspberry Pi 4 Model B Rev 1.4
2. Hardware :
   - BCM2177* [ref]
   - BCM2835 ( 2023/09/05 Updated)
3. Operation System : Respbian Linux 11
4. Arch :
   - armv7l
   - aarch64 ( 2023/09/05 Updated)

There are 2 types of UART interface existed on Raspberry Pi, mini UART & PL011 UART.
By default, /dev/ttyS0 maps to mini UART; on the other hand, /dev/ttyAMA0 maps to PL011 UART.

Hardware Check ( 2023/09/05 Updated)

Thanks to Filip Dobrovolný ’s reply, I had check device tree introduction from the /boot/overlays/README, here is the output related to UART:

Name:   uart0
Info:   Change the pin usage of uart0
Load:   dtoverlay=uart0,<param>=<val>
Params: txd0_pin                GPIO pin for TXD0 (14, 32 or 36 - default 14)

        rxd0_pin                GPIO pin for RXD0 (15, 33 or 37 - default 15)

        pin_func                Alternative pin function - 4(Alt0) for 14&15,
                                7(Alt3) for 32&33, 6(Alt2) for 36&37


Name:   uart1
Info:   Change the pin usage of uart1
Load:   dtoverlay=uart1,<param>=<val>
Params: txd1_pin                GPIO pin for TXD1 (14, 32 or 40 - default 14)

        rxd1_pin                GPIO pin for RXD1 (15, 33 or 41 - default 15)


Name:   uart2
Info:   Enable uart 2 on GPIOs 0-3. BCM2711 only.
Load:   dtoverlay=uart2,<param>
Params: ctsrts                  Enable CTS/RTS on GPIOs 2-3 (default off)


Name:   uart3
Info:   Enable uart 3 on GPIOs 4-7. BCM2711 only.
Load:   dtoverlay=uart3,<param>
Params: ctsrts                  Enable CTS/RTS on GPIOs 6-7 (default off)


Name:   uart4
Info:   Enable uart 4 on GPIOs 8-11. BCM2711 only.
Load:   dtoverlay=uart4,<param>
Params: ctsrts                  Enable CTS/RTS on GPIOs 10-11 (default off)


Name:   uart5
Info:   Enable uart 5 on GPIOs 12-15. BCM2711 only.
Load:   dtoverlay=uart5,<param>
Params: ctsrts                  Enable CTS/RTS on GPIOs 14-15 (default off)

According to the above information, BCM2711 has the capability handled additional 4 UART interface as well.

Software Setup

Open a terminal and change the file config.txt in /boot directory with root privilege.

$ sudo su
# vim /boot/config.txt

add following lines at the end of the file

# enable serial interface
enable_uart=1
dtoverlay=uart0
dtoverlay=uart1
dtoverlay=uart2
dtoverlay=uart3
dtoverlay=uart4
dtoverlay=uart5

save & exit, then reboot !

Check system open the serial by command $ ls -al /dev/ttyAMA*

$ ls -al /dev/ttyAMA*
crw-rw---- 1 root dialout 204, 64 Dec 16 16:01 /dev/ttyAMA0
crw-rw---- 1 root dialout 204, 65 Dec 16 16:01 /dev/ttyAMA1
crw-rw---- 1 root dialout 204, 66 Dec 16 16:01 /dev/ttyAMA2
crw-rw---- 1 root dialout 204, 67 Dec 16 16:01 /dev/ttyAMA3
crw-rw---- 1 root dialout 204, 68 Dec 16 16:01 /dev/ttyAMA4

Let’s take a look into the Raspberry Pi 4 Model B GPIO Pinout:

GPIO Pinout : https://www.raspberrypi.com/documentation/computers/os.html#gpio-and-the-40-pin-header

Hardware Setup

Raspberry Pi Pin pair with uart :

           TXD    PIN  |  RXD      PIN  |  Communication Port
uart0 :  GPIO 14    8  |  GPIO 15   10  |  /dev/ttyAMA0 
uart1 :  GPIO 0    27  |  GPIO 1    28  |  /dev/ttyAMA1
uart2 :  GPIO 4     7  |  GPIO 5    29  |  /dev/ttyAMA2
uart3 :  GPIO 8    24  |  GPIO 9    21  |  /dev/ttyAMA3
uart4 :  GPIO 12   32  |  GPIO 13   33  |  /dev/ttyAMA4

PC setup with USB-to-UART Adapter :

FT232 : https://www.waveshare.com/ft232-usb-uart-board-type-a.htm

Please make sure GND is connected correctly !
USB-to-UART TXD connect to Raspberry Pi uart RXD GPIO Pin
USB-to-UART RXD connect to Raspberry Pi uart TXD GPIO Pin

Take uart3 for example

USB-to-UART TXD <--> GPIO 8
USB-to-UART RXD <--> GPIO 9
USB-to-UART GND <--> GND

Implementation

Once finish setup, open a serial console on PC (ex. Putty) with correct COM Port (ex. COM3) & Baud Rate set to 115200

Then open a terminal in Raspberry Pi, send message from CLI to PC :

$ echo 'hello' > /dev/ttyAMA3

This command should send the string hello to PC via /dev/ttyAMA3 , then the string will be parsed by PC, finally display on serial console.

Test Script ( 2023/09/05 Updated ) from [ref link]

#!/bin/sh

sudo stty -F /dev/ttyAMA0 115200
sudo stty -F /dev/ttyAMA1 115200
sudo stty -F /dev/ttyAMA2 115200
sudo stty -F /dev/ttyAMA3 115200
sudo stty -F /dev/ttyAMA4 115200

while true; do
  sudo sh -c "echo 0 > /dev/ttyAMA0"
  sudo sh -c "echo 1 > /dev/ttyAMA1"
  sudo sh -c "echo 2 > /dev/ttyAMA2"
  sudo sh -c "echo 3 > /dev/ttyAMA3"
  sudo sh -c "echo 4 > /dev/ttyAMA4"
  sleep 1
done

Hope this article can help those makers connect more devices via multiple UART interfaces !! Have Fun !!

Please feel free to contact me by mail anytime if there is anything I could help!
Email : jason19970210@gmail.com