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Renesas Electronics America - Knowledgebase

How long does it take the CPU to read data after data has been received?

Last Updated:12/26/2017


I want to successively transfer serial data. How long does it take the CPU to read data after the data has been received?


This depends on the difference in the configuration of the serial transmission block.
The block diagram of the reception block of a general three-wire serial interface is shown below.

With this configuration, the received data remains in the conversion register and must be read before the next data is transferred (in most of the cases, reading is the condition of starting the next reception for an ordinary reception-only operation).
If a slave receives data, it cannot correctly receive the next data unless it reads the first data before the next data is transferred (also refer to FAQ of handshaking).
Consequently, the period of time from the rising of the serial clock indicating completion of reception to the falling indicating the start of the next transmission is very short.
If this configuration is not sufficient, use a three-wire serial channel having an automatic transmission/reception function.

On the other hand, the general UART for start-stop synchronization communication is of double buffer configuration and has a read register in addition to a conversion register that converts serial data into parallel data, as shown below.

An example of timing with this configuration, where data is 8 bits long and transmitted without parity and with 1 stop bit, is shown below.

As shown in this example, the received data may be read within the time of one character until reception of the next data is completed, which gives sufficient time of processing.
Suitable Products
RL78 Family
78K Family