安全系统控制与MAXQ2000-Security Syste

文章摘要：Figure 4. In each of the four key-scanning states, the application reads the status of four switches and prepares to read the next four.Because the interrupt vector (IV) for the MAXQ2000 can be set on-the-fly, the application holds the next-state value in the interrupt vector register. Whenever the

安全系统控制与MAXQ2000-Security Syste,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

Figure 4. In each of the four key-scanning states, the application reads the status of four switches and prepares to read the next four.

Because the interrupt vector (IV) for the MAXQ2000 can be set on-the-fly, the application holds the next-state value in the interrupt vector register. Whenever the timer interrupt fires, the handler routine for the current key-scanning state sets the interrupt vector address to the next state's handler routine.

org  0000h

Main:
   call    InitializeLCD

   move    PD6, #010h         ; For state 1
   move    PO6, #00Fh         ; For all states
   move    PD7, #000h         ; For state 1
   move    PO7, #000h         ; For all states

   move    IV, #State1
   call    StartTimer
   move    IC, #1            ; Enable global interrupts

   jump    $


State1:
   push    PSF
   push    Acc

   move    Acc, PI6
   and     #000Fh            ; Grab lowest four bits only
   sla4
   move    A[13], Acc

   move    PD6, #020h        ; For state 2
   move    PD7, #000h

   move    T2V0, #RELOAD     ; Set reload value
   move    T2CNB0.1, #0      ; Clear interrupt flags
   move    T2CNB0.3, #0
   move    IV, #State2

   pop     Acc
   pop     PSF

   reti

The handler routines for the other four states are similar, with a slight adjustment to OR in the previously collected switch bits in the A[13] holding register. There are three working accumulators used by the state routines.

A[13] holds the bit array of all the switch states read on the current pass through the keypad. After the State 4 read completes, this register contains the following bits, where a one bit represents an open (released) key switch and a zero bit represents a closed (depressed) key switch.

BIT 15

BIT 14

BIT 13

BIT 12

BIT 11

BIT 10

BIT 9

BIT 8

BIT 7

BIT 6

BIT 5

BIT 4

BIT 3

BIT 2

BIT 1

BIT 0

*

7

4

1

2

5

8

0

3

6

9

#

D

C

B

A

Debouncing Switches

After State 4 is reached and all keys are scanned, a decision must be made whether to accept any keys that are pressed. A simple way to handle debouncing is to maintain a counter value for each of the 16 switches. Every time State 4 is reached and the key is pressed, the counter is incremented. If the key is not pressed, the counter is decremented. When the counter reaches a certain value, the keypress is registered. To prevent a held-down key from repeating (which typically is allowed on computer keyboards, but not on keypads), the counter must be allowed to decrement back to zero (by releasing the key) before that key may be registered again.

As we have the state of all 16 keys in a single register, there is a simpler, less memory-intensive solution for debouncing. The application maintains a single counter value that is incremented each time the bit pattern matches the pattern read on the previous pass.

State4:
   push    PSF
   push    Acc

   move    Acc, PI6
   and     #000Fh            ; Grab low four bits only
   or      A[13]
   cmp     A[15]
   jump    E, State4_End    ; Ignore the last debounced pattern

   cmp     A[14]
   jump    E, State4_Match

   move    LC[0], #DEBOUNCE
   move    A[14], Acc       ; Reset current bit array

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