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 (t_LOW) and high (t_HIGH) times are determined by the Host Baud Rate (TWIn.MBAUD) register, while the rise (t_R) and fall (t_OF) times are determined by the bus topology.

Figure 1. SCL Timing

t_LOW is the low period of SCL clock
t_HIGH is the high period of SCL clock
t_R is determined by the bus impedance; for internal pull-ups. Refer to the Electrical Characteristics section for details.
t_OF 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:

Figure 2. SCL Frequency

fSCL=1tLOW+tHIGH+tOF+tR[Hz]

The SCL clock is designed to have a 50/50 duty cycle, where the low period of the duty cycle comprises of t_OF and t_LOW. t_HIGH 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:

Figure 3. SCL Frequency

fSCL=fCLK_PER10+2×BAUD+fCLK_PER×tR

Figure 3 can be transformed to express BAUD:

Figure 4. BAUD

BAUD=fCLK_PER2×fSCL−(5+fCLK_PER×tR2)

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 t_LOW using the BAUD value from step 1:

Figure 5. t_LOW

tLOW=BAUD+5fCLK_PER−tOF
3.Check if t_LOW from Figure 5 is above the specified minimum of the desired mode (t_{LOW_Sm} = 4700 ns, t_{LOW_Fm} = 1300 ns, t_{LOW_Fm+} = 500 ns).
- If the calculated t_LOW 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 t_{LOW_mode} is either t_{LOW_Sm}, t_{LOW_Fm}, or t_{LOW_Fm+} from the mode specifications:
  
  Figure 6. BAUD
  
  BAUD=fCLK_PER×(tLOW_mode+tOF)−5