
CHAPTER 15 ASYNCHRONOUS SERIAL INTERFACE (UART)
User
’
s Manual U15862EJ3V0UD
472
15.7.5 Transfer rate during continuous transmission
During continuous transmission, the transfer rate from a stop bit to the next start bit is extended two clocks of the
base clock (Clock) longer than normal. However, on the reception side, the transfer result is not affected since the
timing is initialized by the detection of the start bit.
Figure 15-15. Transfer Rate During Continuous Transmission
Start bit
Bit 0
Bit 1
Bit 7
Parity bit
Stop bit
FL
1 data frame
Bit 0
FL
FL
FL
FL
FL
FL
FLstp
Start bit of
second byte
Start bit
Representing the 1-bit data length by FL, the stop bit length by FLstp, and the base clock frequency by f
CLK
yields the following equation.
FLstp = FL + 2/f
CLK
Therefore, the transfer rate during continuous transmission is as follows (when the stop bit length = 1).
Transfer rate = 11
×
FL = 2/f
CLK
15.8 Cautions
Cautions to be observed when using UARTn are shown below.
(1) When the supply of clocks to UARTn is stopped (for example, in IDLE or STOP mode), operation stops with
each register retaining the value it had immediately before the supply of clocks was stopped. The TXDn pin
output also holds and outputs the value it had immediately before the supply of clocks was stopped.
However, operation is not guaranteed after the supply of clocks is restarted. Therefore, after the supply of
clocks is restarted, the circuits should be initialized by setting UARTEn = 0, RXEn = 0, and TXEn = 0 in the
ASIMn register.
(2) UARTn has a 2-stage buffer configuration consisting of transmission buffer register n (TXBn) and the
transmission shift register, and has status flags (the TXBFn and TXSFn bits of the ASIFn register) that
indicate the status of each buffer. If the TXBFn and TXSFn bits are read in continuous transmission, the
value changes 10
→
11
→
01. Read only the TXBFn bit during continuous transmission.