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2016-04-28 - Giant Gecko Family - d0053_Rev1.20
458
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When a high-to-low transition is registered on the input while the receiver is idle, this is recognized as a
start-bit, and the baud rate generator is synchronized with the incoming frame.
For oversampling modes 16, 8 and 6, every bit in the incoming frame is sampled three times to gain
a level of noise immunity. These samples are aimed at the middle of the bit-periods, as visualized in
Figure 17.5 (p. 458) . With OVS=0 in USARTn_CTRL, the start and data bits are thus sampled at
locations 8, 9 and 10 in the figure, locations 4, 5 and 6 for OVS=1 and locations 3, 4, and 5 for OVS=2.
The value of a sampled bit is determined by majority vote. If two or more of the three bit-samples are
high, the resulting bit value is high. If the majority is low, the resulting bit value is low.
Majority vote is used for all oversampling modes except 4x oversampling. In this mode, a single sample
is taken at position 3 as shown in Figure 17.5 (p. 458) .
Majority vote can be disabled by setting MVDIS in USARTn_CTRL.
If the value of the start bit is found to be high, the reception of the frame is aborted, filtering out false
start bits possibly generated by noise on the input.
Figure 17.5. USART Sampling of Start and Data Bits
0
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 1
2
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10 11
Idle
St art bit
Bit 0
0
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8
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13
7
12
O
V
S
=
0
O
V
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=
1
0
1
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1
O
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=
2
1
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1
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O
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=
3
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3
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5
0
If the baud rate of the transmitter and receiver differ, the location each bit is sampled will be shifted
towards the previous or next bit in the frame. This is acceptable for small errors in the baud rate, but for
larger errors, it will result in transmission errors.
When the number of stop bits is 1 or more, stop bits are sampled like the start and data bits as seen in
Figure 17.6 (p. 459) . When a stop bit has been detected by sampling at positions 8, 9 and 10 for normal
mode, or 4, 5 and 6 for smart mode, the USART is ready for a new start bit. As seen in Figure 17.6 (p.
459) , a stop-bit of length 1 normally ends at c, but the next frame will be received correctly as long as
the start-bit comes after position a for OVS=0 and OVS=3, and b for OVS=1 and OVS=2.
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