Page 452 - Programming Microcontrollers in C

Keyboard 437

KPSR.KDSC=ON;
KPSR.KDIE=OFF;
KPSR.KRIE=OFF;
After the key position is detected, and its value stored in the array,
the value 0xff is placed in the next location in the array. This value is
used in the decode routine that changes the key positions to ASCII
characters for display. A termination character is needed to show the
end of the input data has been reached. There is an ‘=’, equal sign on
the keyboard, and it was decided to use it as the termination character.
We will see later that the position value for the ‘=’ symbol is 0x11.
Therefore, if the value c is 0x11 at this point in the code flow, it is
time to decode the buffer contents and proceed further. After the data
are decoded, the semaphore is released and control is returned to the
interrupted program. The COLUMNS bits are all set to zero, the key
depress interrupt is disabled and the key release interrupt is enabled
prior to the return.
It turns out that it is next to impossible to get your finger off a
key soon enough to allow this code to work without detecting the
key release. The key release interrupt is set up in the key depress
interrupt service routine. The main operation that takes place in the
key release isr is to disable the key release interrupt and set up for
a key depress interrupt. Without this approach, each time the key is
depressed, it will get through the isr several times before the key is
released and the buffer will be filled with duplicate values each time
the isr is executed. The last seven lines of code in the isr above
are executed in the key release isr.
The decode function converts a value that represents the position
of the depressed key on the keyboard to an ASCII representation of
the character. The position data is an 8-bit unit with the upper 4 bits
representing the column and the lower 4 representing the row. Only
one column or row bit should be enabled at a time. More than one
row or column is enabled if multiple keys have been depressed. The
acceptable row and column values are 1, 2, 4, and 8. The circuitry
fixes the row value of 1 corresponding to the bottom row with the
value 8 as the top row. Also, the right-most column corresponds to
column 1 and the left-most column corresponds to column 8. The
inner rows and columns follow the pattern established above. A look
at the keyboard has the top row, read from left to right, containing