1. Raspberry Pi <--> Arduino
Paul MacDougal
January 11, 2016

2. Motivation
We want to communicate between python code running on a Pi and C++ code running on an Arduino
Why? Arduino runs realtime. Arduino has analog inputs. Pi talks to the Internet.

3. Hardware
The Pi has two pins (8,10) connected to a UART (Universal Asynchronous Receiver/Transmitter)
Arduino Uno has two pins (0,1) connected to a UART

4. UART Hardware block for handling serial communication
Different baud rates (e.g , 38400, 9600)
Universal Asynchronous Receiver/Transmitter

5. Just hook them up? Pi 5v Uno 8 TXD Tx 1 10 RXD Rx 0 Gnd Gnd
Do not hook up a 5V Arduino Uno pin to a 3.3v Raspberry Pi pin.
It is important to connect the grounds
Gnd
Gnd

6. No!!! Pi 5v Uno 8 TXD Tx 1 10 RXD Rx 0 Gnd Gnd
Do not hook up a 5V Arduino Uno pin to a 3.3v Raspberry Pi pin.
It is important to connect the grounds
Gnd
Gnd

7. Use a 3.3v Arduino Pi 3.3v Arduino 8 TXD Tx 1 10 RXD Rx 0 Gnd Gnd
Yourduino.com sells a UNO clone called RoboRED that can run at 3.3v
Arduino Due runs at 3.3v
Pro Mini runs at 3.3v
Gnd
Gnd

8. Use level shifters Pi Uno 8 TXD 3.3 5 Tx 1 10 RXD 3.3 5 Rx 0 Gnd Gnd
3.3 5
Tx 1
10 RXD
3.3 5
Rx 0
I use Adafruit part #735
74LVC245
Not all chips support 5v inputs when running at 3.3v (check the datasheet)
The 3.3v logic level on Rx …
Gnd
Gnd

9. Demo A Setup USB to PC Pi 3.3v DigiX 8 TXD Tx 1 10 RXD Rx 0 Gnd Gnd
DigiX has SerialUSB (== Serial), Serial0, Serial1, Serial2 UARTs
Gnd
Gnd

10. Demo A Sketch void setup() { SerialUSB.begin(115200);
while(!SerialUSB); // wait for connection
Serial0.begin(115200); // pins 0,1 }
void loop()
while (Serial0.available())
SerialUSB.write(Serial0.read());
while (SerialUSB.available())
Serial0.write(SerialUSB.read());
DigiX has SerialUSB (== Serial), Serial0, Serial1, Serial2 UARTs
is chosen because that is the default UART setting in Raspbian Linux

11. Demo A % cd ~/demos % ./demoa.sh % sudo /sbin/shutdown now
<power off Pi>
<hook up Demo A Setup>
<load demoa.ino onto DigiX>
<start the serial monitor>
<Power cycle the Pi>
<log in and play around>
% cat /etc/motd
<prepare Pi for demoB>
% ./demob.sh

12. Demo A
The individual bits take 1/ of a second

13. Serial Ports are useful
Raspbian Linux uses the serial port for console output and for login.
/boot/cmdline.txt
dwc_otg.lpm_enable=0 console=ttyAMA0, console=tty1 root=/dev/mmcblk0p6 rootfstype=ext4 elevator=deadline rootwait
/etc/inittab
#T0:23:respawn:/sbin/getty -L ttyAMA vt100
Other distros do similar things, I believe.

