The following equations calculate the ratio of the incoming data rate and internal receiver baud rate:
�
SLOW
=
� + 1 �
� − 1 + � ⋅ � + �
�
,
�
FAST
=
� + 2 �
� + 1 � + �
�
•
R
SLOW
is the ratio of the slowest incoming data rate that can be accepted in relation to the receiver
baud rate
•
R
FAST
is the ratio of the fastest incoming data rate that can be accepted in relation to the receiver
baud rate
•
D
is the sum of character size and parity size (
D
= 5 to 10 bits)
•
S
is the number of samples per bit (
S
= 16, 8 or 3)
•
S
F
is the first sample number used for majority voting (
S
F
= 7, 3, or 2) when CTRLA.SAMPA=0.
•
S
M
is the middle sample number used for majority voting (
S
M
= 8, 4, or 2) when CTRLA.SAMPA=0.
The recommended maximum Rx Error assumes that the receiver and transmitter equally divide the
maximum total error. Its connection to the SERCOM Receiver error acceptance is depicted in this figure:
Figure 32-5. USART Rx Error Calculation
+
+
Error Max (%)
Error Min (%)
Baud Rate
SERCOM Receiver error acceptance
from R
SLOW
and R
FAST
formulas
Baud Generator offset error
depends on BAUD register value
+
Clock source error
Recommended max. Rx Error (%)
The recommendation values in the table above accommodate errors of the clock source and the baud
generator. The following figure gives an example for a baud rate of 3Mbps:
Figure 32-6. USART Rx Error Calculation Example
+
+
Error Max 3.3%
Error Min -4.35%
Baud Rate 3Mbps
SERCOM Receiver error acceptance
sampling = x16
data bits = 10
parity = 0
start bit = stop bit = 1
No baud generator offset error
F
baud
(3Mbps) = 48MHz *1
(BAUD=0)
/16
+
DFLL source at 3MHz
+/-0.3%
Recommended
max. Rx Error +/-1.5%
(example)
Error Max 3.3%
Error Min -4.35%
+
Error Max 3.0%
Error Min -4.05%
Transmitter Error*
Accepted
Receiver Error
security margin
*Transmitter Error depends on the external transmitter used in the application.
It is advised that it is within the Recommended max. Rx Error (+/-1.5% in this example).
Larger Transmitter Errors are acceptable but must lie within the Accepted Receiver Error.
Related Links
Clock Generation – Baud-Rate Generator
Asynchronous Arithmetic Mode BAUD Value Selection
on page 597
32.6.3. Additional Features
32.6.3.1. Parity
Even or odd parity can be selected for error checking by writing 0x1 to the Frame Format bit field in the
Control A register (CTRLA.FORM).
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
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