µC/LCD
User’s Manual
6 of 33
useful when you have a low power application and want to turn off the backlight. It’s
actually possible to read the LCD module but µC/LCD doesn’t use that feature.
Four (or eight) of the lines are used for data transfer while the other four are used as
control lines for the LCD module.
The HD44780 takes a certain amount of time to process commands or data sent to it. The
Renesas datasheet provides you with the maximum amount of time required for each type
of data transfer. Because of this, the software can simply send a command or data and
wait at least the amount of time specified before sending the next command or data. Note
that the HD44780 itself allows the microprocessor to read a BUSY status. The BUSY
status can be read by the microprocessor to determine if the HD44780 is ready to accept
another command or more data. If you can, you should make use of the BUSY capability
of the HD44780 because this provides you with a true indication that the HD44780 is ready
to accept another command or more data. As a precaution, however, you should still
provide a timeout loop to prevent hanging up the microprocessor in case of a malfunction
with the interface electronics. Unless the LCD module is directly connected to the
microprocessor bus, implementing read capability with parallel I/O ports is a bit more
complicated since it requires that you change the direction of some of the I/O port lines.
The interface circuit is simplified by choosing to have the CPU wait between commands
and data. It turns out that this scheme also makes the software easier to write. Waiting is
done using µC/OS-II’s OSTimeDly() function or, the time delay of the function RTOS of
the RTOS you are using.
The LCD module appears as two read/write registers. The first register is called the data
register (when RS is high) while the other register is called the instruction register (when RS
is low). µC/LCD calls the instruction register the control register. Characters to display are
written to the data register. The control register allows the software to control the operating
mode of the module: clear the display, set the position of the cursor, turn the display ON or
OFF, etc.
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