I2c interfacing raspberry pi to arduino

Connecting
Raspberry Pi to
Arduino using I2C
Raspberry Pi
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Creative Commons Attribution-
ShareAlike 4.0 International License.
To view a copy of this license, visit
http://creativecommons.org/licenses/b
y-sa/4.0/.
Revision 2
Companion Github

What is I2C?
The Inter-IC Bus (I-IC or I2C) standard defines the hardware and
electrical characteristics of the interface between nearby devices using
only two wires (and a third for ground reference).
SMBus was introduced by Intel as a tight subset of I2C, strictly defining
the interface between devices.
It is commonly used in televisions, computer components, and many
ADC and DAC and GPS devices.
The Raspberry Pi most commonly uses an SMBus implementation
in Python 2.x, and this is what we will concentrate on in this
document.

Connecting Devices
Identify SDA, SCL, and GND pins on both Raspberry Pi and the Arduino.
Connect SDA to SDA, SCL to SCL, and GND to GND between the two devices.
It is possible to connect the two devices directly to one another without a level
shifter, but it is not best practice. I2C levels for a low signal is 0.3Vcc and for a
high, 0.7Vcc. With the Raspberry Pi being a 3.3V device and the Arduino requiring
5V for Vcc, a logic level 'high' coming from the 3.3V Raspberry Pi is slightly below
the 0.7*5= 3.5V required for a definite logic 'high' in the Arduino.

Voltage Levels
The Raspberry Pi is a 3.3V device while the Arduino is a 5V device, but because
both devices are connected in open-drain configuration with relatively high pull-up
resistor values, it is unlikely damage will occur. The problem is that the
communication between the two devices is not guaranteed to work well due to the
3.5V minimum logic level 'high' required by the Arduino, and the Raspberry Pi input
is not 5V resistant.
A level shifter isolates the two different
voltages, while transmitting on/off information
in both directions.
Having said that, I have had reliable
communication between the Raspberry Pi and
the Arduino without any loss of data, but I am
worried about the Arduino accidently driving
one of the SDA or SCL pins to 5V as an output
if the program is not written carefully and
damaging the Raspberry Pi. User beware.

I2C Level Shifter
This rather complicated-looking circuit creates a bi-directional bus that can
automatically translate 3.3V bus signals into 5V signals and vice versa at full bus
speed. This is one of the simplest and cheapest level shifters and can easily be built
on a breadboard or soldered in a few minutes on any of the prototyping stripboards
available.
On the next slide, we look at
this circuit in detail

The Level Shifter
Isolating one of the two identical bus lines, we find an
enhancement-mode n-channel MOSFET the star of the
show. It's Gate is tied directly to the low-voltage Vcc. A
pull-up resistor connects the High voltage side to the
Drain, and another pulls the Source up to the Low voltage
side.
S1 and S2 simulate the Open-Drain connection that drives
the bus at the microcontroller.
With both switches open, Gate and Source are at the
same potential so the transistor is turned off. Both sides
are pulled up to their respective Vcc.
With S1 closed and S1 open, the source is pulled down to
0V, taking Vgs (Gate-Source Voltage) above the threshold
and turning the transistor on hard. This pulls the Drain side
down to 0V as well.
With S2 closed and S1 open, Drain is pulled to 0V and the
internal diode is forward-biased by the 3.3V, dropping the
source to around 0.6V. This is enough to pull Vgs again
beyond the threshold, opening the transistor and dragging
the bus down to 0V.
With both switches closed, both sides will be at 0V.

