Why is the stop bit necessary?

My system reads serial by the following routine, called every 1/baudrate secs.

Is active_word flag high?
Yes: goto NEXT_BIT

START_BIT
Look at input line.
Is it high?
No: Exit routine
Yes: Set active_word flag, set rx_bits=no. of bits in word
Exit routine

NEXT_BIT
Look at input line.
Shift bit into rx_data store
Decrement rx_bits
rx_bits=0?
No: Exit routine
Yes: End of word
Clear active_word flag
Set received_word flag
Exit routine

By this method, the next start bit can come immediately after the last
data bit.
Problems occur if the routine happens to be called on the transition
edges of the data.  I`m already using the edge trigger bit on RB0 for
another serial comms system, and have some maths running "in the
background".  How can I force it to read the centre of the pulses?

Bryan
--
b.crotaz@ic.ac.uk
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