Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

142 Chapter 11
At the start of the execution, the three behaviors (tx_ppp, tx_byte and tx_bit)
waits for data from the upper layer. When tx_ppp receives an IP packet from
the application, it generates and sends a PPP packet to tx_byte. Tx_byte sends
the PPP packet one byte at a time to tx_bit. tx_bit then transmit the data bit
by bit to a serial line(bit_out). The transmission to a serial line is synchronized
with the event from tx_clk (bps_event). The above mentioned process
is repeated infinitely.
The receiver is also divided into behaviors which are executed parallel. The
PPP analysis behavior(rx_ppp) and the byte receive behavior(rx_byte) wait
for data from the lower layer. Detection of the falling edge of the serial line
for start bit detection is performed by the receive clock generation
behavior(rx_clk_gen). When rx_clk_gen detects the falling edge, it sends the
event(bps_event) to the bit receive behavior(rx_bit) after a half cycle of the
baud rate. The rx_clk_gen them continues sending the event at a cycle equal
to the baud rate. When rx_bit receives the first event from rx_clk_gen, rx_bit,
it determines whether the data on the serial line is a start bit. If the data is a
start bit, rx_bit fetches the data on the serial line synchronized with the event
from rx_clk_gen. If the data is not a start bit, tx_bit resets rx_clk_gen for
further detection of a start bit. rx_bit also resets rx_clk_gen after receiving 1
byte of data. The reset functionality of rx_clk_gen was realized using try-trapinterrupt
statement, wrapper behavior and dummy behavior. In the statement