Setting Up Raspberry Pi
Getting the tools for Python2 and/or Python3...
1. Open a terminal window.
Enter “sudo apt-get install python-smbus” (2.x)
Or “sudo apt-get install python3-smbus” (3.x)
And “sudo apt-get install i2c-tools”
Reboot the Pi.
2. Open “Raspberry Pi Configuration” in the GUI or use
a terminal to enter 'sudo raspi-config'.
Enable I2C, save, and reboot.
3. Edit file 'etc/modules' using GUI text editor or
enter “sudo nano etc/modules”
If these two lines are not in there, type them;
i2c-bcm2708
I2c-dev
And save.
4. Edit file 'etc/modprobe.d/raspi-blacklist.comf' using
GUI editor or
Enter “sudo nano etc/modprobe.d/raspi-blacklist.conf”.
If these two lines exist, make sure a '#' is placed before
them, or delete them altogether..
#blacklist spi-bcm2708
#blacklist i2c-bcm2708
And save.
5. Finally, enter “sudo nano /boot/config.txt'. These two
lines should appear:
dtparam=i2c1=on
dtparam=i2c_arm=on
If not, make them so, and save.
Reboot.

Testing Raspberry Pi
Enter “i2cdetect -y 1” in a terminal window to
check that the I2C bus is available and running
on the Pi.
If everything is good, it will show a table of
addresses. If any I2C devices are available, their
addresses will be shown in hexadecimal.
It is worth entering “i2cdetect -F 1” to see which
SMBus calls are supported. It will save hours of
debugging to know that “SMBus Block Read” is
not supported, but “I2C Block Read” is
supported, and that “Block Process Call” is
meaningless as it is also not supported.

Preparing the Raspberry Pi
Start Python, and create a new file.
Import the SMBus library.
Create an SMBus object linked to I2C port 1.
Define the slaves address(es) to connect to.
Collect the data to be transmitted, or prepare the
data variables for reading back from the slave.
These instructions are for Python 2. Python 3 may have small differences.
import smbus
i2c_object = smbus.SMBus(1)
# Link to SMBus(0) on older Pi
addrSlave1 = 8
command = 0
dataList = [1,2,3,4,5]
i2c_object.write_i2c_block_data(addrSlave1,
command,
DataList)
getDataList = i2c_object.read_i2c_block_data(
addrSlave1,
command)

Preparing the Arduino
Start Arduino IDE
Include the Wire.h standard library for Arduino.
Set up the slave address.
Register a Wire function that will run when the
interrupt for receiving information from the Master
triggers.
The event is onReceive(function)
This event will react to information coming from
the master, and prepare for sending data back to
the master, if necessary.
Register a Wire function that will run when the
interrupt for returning data to the Master triggers.
This event is on onRequest(function)
When the interrupt fires, send data to the Master
that was requested.
#include <Wire.h>
const byte slaveAddr = 8;
void setup() {
Wire.begin(slaveAddr);
Wire.onReceive(i2cReceive);
Wire.onRequest(i2cTransmit);
}
void loop() {
}
void i2cReceive(int numBytes) {
byte rxByte;
while Wire.available() {
rxByte = Wire.read();
// Do something with the byte
}
}
Void i2cTransmit() {
Wire.write(...) // send requested data
}

Example Project
Using a Raspberry Pi 2 and an Arduino
Leonardo, we will connect up the I2C bus,
and connect an RGB LED to the Arduino. The
LED color and brightness will be controlled by
a Python program running on the Pi.
We will also simulate analog data to send
back to the Pi from the Arduino, a
temperature sensor and a light-level sensor.
The source code for both Arduino and
Raspberry Pi can be found on github at
mikeochtman.github.io/Pi_Arduino_I2C.
This drawing was created in Fritzing.

Interface Specification
Before we can communicate with the Arduino, we
need an interface specification which describes, in
detail, how and what data should be moved over
the bus from Master to Slave, and from Slave to
Master.
Type Description Access
Byte Command private
Byte Control private
Real Temperature Read Only
Real Light Level Read Only
Byte Brightness Red Read/Write
Byte Brightness Green Read/Write
Byte Brightness Blue Read/Write
Arduino Internal Data Table
Command Action
0x81 Read Temperature. Return int(round(temperature*100))
0x82 Read Light. Return int(round(light*100))
0x0A Write three bytes for brightness RGB
0x0B Write single byte, brightness Red channel
0x0C Write single byte, brightness Green channel
0x0D Write single byte, brightness Blue channel
0x90 Read three bytes brightness RGB
0x91 Read single byte, brightness Red channel
0x92 Read single byte, brightness Green channel
0x93 Read single byte, brightness Blue channel
Read,
no command
Return Slave Address, one Byte
Write,
No command
Master interrogating slave presence. Ignore
Other commands
Ignore. Master software has to deal with communication
exceptions, if any.

