| Main | Assembly Programming | Input/Output | Memory | Computer Architecture | Advanced Topics |
Every rotation of a motor causes a switch to be pushed. The switch, like most mechanical switches, ‘clatters’ or bounces when the contacts come together, causing an interval when a single press causes multiple closures. This bouncing can continue for just under 3 milliseconds.
The switch is linked into the memory map of a 68000 processor at bit 0 of location $FFFFF000. It is connected so that the bit will read a ‘1’ when the switch is open, and a ‘0’ when closed and not bouncing. The bouncing will cause alternating zeros and ones.
For ease of any analysis you do, assume that every instruction takes 1 usec., regardless of addressing mode.
Write a commented program that continuously counts the number of times the switch is pushed. It must ignore the bouncing of the switch. This is all your program needs to do; don’t worry about other functions that the 68000 may have to perform at this point.
Assumptions: The switch is like a switch on a calculator, where there is no latched 'on' position. At one point in the turn of the motor the switch is pressed and bounces. Later on in the turn the switch is released. This repeats every revolution of the motor. There is no bounce when the switch is released.
Different reasonable interpretations of the mechanics of the problem are possible.
SwitchPlace EQU $FFFFF000 SwitchBit EQU $00 DelayCount EQU 1500 ;1500 * 2 instr * 1 usec = 3 msec. org $10000 Start moveq #0,d0 ;d0 is the counter ; The main counting loop starts here Open btst.b #SwitchBit,SwitchPlace ;see if switch open bne Open ;back if so ; When we get here, the switch has just been pressed addq.l #1,d0 ;advance the count ; Delay over 3 msec to ignore bounce movei #DelayCount,d1 ;loop this many times Loop subq #1,d0 ;d0 will be zero by 3 msec. bne Loop ; Now must wait for switch to be released Closd btst.b #SwitchBit,SwitchPlace ;see if switch closed bne Open ;back to start if open (released) bra Closd ;keep looping here if not