14. Uno Pins 0,1 are in use for serial communication over USB
Pins 0,1 are used to communicate with the IDE
We cannot do both USB and Pi on the same port
When not connected to USB, we can hope that the other end of those 1k resistors is high impedance

15. SoftwareSerial
Arduino Library for bit-banging on digital I/O lines to emulate a UART
Not as fast as a real UART

16. Demo B Setup
USB to PC Pi
Uno
8 TXD
3.3 5
Tx 3
0,1
10 RXD
Rx 2
Gnd
Gnd

17. Demo B Sketch #include <SoftwareSerial.h>
SoftwareSerial mySerial(2,3); // RX, TX
void setup() {
Serial.begin(38400); // HW UART to PC host
mySerial.begin(9600); // bit-banging to Pi }
void loop()
while (mySerial.available())
Serial.write(mySerial.read());
while (Serial.available())
mySerial.write(Serial.read());
Bit banging a serial port on the digital I/O lines is not as fast as a HW UART
38400 works for a few characters, but falls behind on large bursts

18. DemoB öænuóÿn:—ûøöænuóÿÿ  some spew at 115200 baud
Raspbian GNU/Linux 7 rpiB ttyAMA0
rpiB login: pi
Password:
Last login: Sat Jan 9 10:07:31 EST 2016 on ttyAMA0
Linux rpiB #691 PREEMPT Wed Jun 18 18:29:58 BST 2014 armv6l
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law. ls
crontab.txt demos ocr_pi.png RPi.GPIO tar.gz
crontab.txt.save Desktop python_games rs485
datalog.sh foo.log RPi.GPIO 
% cd ~/demos
% ./demob.sh <change baud rate to 9600 and turn off console spew>
% sudo /sbin/shutdown now
<power off Pi>
<hook up Demo B config>
<load DemoB sketch into Uno>
<start Serial Monitor>
<power on Pi>
<log in>
<prepare Pi for demo C>
% ./democ.sh
% sudo /sbin/shutdown –r now

19. Demo C Same hardware setup as demo B
Turn off the default Rasbian uses of /dev/ttyAMA0
Transmit from Python program
Receive on Arduino and echo to PC
If you don’t turn off ttyAMA0, your python code will fail to connect to the serial port

20. Demo C Python program import serial import time
ser = serial.Serial('/dev/ttyAMA0', 9600, timeout=0)
ser.open()
ser.write("democ.py")
value = 0
try:
while 1:
ser.write(chr(value))
value = (value+1) & 255
time.sleep(0.1)
except KeyboardInterrupt:
ser.close()

21. Demo C Initialized Pi_talk democ.py
<restart Serial Console>
% cd demos
% sudo python democ.py

22. Demo D
Transmit from Arduino
Receive on Pi

23. Demo D Sketch #include <SoftwareSerial.h>
SoftwareSerial mySerial(2,3); // RX, TX
void setup() {
mySerial.begin(9600); // bit-banging to Pi }
uint8_t c = 0;
void loop()
mySerial.write(c++);
delay(1);

24. Demo D Python program import serial import time
ser = serial.Serial('/dev/ttyAMA0', 9600, timeout=0)
ser.open()
ser.write("demod.py")
try:
while 1:
# Nonblocking read of serial port
response = ser.read()
if len(response) > 0:
print("got %d" % (ord(response[0])))
except KeyboardInterrupt:
ser.close()

25. DemoD % sudo python demod.py … got 163 got 164 got 165 got 166 got 167
% sudo python demodcheck.py
Expected 0, got 220
1/1
1/2
1/3
1/4
1/5
1/6
1/7
1/8
<load DemoD sketch>
% sudo python demod.py
% ^C
% sudo python demodcheck.py

26. Software Roll your own (e.g. ‘a’ means LED on, ‘b’ means LED off)
MOSS (Message Over SoftwareSerial)
Start byte for synchronizing to message
Length / Data / Checksum
Firmata (OpenSource package)
Initial impression is quite favorable
I developed MOSS for my rs485 home automation use

27. Q/A
Questions?

28. Backup Slides

29. 5 to 3.3 Level Shifter Adafruit part #735
Check the datasheet to be sure that input pins can accept greater than Vcc (i.e. 5v > 3.3v)

30. 3.3 to 5 Level Shifter
I have not tried this.

31. Level Shifter