Arduino Software//The full software can be found on github at
// https://github.com/MikeOchtman/Pi_Arduino_I2C
//This is a simplified collection of
//the most important parts.
#include <Wire.h>
#define rxFault 0x80
#define txFault 0x40
#define slaveAddress 8
struct {
byte volatile command;
byte volatile control;
float volatile temperature;
float volatile light;
byte volatile brightR;
byte volatile brightG;
byte volatile brightB;
} commsTable;
byte volatile txTable[32];
// prepare data for sending over I2C
Void setup() {
Wire.begin(slaveAddress);
Wire.onReceive(i2cReceive);
Wire.onRequest(i2cTransmit);
}
Void loop() {
}
void i2cReceive(int byteCount) {
if (byteCount == 0) return;
byte command = Wire.read();
commsTable.command = command;
if (command < 0x80) {
i2cHandleRx(command, byteCount - 1);
} else {
i2cHandleTx(command);
}
}
byte i2cHandleRx(byte command, int numBytes) {
switch (command) {
case 0x0A:
if (Wire.available() == 3)
commsTable.brightR = Wire.read();
commsTable.brightG = Wire.read();
commsTable.brightB = Wire.read();
result = 3;
} else {
result = 0xFF;
}
break;
// and so on, one case for each command according to the
Interface Specification
default:
result = 0xFF;
}
if (result == 0xFF) commsTable.control |= rxFault;
return result;
}

Arduino Software, cont'd
byte i2cHandleTx(byte command) {
// If you are here, the I2C Master has requested
information
// If there is anything we need to do before the interrupt
// for the read takes place, this is where to do it.
return 0;
}
void i2cTransmit() {
// byte *txIndex = (byte*)&txTable[0];
byte numBytes = 0;
int t = 0;
switch (commsTable.command) {
case 0x00: // send slaveAddress.
txTable[0] = slaveAddress;
numBytes = 1;
break;
case 0x81: // send temperature
t = int(round(commsTable.temperature*100));
txTable[1] = (byte)(t >> 8);
txTable[0] = (byte)(t & 0xFF);
numBytes = 2;
break;
case 0x91: // send RGB Red channel
txTable[0] = commsTable.brightR;
numBytes = 1;
break;
// Again, create a case statement for each legal command
default:
commsTable.control |= txFault;
}
if (numBytes > 0) {
Wire.write((byte *)&txTable, numBytes);
}
}

