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PIR (PASSIVE INFRARED) DETECTOR USING ST7FLITE05/09/SUPERLITE

2.2 SCI IMPLEMENTATION USING THE MICROCONTROLLER STANDARD I/O PORTS

A Serial Communications Interface (SCI) is used to allow the user to check different variables

Hyperterminal application. SCI is implemented using standard I/O ports. We just need to

transmit the data from the microcontroller to a PC, so only SCI transmit is implemented for

4800 or 2400 baud rate (selectable by a macro in the software). This is implemented as fol-

lows:

1. Convert the variable to be transmitted into ASCII data. This is required because Hyperter-

minal displays ASCII data only.

2. The ASCII data is transmitted bit-by-bit with a bit duration depending on the Baud Rate

selected in the software.

For example:

1 bit duration = 416µs (1/2400 seconds) for 2400 baud

= 208µs (1/4800 seconds) 4800 baud

This bit duration is calculated with the help of the Timer interrupt which is configured for 52µs.

So we can change the status of PA5 Pin after 416µs(52x8) or 208µs(52x4). The time period

for each data bit will be the same.

3. Transmit the Start bit. To do this, hold the PA5 pin LOW for 1 bit time.

4. After the start bit, change the PA5 pin level according to the data (ASCII) to be transmitted.

5. Then, transmit the Stop bit. To do this, hold the PA5 pin HIGH for 1 bit time.