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How is the start bit actually detected?

Latest Updated:03/07/2012

Question:

How is the start bit actually detected?

Answer:

The start bit is detected in the following steps.

  1. The falling edge of the RxD pin is detected.
  2. When the falling edge of the RxD pin is detected, the transmit/receive clock gets activated.
  3. The level of the RxD pin is checked again on the rising edge of the transmit/receive clock.
  4. If the level is "L", it will be determined as the start bit.

If it is "H", reception will not be performed by the determination as noise. Also, the reception interrupt request bit or error flags will not be set.
When a RTS is output, the output of the RTS pin becomes "H" once, when detecting the falling edge of the RxD pin in the Step 1. However, if the level of the RxD pin is "H" on the rising edge of the transmit/receive clock in the Step 3, the output of the RTS pin returns to "L".

Suitable Products
M32C/8A
M32C/88
M32C/87
M32C/85
M32C/84
M32C/83
M32C/80
M16C/62A
M16C/62A(M16C/62T)
M16C/62M
M16C/62N
M16C/62N(M3062GF8NFP, M3062GF8NGP)
M16C/62P
M16C/63
M16C/64A
M16C/64C
M16C/65
M16C/65C
M16C/6C
M16C/6N4, M16C/6N5
M16C/6NK, M16C/6NL, M16C/6NM, M16C/6NN
M16C/5L, M16C/56
M16C/5LD, M16C/56D
M16C/5M, M16C/57
M16C/30P
M16C/29
M16C/28
M16C/26A
M16C/1N