NXP Semiconductors freescale KV4 Series, Reference Manual

Page: 1309 / 1359

The following figure shows receiver hardware flow control functional timing. Along with
the actual character itself, RXD shows the start bit. The stop bit can also indicated, with a
dashed line, if necessary. The watermark is set to 2.
C1
C2
C3
C4
RXD
C3
data
buffer
read
S1[RDRF]
RTS_B
C1 in reception
1
C1 C3
Status
Register 1
read
C1 C2
Figure 46-74. Receiver hardware flow control timing diagram
46.5.2.8 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
(RT8, RT9, and RT10) to fall outside the actual stop bit. A noise error will occur if the
RT8, RT9, and RT10 samples are not all the same logical values. A framing error will
occur if the receiver clock is misaligned in such a way that the majority of the RT8, RT9,
and RT10 stop bit samples are a logic 0.
As the receiver samples an incoming frame, it resynchronizes the RT clock on any valid
falling edge within the frame. Resynchronization within frames corrects a misalignment
between transmitter bit times and receiver bit times.
46.5.2.8.1 Slow data tolerance
The following figure shows how much a slow received frame can be misaligned without
causing a noise error or a framing error. The slow stop bit begins at RT8 instead of RT1
but arrives in time for the stop bit data samples at RT8, RT9, and RT10.
Chapter 46 Universal Asynchronous Receiver/Transmitter (UART) / FlexSCI
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
1309