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

The Host Baud Rate (TWIn.MBAUD) register must be written 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 the SCL clock
t_HIGH is the high period of the SCL clock
t_R is determined by the bus impedance; for internal pull-ups. Refer to Electrical Characteristics for details.
t_OF is the output fall time and is determined by the open-drain current limit and bus impedance. Refer to Electrical Characteristics for details.

Properties of the SCL Clock

The SCL frequency is given by:

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 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:

fSCL=fCLK_PER10+2×BAUD+fCLK_PER×tR(1)

Equation 1 can be transformed to express BAUD:

BAUD=fCLK_PER2×fSCL−(5+fCLK_PER×tR2)(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 equation 2
2.Calculate t_LOW using the BAUD value from step 1:

tLOW_Fm=tLOW_Fm+=BAUD+6+min(SCLDUTY,BAUD)fCLKPER-tOF(3.1)

tLOW=BAUD+6fCLK_PER−tOF(3.2)
3.Check if your t_LOW from equation 3 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 equation 2
- If the limit is not met, calculate a new BAUD value using equation 4, below, where t_{LOW_mode} is either t_{LOW_Sm}, t_{LOW_Fm}, or t_{LOW_Fm+} from the mode specifications:
  
  BAUD=fCLK_PER×(tLOW_mode+tOF)−3(4)