Datasheet

bit,ideal,TX
1
t [i] = (i + 1)
Baud rate
i
bit,TX bit,TX
j=0
t [i] = T [j]
å
15
UCBRFx
j=0
m [j]
å
( )
15
bit,TX UCBRSx UCBRFx
BRCLK
j=0
1
T [i] = 16 + m [i] × UCBRx + m [j]
f
æ ö
ç ÷
ç ÷
ç ÷
è ø
å
bit,TX UCBRSx
BRCLK
1
T [i] = (UCBRx + m [i])
f
USCI Operation: UART Mode www.ti.com
When greater accuracy is required, the UCBRSx modulator can also be implemented with values from 0
to 7. To find the setting that gives the lowest maximum bit error rate for any given bit, a detailed error
calculation must be performed for all settings of UCBRSx from 0 to 7 with the initial UCBRFx setting and
with the UCBRFx setting incremented and decremented by one.
15.3.11 Transmit Bit Timing
The timing for each character is the sum of the individual bit timings. Using the modulation features of the
baud rate generator reduces the cumulative bit error. The individual bit error can be calculated using the
following steps.
15.3.11.1 Low-Frequency Baud Rate Mode Bit Timing
In low-frequency mode, calculate the length of bit i T
bit,TX
[i] based on the UCBRx and UCBRSx settings:
Where,
m
UCBRSx
[i] = Modulation of bit i from Table 15-2
15.3.11.2 Oversampling Baud Rate Mode Bit Timing
In oversampling baud rate mode calculate the length of bit i T
bit,TX
[i] based on the baud rate generator
UCBRx, UCBRFx and UCBRSx settings:
Where,
= Sum of ones from the corresponding row in Table 15-3
m
UCBRSx
[i] = Modulation of bit i from Table 15-2
This results in an end-of-bit time t
bit,TX
[i] equal to the sum of all previous and the current bit times:
To calculate bit error, this time is compared to the ideal bit time t
bit,ideal,TX
[i]:
This results in an error normalized to one ideal bit time (1/baudrate):
Error
TX
[i] = (t
bit,TX
[i] – t
bit,ideal,TX
[i]) × Baudrate × 100%
15.3.12 Receive Bit Timing
Receive timing error consists of two error sources. The first is the bit-to-bit timing error similar to the
transmit bit timing error. The second is the error between a start edge occurring and the start edge being
accepted by the USCI module. Figure 15-11 shows the asynchronous timing errors between data on the
UCAxRXD pin and the internal baud-rate clock. This results in an additional synchronization error. The
synchronization error t
SYNC
is between -0.5 BRCLKs and +0.5 BRCLKs independent of the selected baud
rate generation mode.
422
Universal Serial Communication Interface, UART Mode SLAU144J–December 2004–Revised July 2013
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