TWI Library

I2C/TWI library for AVR-GCC. More...

Other definitions
void	twi_init (void)
	Initialize TWI unit, enable internal pull-ups, and set SCL frequency.
void	twi_start (void)
	Start communication on I2C/TWI bus.
uint8_t	twi_write (uint8_t data)
	Write one byte to the I2C/TWI bus.
uint8_t	twi_read (uint8_t ack)
	Read one byte from the I2C/TWI bus and acknowledge it by ACK or NACK.
void	twi_stop (void)
	Generates Stop condition on I2C/TWI bus.
uint8_t	twi_test_address (uint8_t addr)
	Test presence of one I2C device on the bus.
void	twi_readfrom_mem_into (uint8_t addr, uint8_t memaddr, volatile uint8_t *buf, uint8_t nbytes)
	Read into buf from the peripheral, starting from the memory address.
#define	TWI_WRITE   0
	Mode for writing to I2C/TWI device.
#define	TWI_READ   1
	Mode for reading from I2C/TWI device.
#define	TWI_ACK   0
	ACK value for writing to I2C/TWI bus.
#define	TWI_NACK   1
	NACK value for writing to I2C/TWI bus.
#define	DDR(_x)
	Address of Data Direction Register of port _x.
#define	PIN(_x)
	Address of input register of port _x.

Definition of frequencies
#define	F_CPU   16000000
	CPU frequency in Hz required TWI_BIT_RATE_REG.
#define	F_SCL   100000
	I2C/TWI bit rate. Must be greater than 31000.
#define	TWI_BIT_RATE_REG   ((F_CPU/F_SCL - 16) / 2)
	TWI bit rate register value.

Definition of ports and pins
#define	TWI_PORT   PORTC
	Port of TWI unit.
#define	TWI_SDA_PIN   4
	SDA pin of TWI unit.
#define	TWI_SCL_PIN   5
	SCL pin of TWI unit.

Detailed Description

I2C/TWI library for AVR-GCC.

#include <twi.h> 

This library defines functions for the TWI (I2C) communication between AVR and Slave device(s). Functions use internal TWI module of AVR.

Note

Only Master transmitting and Master receiving modes are implemented. Based on Microchip Atmel ATmega16 and ATmega328P manuals.

Copyright

(c) 2018-2025 Tomas Fryza, MIT license

Macro Definition Documentation

DDR

#define DDR

(

_x

)

Value:

(*(&_x - 1))

Address of Data Direction Register of port _x.

PIN

#define PIN

(

_x

)

Value:

(*(&_x - 2))

Address of input register of port _x.

Function Documentation

twi_init()

void twi_init

(

void

)

Initialize TWI unit, enable internal pull-ups, and set SCL frequency.

Implementation notes:

• AVR internal pull-up resistors at pins TWI_SDA_PIN and TWI_SCL_PIN are enabled
• TWI bit rate register value is calculated as follows fscl = fcpu/(16 + 2*TWBR)

Returns

none

twi_read()

uint8_t twi_read

(

uint8_t

ack

)

Read one byte from the I2C/TWI bus and acknowledge it by ACK or NACK.

Parameters

ack	- ACK/NACK value to be transmitted

Returns

Received data byte

twi_readfrom_mem_into()

void twi_readfrom_mem_into	(	uint8_t	addr,
		uint8_t	memaddr,
		volatile uint8_t *	buf,
		uint8_t	nbytes )

Read into buf from the peripheral, starting from the memory address.

Parameters

addr	Slave address
memaddr	Starting address
buf	Buffer to be read into
nbytes	Number of bytes

Returns

none

twi_start()

void twi_start

(

void

)

Start communication on I2C/TWI bus.

Returns

none

twi_stop()

void twi_stop

(

void

)

Generates Stop condition on I2C/TWI bus.

Returns

none

twi_test_address()

uint8_t twi_test_address

(

uint8_t

addr

)

Test presence of one I2C device on the bus.

Parameters

addr	Slave address

Returns

ACK/NACK received value

Return values

0	- ACK has been received
1	- NACK has been received

twi_write()

uint8_t twi_write

(

uint8_t

data

)

Write one byte to the I2C/TWI bus.

Parameters

data	Byte to be transmitted

Returns

ACK/NACK received value

Return values

0	- ACK has been received
1	- NACK has been received

Note

Function returns 0 if 0x18, 0x28, or 0x40 status code is detected

• 0x18: SLA+W has been transmitted and ACK has been received
  
• 0x28: Data byte has been transmitted and ACK has been received
  
• 0x40: SLA+R has been transmitted and ACK has been received