MMC2001
UNIVERSAL ASYNCHRONOUS RECEIVER/TRANSMITTER MODULE
MOTOROLA
REFERENCE MANUAL
11-19
Figure 11-12 Start Bit — Ideal Case
Figure 11-12 shows the details of the ideal case of start-bit recognition. All samples
taken at [1] detect logic ones on the RXD line and correspond to the idle-line time or a
stop-bit time prior to this start bit. At [2] a logic zero sample is preceded by four logic
one samples. These five samples are the start-bit qualifiers. The beginning of the
start bit time is tentatively perceived to occur between the fourth logic one sample
and the logic zero sample of the start qualifiers. Next, the samples at RT2, RT3, RT4,
RT5, RT6, and RT7 [3] are taken to verify that this bit time is indeed the start bit. The
samples at RT8, RT9, and RT10 (or RT9, RT10, and RT11) are called the data sam-
ples [4]. These samples drive a majority voting circuit to determine the logic sense of
the bit time.
In this ideal case, the actual start bit and the perceived start bit match. The resolution
of the RT clock leads to an uncertainty about the exact placement of the leading edge
of the start bit. The uncertainty in the placement of the edge will be one-sixteenth of a
bit-time.
RT1
RT1
RT1
RT1
RT1
RT1
RT2
RT4
RT5
RT6
RT7
RT8
RT9
RT3
RT1
0
RT1
1
RT1
2
RT
1
3
RT
1
4
RT1
5
RT
1
6
RT1
RT2
RT3
RT4
RT5
RT6
RT7
1 1 1 1 1 0 0 0 0 0 0 0 0 0 0
RXD Pin
Samples
RT CLK
(16X Bit
Rate)
RT CLK
State
Reset RT
Actual Start Bit
Perceived Start Bit
LSB
[1]
[2]
[3]
[4]
Freescale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
nc.
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