How To Read I2c Register Arduino

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Intro to Arduino: I2C Serial Advice

Created On January two, 2021

Concluding Updated On March 23, 2021

Impress

Inter-integrated Circuit serial communication, or I²C, is a two-wire serial interface (TWI) that was invented by Phillips Semiconductor (at present NXP Semiconductors). It was designed for communication with lower-speed peripheral devices on the same board. The protocol is a master-slave protocol, where masters command the communication and the slaves just answer. The protocol allows for multiple principal and multiple slaves, while it is more than common to just have one principal and multiple slaves.

Hardware

All of the devices on an IⁱⁱC bus share the same Serial Data (SDA) line and the same Series Clock (SCL). The I²C lines have to have a pull-up to Vcc because IⁱⁱC devices operate with open up-drain outputs. This means that their connexion is either open (not connecting the wire to anything) or draining (connecting the line to footing). The open-bleed output is non able to drive the line to a high voltage so, a pull-up is provided to pull the line loftier when it is released (open up state). These pull-up resistors are usually 4.7kΩ, only they can exist anywhere from 2.2kΩ to 10kΩ depending on the needs of the organization.

In the Arduino IDE

The Wire library for Arduino is used to perform I^twoC/TWI advice. Here we will await at some bones examples of using the Arduino as the autobus master to write to and read from a slave device with the address 0x55 (85). You will have to include the Wire library into your sketch to use these functions. (Click Sketch -> Include Library -> Wire)

Setup

To initialize the Wire library, y'all will have to call the Wire.begin() function. This is ordinarily placed in the setup() area of your sketch.

Writing to the Slave

To write a register on a slave device, you have to initiate communication with the device (slave accost), write the address of the annals that you wish to write, and so write the value that you lot wish the register to be.

                Wire.beginTransmission(85); // Begin advice with slave address 85 (0x55)  Wire.write(12); // Write the annals address that nosotros want to target  Wire.write(42); // Write the value that we want in register 12  Wire.endTransmission(); // Send the stop condition and end the transaction

Reading from the Slave

In club to read a register from the slave device, we begin by initiating communication with the slave, writing the register accost to the pointer register, and sending a restart condition. Then, a read is performed, reading the desired number of bytes from the slave accost.

                Wire.beginTransmission(85); // Begin communication with slave accost 85 (0x55) Wire.write(12); // Write the register address that nosotros want to target Wire.endTransmission(Simulated); // Send the restart status  Wire.requestFrom(85, 1); // Perform read request from slave address 85 and read 1 byte from the register that it is pointed at (12) receivedByte = Wire.read(); // Read the byte that was received

Reading and Writing Multiple Bytes

The I²C protocol allows for multiple bytes to be written or read in one transaction. This is accomplished by having the accost register on the slave increment with each byte sent or received. The transaction begins past writing the first (lowest value) register address that y'all desire to read/write. Then, equally the bytes are read/written the address annals will increment the target accost.

                // Write 3 bytes to registers 12, 13, and 14 Wire.beginTransmission(85); // Brainstorm communication with slave accost 85 (0x55) Wire.write(12); // Write the register accost that we want to target. Wire.write(42); // Write the value that nosotros want in annals 12 Wire.write(23); // Write the value that nosotros want in register 13 Wire.write(14); // Write the value that we want in register 14 Wire.endTransmission(); // Send the stop condition and stop the transaction  // Read 3 bytes from registers 12, thirteen, and 14 Wire.beginTransmission(85); // First communication with slave address 85 Wire.write(12); // Write pointer register to 12 Wire.endTransmission(False); // Send restart condition  Wire.requestFrom(85, 3); // Read iii byte from slave address 85, starting at register 12 receivedByte1 = Wire.read(); // Read byte from register 12 receivedByte1 = Wire.read(); // Read byte from register xiii receivedByte1 = Wire.read(); // Read byte from register 14

Visit Arduino'southward reference page to learn more than most using the Arduino Wire library.

Protocol

In the I²C protocol, all of the advice is controlled past a master. Slave devices accept retentiveness registers that tin be written or read by a primary and each slave device has a unique accost. Every advice begins with a start status, followed by the accost of the slave that the principal is targeting.

Annotation: In all of the protocol diagrams, any action that is bold and underlined is performed past the slave device.

Slave Addresses

You volition run across three different length I^twoC addresses used in documentation; 7-bit, 8-flake, and 10-bit.

7-bit Addresses

This is the standard length I^twoC accost. The 7-bit address is transmitted immediately later on the first condition and it is immediately followed past the read/write (RW) bit. The RW bit indicates whether the primary is writing to the slave (0) or reading from the slave (1).

