Microcomputer Circuits and Processes

Figure 4.35 shows how the letter 'D' would be sent on a serial line.
First a 'start' bit is sent out, and then the 7 bits of the data, starting with
the least significant bit. Finally, a 'stop' bit is sent.
I
Figure 4.35 •.... ------. r----, II I
Sending a letter 'D' on a single line. The -.J L-.J L....-_---I L...__ .....J
ASCII code for D is combined with serial I start I 1 0 0 0 0 I stop I
control bits, start and stop. bit bit
You cannot stick this signal straight into a cassette recorder, since it
does not have enough bandwidth to cope with the square edges of the
pulses. The best thing to do is to pass the pulse train through a
modulator (a modem). When a logical 1 goes in to the modem, it may
produce a burst of sound at frequency 2400 Hz. And when a logical 0 is
picked up by the modem, it may produce a 1200Hz tone burst. These
sounds are easily recorded by a typical cassette recorder.
It is quite interesting to see how the CPU converts an 8-bit lump of
data into a serial string. The 8-bit data is loaded into the accumulator
with a MOV M-+Acc instruction. Then a SHR ('shift accumulator
right') instruction is performed. This is shown in figure 4.36.
accumulator
carry
after shift
L...-L...-L...-L...-L...-L...-.L--.L......J
Figure 4.36
How a 'shift accumulator right' (SHR) instruction shifts the lowest bit of the 8-bit
number out of the accumulator and into the carry cell.
You can see that all the bits are shifted one place to the right. The
rightmost bit is placed into the carry cell. Now the CPU looks at the
carry cell, and it makes its serial output line (SOD-line) equal to the
carry bit, in this case a 1. If the CPU repeats this another seven times,
the 8-bit data will come out of the SOD pin in serial format. This is
shown in figure 4.37.
The reverse question, how to convert incoming serial data into 8-bit
data, is a little more complex, at least from the point of view of writing
the software. Most CPUs will have a SID connection, a serial input line,
and the rest is programming.
DIRECT MEMORY ACCESS (DMA)
Have you ever thought about writing programs to make those beautiful
animated pictures or coloured processed pictures you normally associate
with NASA? Before you start, think about this calculation. To
make a medium-resolution picture of 256 by 256 dots needs 256 x 256
= 65 536 bits of information which is 65536/8 =8192 bytes. (If you
wanted colour, you would need 8192 x 3=24 K.) To make a flicker-free
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