Page 451 - Programming Microcontrollers in C

436 Chapter 8 MCORE, A RISC Machine

{
decode(savedata);
release_semaphore(semaph);
}
KPDRB.COLUMNS=0X0; /* write 0 to KPDR[15:8] */
KPSR.KDIE=OFF;/* disable key depress interrupt */
KPSR.KRIE=ON; /* enable key release interrupt */
}
else /* key release was detected */
{
KPSR.KPKR=ON;
KPSR.KPKD=ON; /* clear key release and depress */
KPSR.KRSS=ON; /* set key release synchronizer */
KPSR.KDSC=ON; /* clear key depress synchronizer */
KPDRB.COLUMNS=0X0;/* write 0 to KPDR[15:8] */
KPSR.KDIE=ON; /* enable key depress interrupt */
KPSR.KRIE=OFF;/* disable key release interrupt */
}
}
Notice in the code above that after the columns value is set and
before the row values are tested, there is a comment to kill some
time. It turns out that some time is required to allow propagation
from the command to the electrical connection on the external switch.
The delay() routine with an argument of 1, a 1-ms delay, worked
well, but this routine also uses the PIT. It is intended to use this
routine with the clock shown in the earlier section, so it is very
inconvenient to use the clock. Therefore, a series of instructions
around the keyboard controller were put together that would use up
the necessary time. These instructions would have to be executed to
set up correct conditions prior to the return from interrupt, so it is not
too much a waste of computer resources. To get enough delay, I used
some of these instructions more than one time. The code for this
time delay is shown below. The last six lines of code are required to
set up for the exit from the isr, and the first four are duplicates needed
to round out the time.
KPSR.KRSS=ON;
KPSR.KPKD=ON; /* flip and flop some innocuous bits */
KPSR.KDIE=OFF; /* just to kill time to synchronize */
KPSR.KRIE=OFF; /* the command with data arrival at */
KPSR.KPKR=ON; /* the chip’s edge */
KPSR.KPKD=ON;
KPSR.KRSS=ON;