The popularity and easy access of RS-232 ports lend them to many communication projects. You can use a port as is” or as a tiny parallel port when the exchange uses only control lines. Before the asynchronous serial-data transfer between two devices can take place, you must ensure that both devices are configured to the same data format and transmission rate. Usually, the OS utility or application program on the host computer performs this task, which the OS or a combination of switches selects in the peripheral.
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The design in Figure 1 and the accompanying software realize the requirement of automatically equalizing the speed of the parties by adapting the bit rate of
a mC’s serial port to that of a
host computer. The host sends a known
ASCII code to a mC that estimates the bit
rate of the transmission. The mC samples
the receiving end of a transmission line.
As soon as the line goes low, indicating a
start bit, the mC clears the timer. After the
line again goes high, the mC reads the
timer’s value and uses this value to calculate the bit rate of a transmission. The
mC uses the calculated bit rate in a timing-delay loop for clocking out the data
from the mC to the host computer during transmission and for sampling the incoming data during receiving mode. The
time between those falling and rising
edges on a transmission line depends, of
course, on the transmitted ASCII symbol.
Its good practice to use a symbol with
21 zeros on the low side. Then, when
you include the start bit, there are 2
trailing zeroes. One timing unit is easier to
derive by substituting a division with k
The application in Figure 1 uses an 8-
bit RISC mP and an ASCII code of 8 (38
hex) to establish a serial communication.
Any symbol that ends with 8H also
works. The timer increments its value
seconds, where P is a
prescaler factor and FOSC
is the mP’s clock
frequency. Thus, the timer’s value...
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