Python Softwareimport smbus
import time
i2c = smbus.SMBus(1)
addr = 8 # address of the arduino I2C
##/* Interface Specification
## Data in a table thus:
## byte purpose
## 0: command
## 1: control
## 2-5: Current Temperature (read-only)
## 6-9: Current light level (read only)
## 10: Brightness for RED r/w
## 11: Brightness for GREEN r/w
## 12: Brightness for BLUE r/w
## Commands:
## Write with no command: Ignore
## Read with no command: Return slave address
## Command 0x81: read temperature;
## Integer returned, int(round(temp*100))
## Command 0x82: read light level;
## Integer returned, int(round(lux*100))
## Command 0x0A: Write three bytes to RGB
## Command 0x0B: Write single byte brightness red;
## Command 0x0C: Write single byte brightness green;
## Command 0x0D: Write single byte brightness blue;
## Command 0x90: read three bytes brightness RGB
## Command 0x91: read single byte brightness red;
## Command 0x92: read single byte brightness green;
## Command 0x93: read single byte brightness blue;
##
## All other values are ignored, no data returned.
RGB = [20,200,128]
temperature = 0
light_level = 0
i2c.write_quick(addr) # no data expected back
time.sleep(0.5)
print i2c.read_byte(addr) # expect the slave address
time.sleep(0.5)
print i2c.read_word_data(addr,0x81)/100.0 # temperature
time.sleep(0.5)
print i2c.read_word_data(addr,0x82)/100.0 # light level
time.sleep(0.5)
i2c.write_byte_data(addr, 0x0B, 12) # write Red value
time.sleep(0.5)
print i2c.read_byte_data(addr, 0x91) # write Green value
time.sleep(0.5)
i2c.write_byte_data(addr, 0x0C, 123) # write Blue value
time.sleep(0.5)
print i2c.read_byte_data(addr, 0x92) # get Green value
time.sleep(0.5)
i2c.write_byte_data(addr, 0x0D, 234) # write Blue value
time.sleep(0.5)
print i2c.read_byte_data(addr, 0x93) # get Blue value
time.sleep(0.5)
print i2c.read_i2c_block_data(addr, 0x90, 3) # get RGB
time.sleep(0.5)
i2c.write_i2c_block_data(addr, 0x0A, RGB) # set all RGB
time.sleep(0.5)
print i2c.read_i2c_block_data(addr, 0x90, 3) # get all RGB
time.sleep(0.5)
print i2c.read_i2c_block_data(addr, 0x10, 3) # force error
time.sleep(0.5)

The Python ResultsPython 2.7.9 results.
>>>
8
23.95
6.97
12
123
234
[12, 123, 234]
[20, 200, 128]
[0, 255, 255]
>>>
No event for write_quick()
Got back the slave Address.
Got the simulated temperature
Got the simulated light level
Set and read back Red channel
Set and read back Green channel
Set and read back Blue channel
Got back RGB values as a list.
Set new RGB values from a list
Read back the RGB values as a list
Forced a read error by sending wrong command.
RGB = [20,200,128]
i2c.write_quick(addr) # no data expected back
print i2c.read_byte(addr) # expect the slave address
print i2c.read_word_data(addr,0x81)/100.0 # temperature
print i2c.read_word_data(addr,0x82)/100.0 # light level
i2c.write_byte_data(addr, 0x0B, 12) # write Red value
print i2c.read_byte_data(addr, 0x91) # get Red value
i2c.write_byte_data(addr, 0x0C, 123) # write Green value
print i2c.read_byte_data(addr, 0x92) # get Green value
i2c.write_byte_data(addr, 0x0D, 234) # write Blue value
print i2c.read_byte_data(addr, 0x93) # get Blue value
print i2c.read_i2c_block_data(addr, 0x90, 3) # get RGB
i2c.write_i2c_block_data(addr, 0x0A, RGB) # set all RGB
print i2c.read_i2c_block_data(addr, 0x90, 3) # get all RGB
print i2c.read_i2c_block_data(addr, 0x10, 3) # force error

Instructions in Detail
Master (Rpi, Python) Slave (Arduino
Bus.write_quick(slave)
Used to check presence, or trigger event in slave without data
transfer.
Wire.onReceive() event triggered, but no data is sent.
Bytes received: 0. Bytes returned: 0.
Bus.write_byte(slave, data)
Can be used to instruct the slave to do any of up to 255 different
actions, or to place this byte in one predefined byte variable.
Wire.onReceive() event triggered. Single byte received.
ByteValue = Bus.read_byte(addr)
Read slave status or a single predefined value
Wire.onRequest() event triggered only. No data received.
Slave must return one defined byte.
Bus.write_byte_data(addr, command, byteValue)
Send slave some information to accompany a command.
Wire.onReceive() event triggered. Two bytes sent. First byte is a
command, second byte is the byte data to accompany it.
ByteValue = Bus.read_byte_data(addr, command)
Interrogate a slave about a specific piece of information
Wire.onReceive event triggered. One byte sent, a command for
which byte is requested from slave. Wire.onRequest() then
triggered, and one byte to be returned.
Bytes received: 1. Bytes returned: 1

