of Microprocessors

304 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Software Functions
Of course, with a microprocessor-based interface, hardware is only half
the batde. The fundamental software job is to scan the 61 keys as fast as
possible and handle any that are not contacting the upper bus (i.e., depressed)
in an appropriate manner. In the fundamental program structure,
time is broken into fixed length segments, and an attempt is made to do
everything necessary in each time period. The most important housekeeping
functions are done first and then the keyboard is scanned starting at the low
end. In the rare event that keyboard activity is such that processing takes
longer than a time segment, an interrupt from the timer will abort scanning
and start the next cycle. Thus, the highest few keys might experience an
occasional one count error in event or velocity timing under very heavy
keyboard activity conditions.
One of the first software design steps (and in fact the first feasibility
study step) is to determine how fast the scanning can be done and therefore
the length of a time segment. If a straightforward scanning loop is not fast
enough for adequate velocity timing resolution, then something less
straightforward will be necessary.
As it turns out, a simple scanning loop can test a key and go to the next
if it is up in 12 f-Lsec, which means that a scan of the entire keyboard would
take 732 f-Lsec. Allowing 70 f-Lsec of additional overhead for handling the
timer and setting up for a scan, it would seem to be possible to complete a
scan every millisecond, which is the desired resolution of key timing. However,
if a key is found to be down or was down in the previous scan, then
additional processing is necessary. Further study of key-down processing
reveals that the time used varies from a minimum of 37 f-LSec when the key
is fully depressed and held to a maximum of 100 f-Lsec when an event is
queued. Thus, a time allotment of a millisecond would only cover a
maximum offive fully down keys and less than that when events are queued,
which is not really satisfactory. Although several courses of action are possible
(2-MHz version of 6502 or external hardware to make key scanning
faster), for this example the scan period will be increased to 1.28 msec.
Velocity timing will therefore have a resolution of 1.28 msec, which is
adequate when compared with normal variation in the velocity time differential.
Software Flowchart
An overall program flowchart is given in Fig. 9-7. When power is first
applied or after a reset, all of the important program variables are initialized
and then the main loop is entered. In the loop, the "time of day" is updated
first taking into account the 1. 28-msec update interval and 1.0-msec time of
day units. Next the queue of events is checked. If an event is in the queue,
then a check is made to determine if the previous event is still awaiting