USART Operation: UART Mode
14-14 USART Peripheral Interface, UART Mode

Receive Bit Timing

Receive timing consists of two error sources. The first is the bit-to-bit timing
error. The second is the error between a start edge occurring and the start
edge being accepted by the USART. Figure 149 shows the asynchronous
timing errors between data on the URXDx pin and the internal baud-rate clock.

Figure 149. Receive Error

123456
0
i
t0
tideal
78
1
t1
2
9 10 11 1213 14 1 2 3 4 5 6 7 8 9 10 11 1213 14 1 2 3 4 5 6 7
t0t1t2
ST D0 D1
D0 D1
ST
Synchronization Error ±0.5x BRCLK
Int(UxBR/2)+m0 =
Int (13/2)+1 = 6+1 = 7
Majority Vote Taken Majority Vote Taken
UxBR +m1 = 13+1 = 14 UxBR +m2 = 13+0 = 13
Majority Vote Taken
BRCLK
URXDx
URXDS
tactual
Sample
URXDS
The ideal start bit timing tideal(0) is half the baud-rate timing tbaud rate because
the bit is tested in the middle of its period. The ideal baud rate timing tideal(i) for
the remaining character bits is the baud rate timing tbaud rate. The individual bit
errors can be calculated by:
Error [%] +ȧ
ȡ
Ȣbaudrate
BRCLK NJ2 ƪm0)int ǒUxBR
2Ǔƫ)ǒi UxBR )S
j
i+1miǓNj*1*jǓ 100%
Where: baud rate is the required baud rate
BRCLK is the input frequencyselected for UCLK, ACLK, or SMCLK
j = 0 for the start bit, 1 for data bit D0, and so on
UxBR is the division factor in registers UxBR1 and UxBR0