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TWI Bit Rate Configuration — ATmega328

This document explains how to configure the I²C (TWI) clock speed on the ATmega328 microcontroller using the TWBR (TWI Bit Rate Register) and TWPS (Prescaler bits in TWSR). It includes the formula for calculating the SCL frequency and practical examples for common I²C speeds.

Registers Involved

  • TWBR — TWI Bit Rate Register
    Defines the bit rate for SCL clock generation.
Bit 7 6 5 4 3 2 1 0
TWBR7 TWBR6 TWBR5 TWBR4 TWBR3 TWBR2 TWBR1 TWBR0
  • TWSR — TWI Status Register
    Contains the TWPS1 and TWPS0 bits that set the prescaler value.
Bit Description
7 TWS7 (TWI Status, part of status code)
6 TWS6 (TWI Status, part of status code)
5 TWS5 (TWI Status, part of status code)
4 TWS4 (TWI Status, part of status code)
3 TWS3 (TWI Status, part of status code)
2 Reserved
1 TWPS1 (TWI Prescaler Bit 1)
0 TWPS0 (TWI Prescaler Bit 0)

SCL Frequency Formula

The SCL (Serial Clock Line) frequency is calculated using the following formula:

SCL Frequency = F_CPU / (16 + (2 × TWBR × Prescaler))

Where:

  • F_CPU is the system clock frequency (e.g., 16 MHz)
  • TWBR is the value in the TWI Bit Rate Register
  • Prescaler is determined by TWPS bits in TWSR

Prescaler Values (TWPS1:TWPS0)

TWPS1 TWPS0 Prescaler
0 0 1
0 1 4
1 0 16
1 1 64

Example 1: 100 kHz (Standard Mode)

Assuming F_CPU = 16 MHz and Prescaler = 1:

TWBR = ((F_CPU / SCL) - 16) / (2 × Prescaler)
TWBR = ((16000000 / 100000) - 16) / (2 × 1) = 72

Configuration:

/* Set Prescaler to 1 */
bitClear(TWSR, TWPS0);
bitClear(TWSR, TWPS1);

/* Configure for 100kHz with 16MHz clock */
TWBR = 72;

This sets the SCL frequency to approximately 100 kHz.

Example 2: 400 kHz (Fast Mode)

Assuming F_CPU = 16 MHz and Prescaler = 1:

TWBR = ((16000000 / 400000) - 16) / (2 × 1) = 12

Configuration:

/* Set Prescaler to 1 */
bitClear(TWSR, TWPS0);
bitClear(TWSR, TWPS1);

/* Configure for 400kHz with 16MHz clock */
TWBR = 12;

This sets the SCL frequency to approximately 400 kHz.

Tips for Reliable Communication

  • Always match the SCL frequency with the slave device's supported speed.
  • Use lower prescaler values for better resolution.
  • Avoid using very low TWBR values with high prescalers, as it may cause timing inaccuracies.

CPU Clock Dependencies

The bit rate calculation depends on the CPU clock frequency. Common configurations:

CPU Clock 100 kHz TWBR 400 kHz TWBR
8 MHz 32 2
16 MHz 72 12
20 MHz 92 17

Note

After setting the bit rate, ensure the TWI module is properly initialized and enabled before starting communication.

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