Holtek HT-IDE, User Manual

Page: 222 / 258

mov tmr0h,a ;
set tmr0c.4 ;start timer0
key_halt: ;(7) ;check key holding state
mov a,pa ;test pa
cpl acc ;
sz acc ;is holding?
jmp key_halt ;yes. check again
clr tmr0c.4 ;(8) ;no. stop timer (stop sound two)
clr pb ;
ret ;
whichkey endp ;
org 0f00h ;sound freq.
dc 21h,0feh,58h,0feh
dc 84h,0feh,99h,0feh
dc 0c1h,0feh,0e3h,0feh
dc 02h,0ffh,11h,0ffh
end
Program description
The program begins (1) by setting all pins on Port A to inputs by setting the
Port A control register PAC high. Port B is setup as outputs by clearing its
control register PBC. After this the program enters a program loop (2) to
detect if a key has been pressed. If a key has been pressed the program
jumps out of this loop otherwise it will remain in this loop until a key is
pressed. The next stage is a program to determine which key was pressed
(3). Determining which key is pressed enables the correct frequency value
to be obtained from the table. This is done by incrementing the table pointer
until the correct location is reached. The value is divided into a high byte
and low byte, these two values have to be placed in the correct high/low byte
position of the timer/counter (5). After the value is placed and counted an
interrupt is generated by the timer/counter and the interrupt routine is run
and the correct output (6) placed on Port B. In this way the correct
frequency can be generated. The Port B output pin is connected to a suitable
amplifier and speaker to generate the correct tone.
After detecting the key and obtaining the correct frequency value the key
is again examined (7) to see if it has been released. This is because the
timer/counter will continue to be reloaded so if the key is not released the
note will continue to appear on Port B. If the key has been released (8) then
the control bit is set to turn off the timer/counter.
Clock
This application shows the use of the 16 bits of the timer counter to
generate internal interrupts and consequently generate a timing function.
HT-IDE User’s Guide
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