Holtek HT-IDE, Руководство пользователя

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C h a p t e r 1 7
Parallel Port
This section describes a parallel port application. The speed of parallel
ports is higher than serial ports as there are more lines involved in the data
transfer. Most parallel port line applications use TTL standard voltage
levels, therefore operate best within a line range of several feet. Lines of
excessive length, due to the increases resistance of the lines, can generate
erroneous data transfers. For these reasons, parallel port applications are
normally only used for short length data transfer operations. Because
parallel port applications require several data lines to transmit data and
because several control lines are also required the application is well suited
for the HT48x00 series of controllers.
ROM Emulator
During the first stages of system development it is normally impractical to
burn the required ROM. The usual procedure is to use a suitable EPROM
to replace the ROM during the stages of program and system development.
In this way, by using an ultra-violet light source, the internal EPROM
programmed data can be erased and the device reused. The inconvenience
in this approach is however, using an ultraviolet light source and the time
required to erase the data. An alternative solution, which overcomes these
EPROM limitations, is to use a ROM emulator. This application uses the
PCs printer port to download data into the ROM emulator SRAM. Such a
ROM emulator can reduce significantly early program development effort.
In this application the design is implemented using the HT48300, 28-pin
microcontroller.
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Chapter 17 Parallel Port
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