Bus.write_word_data(addr, command, integerVal)
Send slave extra information accompanying a specific command
Wire.onReceive() event triggered, three bytes send. First byte is
command, second is LSB, third is MSB of integer.
WordValue = Bus.read_word_data(addr, command)
Retrieve two bytes, first LSB, then MSB of an integer. Python knows
to rebuild the integer. Retrieve integer data from slave.
Wire.onReceive() event triggered. Single byte received, a
command. Wire.onRequest() event triggered, slave must send back
two bytes, first LSB, then MSB, of an integer.
Bus.write_block_data(addr, command, [list of bytes])
Sending unstructured data.
Wire.onReceive() event triggered. Several bytes sent. First byte is
command, second is number of following bytes, remainder of bytes
must be processes according to interface list.
Bytes received: > 2. Bytes returned: 0.
Bus.write_i2c_block_data(addr, command, [list of bytes])
Use this to fill a table of related bytes quickly.
Wire.onReceive() event triggered. Several bytes sent. First byte is a
command, remaining bytes to be processed according to interface.
Bytes received: > 1. Bytes returned: 0.
ByteList = Bus.read_i2c_block_data(addr, command)
Use this to read a table of related bytes quickly
Wire.onReceive event triggered. One byte sent, a command for
which byte is requested from slave. Wire.onRequest() then
triggered, requested bytes returned as per interface list.
Bytes received: 1. Bytes returned: > 1

List = bus.read_block_data(addr, command, bytes)
will fail. Always returns the empty list. Do not use this function.
Wire.onReceive() event triggered, two bytes send. First byte is
command, second is number of bytes to return. OnRequest() event
is triggered, sending back the correct number of bytes
Bytes received: 2. Bytes returned: > 1.
Bus.block_process_call(addr, command, [vals])
Following the protocol, it reads the correct number of bytes sent
back, but random data appears in the list returned. Do not use.
Wire.onReceive() event triggered. Several bytes received, first
command, then count of bytes in [vals], then vals.
Wire.onRequest() triggered, return first count, then number of bytes
Bytes received: > 2. Bytes returned: > 2.
process_call(addr, command, val) does sent the integer 'val', but
does not trigger the interrupt to return data, so it returns the empty
list
Wire.onReceive() event triggered. Three bytes sent. First byte is
command, second is LSB and last is MSB. SMBus Specification is
to return two byte integer, but onRequest() event never triggered.
Bytes received: 3. Bytes returned: n/a.

Conclusions
Communication over I2C is quite simple to
implement using the SMBus library on the
Raspberry Pi, and the Wire.h library on the
Arduino..
Knowing what each SMBus Write and Read
instruction sends to the Arduino and what data
the Arduino is expected to send back is crucial
to successful implementation of the I2C/SMBus
protocol.
Before coding, have a clear Interface
Specification with a list of commands to be sent
to the Arduino, and clear instructions of what
should happen with the data sent and how to
return requested data.
It is also important to specify the reaction to
bad data.
The SMBus protocol document can be found at
http://smbus.org/specs/smbus20.pdf
The linux Python implementation document can be
found at
https://www.kernel.org/doc/Documentation/i2c/smb
us-protocol
Some of the SMBus protocol is not fully
programmed in the existing SMBus python
libraries, even though the functions are
available to include in the program.
These functions do not appear to work with the
arduino:
● read_block_data(addr, command, bytes)
● block_process_call(addr, command, [vals])
● process_call(addr, command, val)
All the other functions appear to operate
according to the specification.
This work is licensed under the
Creative Commons Attribution-
ShareAlike 4.0 International License.
To view a copy of this license, visit
http://creativecommons.org/licenses/b
y-sa/4.0/.
Compiled by Mike Ochtman.
Find me at
https://za.linkedin.com/in/mikeochtman

I2c interfacing raspberry pi to arduino

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