Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 178)

164 Assembly Language Programming for the 68000 Family

serial and parallel refer to the methods used to transfer data from the

input/output device to the interface. A parallel interface transfers a com

plete byte or word at a time. A serial interface transfers a byte or word

a single bit at a time. The advantage of a serial interface is that fewer

wires are needed to make the connection. A parallel interface requires

at least one wire for each bit. A disadvantage of a serial interface is that

data cannot be transferred as rapidly as with the parallel interface.

At the heart of the serial interface is a chip commonly known as a

universal asynchronous receiver transmitter (UART). Each byte of data

is sent or received as a stream of bits. Two additional bits are included

for each byte of data. A start bit is included to tell the UART that a byte

is to follow. A stop bit is used to verify to the UART that we are finished

with the current byte. Without start and stop bits, the UART would not

be able to ensure framing. When a framing error occurs, some of the

bits from one byte are mixed up with some of the bits from a previous

byte. Figure 14 shows this typical asynchronous serial format. The most

popular format is for eight data bits, one start bit, and one stop bit, giving

a total of 10 bits for each byte of data sent or received.

If each byte is to represent an ASCII character code, an extra bit is

available. This bit can be permanently set to a 1 or 0, or it may be used

as a parity bit. A parity bit is used to help verify that the data has been

received without error. The way a parity bit is used is very simple. We

--------------

------

-------

--------

I I I I I I ■ I I

I I I I I I I I I I

s I

____

--------

1---------1---------1

--------

---------

Time

M—Marking or a logical one.

S—Spacing or a logical zero.

Figure 14 Asynchronous serial I/O format

1 2 ... 173 174 175 176 177 178 179 180 181 182 183 ... 255 256

Žádné komentáře

Motorola M68000 Uživatelská příručka Strana 178