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What does the F in and the formula itself refer to in Usage Notes of SCI?

Latest Updated:03/17/2011

Question:

In  Usage Notes in the Serial Communication Interface chapter in the Hardware Manual, we find the description of a calculation formula for the receive margin (M) in asynchronous mode. What does the  formula refer to?
What does the F (absolute deviation of clock frequency) in the formula refer to specifically?

Answer:

Receive margin (M) indicates in which position in one bit period the data is sampled.
M = 50% represents the sampling at the center of one bit period. M = 0%, at the start or the end of one bit period.
M = 0% indicates there is no margin. So, we recommend setting the receive margin as close to 50% as possible.

Typically, the deviations of the clock frequencies are as follows:

  1. Sampling clock frequency deviation
    Frequency deviation between the base clock to achieve the assumed bit rate (BPS), and the base clock which can be specified by the bit rate register (SCBRR)
  2. Source clock frequency deviation
    Frequency deviation of the EXTAL frequency and of the source clock input to the SCK pin
  3. Clock frequency deviation of the other device
    Frequency deviation of the clock to generate the transmit data of the other device
  4. Noise on the transmission line
    e.g. waveform degradation, noise, etc.

"F (absolute value of the deviation of the clock frequency)" in the formula equals to sampling clock frequency deviation described above as (1.), and it is expressed by the following formula:

F = |(BPS which can be set by the SCBRR) − (assumed BPS)| ÷ (assumed BPS)

The above value is same as the absolute value of the "bit rate error (%)" which is described in the bit rate register (SCBRR) in the Hardware Manual.

 

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