NightShade Electronics - Intro to Arduino: I2C Serial Communication

eight-fleck Addresses

When you see an 8-bit address, you should meet two addresses listed for the same device; ane for reading and i for writing. This is truly an inappropriate manner to list I²C addresses because they are just taking the real 7-bit address of the slave and adding the read/write bit on to the cease of it. The real address is the upper 7 bits.

10-bit Addresses

The standard seven-flake address system of I²C but allows for 127 unique addresses. In social club to expand the number of available device addresses, a method was created to utilise 10-fleck addresses. This arrangement was designed to exist compatible with 7-bit addresses and then that y'all tin can use both seven-bit addressed devices and 10-bit addressed devices.

The start byte sent past the main, which is unremarkably the only address byte, begins with a 10-flake accost identifier. This the sequence 11110 is never used on the front end of a seven-bit accost, so this kickoff byte will never match a 7-scrap slave address. Later the ten-bit identifier, the starting time two bits of the address are sent, followed by the read/write bit. The second byte contains the rest of the x-bit address. These 2 bytes will replace the address byte of a seven-fleck device in whatsoever of the transactions.

Write Slave Register (1 Byte)

When the master is writing a register to a slave device, it follows this protocol:

A write operation begins with the start condition, generated by the chief. This is followed past the vii-bit accost and a 0, to indicate that the master will be writing to the slave. After the address and RW bit, the slave will ship an acknowledge. Then the master will send the address of the register that it wishes to write. Once more, the slave will transport an acknowledge. Finally, the primary will send the byte that is to be written to the register on the slave. The slave will acknowledge and and then the chief sends the stop condition to indicate that the transaction is complete.

Read Slave Register (1 Byte)

Reading a annals on a slave device is slightly more complicated. The process requires ii steps. In the first footstep, the master writes the address of the register to be read. The communication is so restarted and the main initiates a read command. The slave then begins sending data from the address that was written to the slave in the first footstep.

The read transaction begins exactly like the write transaction. The master sends the slave address with the write bit and the slave acknowledges. Then the master writes the register accost that it wishes to read and the slave acknowledges. Then the main starts a new transaction by sends a stop condition and a starting time condition, or past sending a restart. The difference is that the master maintains control of the passenger vehicle when it performs a restart, while information technology may lose control of the charabanc if it stops and then tries to start. This is non an issue on single master buses. The new transaction begins with the chief sending the slave address with the read scrap. The slave acknowledges and then begins to ship the byte located at the annals address that the master wrote previously. One time the slave finishes transmitting the whole byte (8 bits) the primary does not acknowledge (NA) and sends the stop condition to indicate that the transaction is complete.

Writing or Reading Multiple Bytes

Writing or reading multiple bytes with the IⁱⁱC protocol is quite natural. When the master writes a annals accost to the slave, the address is stored in a arrow register that tells the slave where to read or write next. Afterwards the slave performs a read or write operation to that retention location, the slave increments the address in the pointer register the next retentiveness address. At present, the principal tin can continue by reading or writing another byte, rather than issuing a end status.

Sometimes, it is critical to read multiple registers at ane time. In certain cases, a unmarried, large (multi-byte) value can be stored across several byte addresses in the slave. To brand sure that all of the bytes are read together (representing the register value at the aforementioned point in time) all of the bytes that make of one value are locked during a read functioning. This is a common occurrence in devices like a real-time clock (RTC) because some of the values are then large that they crave more than than one byte and the values are constantly changing.

Multi-Byte Write

A multi-byte write is exactly the same as a unmarried byte right upward to the point where the slave acknowledges the transmission of the data byte. Usually, after the slave acknowledges the manual, the principal would send a stop condition. Instead, the master gain to transmit a seconds byte. This byte volition write to the next annals in the memory of the slave. In this style, this master tin can continue writing successive bytes into the slave's memory. Once the chief has finished write, it then sends the end condition to cease the transaction.

Multi-Byte Read

The multi-byte read is as well exactly like the unmarried byte read upward to the finish of the first byte transmitted. When reading a unmarried byte, the main would not acknowledge (NA) the manual of the first byte, which tells the slave to end transmitting. Instead, in a multi-byte read, the master acknowledges (A) the byte transmitted and the slave proceeds to transmit the value of the side by side retentiveness address. The master continues acknowledging the transmitted bytes as long as information technology wants the slave to continue transmitting. When the master has received the last byte that it wants, it completes the transaction by not acknowledging and so sending the stop status.