25.3.2.2.1 Clock Generation

The TWI supports several transmission modes with different frequency limitations:
  • Standard mode (Sm) up to 100 kHz
  • Fast mode (Fm) up to 400 kHz
  • Fast mode Plus (Fm+) up to 1 MHz
Write the Host Baud Rate (TWIn.MBAUD) register to a value that will result in a TWI bus clock frequency equal to, or less than, those frequency limits, depending on the transmission mode.

The low (tLOW) and high (tHIGH) times are determined by the Host Baud Rate (TWIn.MBAUD) register, while the rise (tR) and fall (tOF) times are determined by the bus topology.

Figure 25-3. SCL Timing
  • tLOW is the low period of SCL clock
  • tHIGH is the high period of SCL clock
  • tR is determined by the bus impedance; for internal pull-ups. Refer to the Electrical Characteristics section for details.
  • tOF is the output fall time and is determined by the open-drain current limit and bus impedance. Refer to the Electrical Characteristics section for details.

Properties of the SCL Clock

The SCL frequency is given by:
Equation 25-1. SCL Frequency
f SCL = 1 t LOW + t HIGH + t OF + t R [Hz]

The SCL clock is designed to have a 50/50 duty cycle, where the low period of the duty cycle comprises of tOF and tLOW. tHIGH will not start until a high state of SCL has been detected. The BAUD bit field in the TWIn.MBAUD register and the SCL frequency are related by the following formula:

Equation 25-2. SCL Frequency
f S C L = f C L K _ P E R 10 + 2 × B A U D + f C L K _ P E R × t R
Figure   3 can be transformed to express BAUD:
Equation 25-3. BAUD
B A U D = f C L K _ P E R 2 × f S C L − ( 5 + f C L K _ P E R × t R 2 )

Calculation of the BAUD Value

To ensure operation within the specifications of the desired speed mode (Sm, Fm, Fm+), follow these steps:
  1. Calculate a value for the BAUD bit field using Figure   4.
  2. Calculate tLOW using the BAUD value from step 1:

    Equation 25-4. tLOW
    t L O W = B A U D + 5 f C L K _ P E R − t O F
  3. Check if tLOW from Figure   5 is above the specified minimum of the desired mode (tLOW_Sm = 4700 ns, tLOW_Fm = 1300 ns, tLOW_Fm+ = 500 ns).
    • If the calculated tLOW is above the limit, use the BAUD value from Figure   4
    • If the limit is not met, calculate a new BAUD value using Figure   6 below, where tLOW_mode is either tLOW_Sm, tLOW_Fm, or tLOW_Fm+ from the mode specifications:

      Equation 25-5. BAUD
      B A U D = f C L K _ P E R × ( t L O W _ m o d e + t O F ) − 5