PXR40RM Datasheet, PDF (954/1434 Pages) Freescale Semiconductor, Inc – PXR40 Microcontroller

Enhanced Serial Communication Interface (eSCI)
The frequency of the receiver clock is 16 times the frequency of the transmitter clock, this each bit is
sampled with 16 samples. Each of the 16 samples of a bit has a sample number assigned, which is defined
by the receiver sample counter RSC. The n-th sample is denoted by RSn. The receiver sample counter RSC
is updated with each rising edge of the receiver clock RCLK.
26.4.4 Baud Rate Tolerance
A transmitting device may be operating at a baud rate below or above the receiver baud rate. Accumulated
bit time misalignment can cause one of the three stop bit data samples RS8, RS9, and RS10 to fall outside
the actual stop bit. A noise error will occur if the stop bit sample RS8, RS9, and RS10 samples are not all
the same logical value 1. A framing error will occur if the receiver clock is misaligned in such a way that
the majority of the RS8, RS9, and RS10 stop bit samples are a logic zero.
26.4.4.1 Faster Receiver Tolerance
In this case the receiver has a higher baud rate than the transmitter, thus the stop bit sampling starts already
in the last transmitted payload bit. To ensure the correct, noise and framing error free reception of the stop
bit, the samples RS8, RS9, and RS10 must be located in the transmitted stop bit as shown in Figure 26-23.
RXD
START BIT
PAYLOAD
STOP BIT
START BIT
QUALIFICATION
DATA
VOTING
RCLK
RSC 6 7 8 1 2 3
6 7 8 9 10
Figure 26-23. Faster Receiver
The maximum tolerance that ensures error free reception can be calculated with the assumption, that RS7
is sampled during the last transmitted payload bit and RS8 is sampled in the stop bit.
For an frame with n payload bits the transmitter starts the transmission of the stop bit
tx_STOP = n + 1  16  RT_TR
after the start of the transmission of the start bit.
For an frame with n payload bits the receiver samples the RS8 sample of the stop bit
Eqn. 26-3
rx_STOP = n + 1  16  RT_RE + 7  RT_RE
after the successful qualification of the start bit.
Eqn. 26-4
To ensure error free reception of the stop bit, the transmitter must start the transmission of the stop bit
before the receiver samples RSC8.
tx_STOP  rx_STOP
Eqn. 26-5
26-26
PXR40 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor