MC68HC711G5 MOTOROLA [Motorola, Inc], MC68HC711G5 Datasheet - Page 96

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MC68HC711G5

Manufacturer Part Number
MC68HC711G5
Description
High-density Complementary Metal Oxide Semiconductor (HCMOS) Microcontroller
Manufacturer
MOTOROLA [Motorola, Inc]
Datasheet
When the RXD input is detected low, it is tested for three more sample times (referred to as the start
edge verification samples in Figure 7-5). If at least two of these three verification samples detect a
logic zero, a valid start bit has been detected, otherwise the line is assumed to be idle. A noise flag
is set if all three verification samples do not detect a logic zero. A valid start bit could be assumed
with a set noise flag present.
If there has been a framing error without detection of a break (10 zeros for 8-bit format or 11 zeros
for 9-bit format), the circuit continues to operate as if there actually was a stop bit and the start edge
will be placed artificially. The last bit received in the data shift register is inverted to a logic one, and
the three logic one start qualifiers (shown in Figure 7-5) are forced into the sample shift register
during the interval when detection of a start bit is anticipated (see Figure 7-6); therefore, the start
bit will be accepted no sooner than it is anticipated.
If the receiver detects that a break produced the framing error, the start bit will not be artificially
induced and the receiver must actually detect a logic one before the start bit can be recognised (see
Figure 7-7).
MOTOROLA
7-6
RXD
RXD
RXD
7.6
16X Internal Sampling Clock
RT Clock Edges (For all three Examples)
START BIT DETECTION
1
1
1
Idle
Idle
Idle
1
1
1
1
1
1
Figure 7-5. Examples of Start Bit Sampling Techniques
1
1
1
Noise
1
1
0
SERIAL COMMUNICATIONS INTERFACE
1
1
1
1
1
1
1
1
1
Qualifiers
Start
1
1
1
1
1
1
Start
Start
Start
0
0
0
R
1
T
R
2
T
0
0
0
R
3
T
Verification Samples
R
4
T
Start Edge
Noise
0
1
0
R
5
T
R
6
T
MC68HC11G5
0
0
0
R
7
T
R
8
T

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