UM0239 datasheet(38/44 Pages) STMICROELECTRONICS

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Communication session examples

UM0239

38/44

Add the text string “transmission without synchronization” in the ASCII textbox.

Select Sequence Mode and set Nr. to “5”.

Click ON.

Five messages are sent with a temporal distance of 200 milliseconds.

Figure 23.

Communication without Frame Synchronization example

In Figure 23 (COM3 section), the five transmitted sequences do not appear to be received

correctly. This behavior is due to fact that the ST7540 starts writing data on the RX line at

different time for every message, because in Synchronous Mode the PLL that provides the

CLR/T signal must reach the lock-in condition.

Lock-in condition can only be reached after an undetermined number of demodulated data

transitions (maximum 5) and during this time one or more bits can be lost. For this reason, if

a sequence of bits that starts like: "1010-1010-1010-1010-…." (0xAAAA… in hexadecimal)

is transmitted, the receiver can only demodulate with correct timings a part of this sequence.

For example, if the first "10101" is lost, the PC will receive the sequence "0101-0101-

010….." (or 0x55 in hexadecimal). So, even if all bits are correct, byte synchronization is

lost.

Figure 24 shows an example of timing diagram for a baud rate of 2400.

Figure 24.

Demodulation timings for RX with preamble detection (@2400 baud)

COM 1

COM 3

Synchronized message

COM 3

COM 1

Synchronized

Message

DATA DEMOD.

PLL LOCK-IN

CD/PD

RxD

INTERNAL

SIGNALS

EXTERNAL

SIGNALS

010101010

PLL lock-in (variable)

preamble detected

CD/PD detection time (1ms)

bit time (0.417ms)

RxD NOT ACTIVE

RxD ACTIVE

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