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(3) Continuous transmission of 3 or more frames
In addition to the 1-frame/2-frame transmission function, UARTn also enables continuous transmission of 3 or
more frames, using the method shown below (n = 1, 2).
(a) How to continuously transmit 3 or more frames (when the stop bit is 1 bit (SL bit = 0))
Three frames can be continuously transmitted by writing transmit data to the TXSn/TXSLn register in the
period between the generation of the transmission completion interrupt request (INTSTn) and 4 × 2/f
before the output of the last stop bit.
The INTSTn interrupt becomes high level 2/f
TXSn/TXSLn can only be written after the INTSTn interrupt level has fallen. The time from INTSTn
interrupt generation to the completion of transmit data writing (t) is therefore indicated by the following
expression.
t = (Time of one stop bit) – (2 × 2/f
f
= Internal system clock
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Caution 4 × × × × 2/f
Example Count clock frequency = 32 MHz = 32,000,000 Hz
Target baud rate in synchronous mode = 9,600 bps
t = (1/9615.385) − ( (4 + 8) /32,000,000)
= 104.000 − 0.375
= 103.625 [ µ s]
Therefore, be sure to write transmit data to TXSn/TXSLn within 103 µ s of the generation of the INTSTn
interrupt.
Note, however, that because writing to TXSn/TXSLn may be delayed depending on the priority order of
the INTSTn interrupt or the interrupt servicing time, be sure to allow sufficient time for writing transmit
data after the INTSTn interrupt has been generated.
transmission due to a delay in writing to TXSn/TXSLn, a 1-bit high level is transmitted.
Note also that if the stop bit length is 2 bits (SL bit = 1), the INTSTn interrupt will be generated when the
second stop bit is output.
Figure 10-18. Continuous Transmission of 3 or More Frames (When SL Bit = 0)
2/f
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Stop bit
INTSTn interrupt
2/f
CHAPTER 10 SERIAL INTERFACE FUNCTION
+ 4 × 2/f
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has a margin of double the clock that can actually be used for operation.
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2/f
TXSn/TXSLn write period for 3-frame
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continuous transmission
User's Manual U14492EJ3V0UD
after being output and returns to low level 2/f
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)
If there is not enough time for continuous
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later.
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4 × 2/f
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463
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