An example Program EXAMPLE Program how to the user java program source code assignment statements nonzero elements Java programs the Squares by mariolopez


									An example Program

 switch reading, the most
  basic computer input
A computer keyboard typically uses a
  microprocessor to scan the switches.
  Similarly in remote control units for TVs
      lines               switches


                      column select lines
one column at a time has a voltage
  applied. If a switch on the column is
  pressed the corresponding row has the
   row select
      lines             switches


                    column select lines
 This requires some of the lines on the
  MPU to be configured as input or
  output lines. These collections of wires
        termed ‘ports’.
  are select
       lines              switches

port                  column select lines
Port scanning
   Typically each port is several bits wide,
    we use a shifting bit to scan
outport:=1                          --- set col 1 on
   if inport>0 then processs switch
  outport:=outport+outport;         --- shift column bit up
until outport>32
The problem
Mechanical switches play an important and
   extensive role in practically every computer,
   microprocessor and microcontroller
   application. Mechanical switches are
   inexpensive, simple and reliable.
In addition, switches can be very noisy. The
   apparent noise is caused by the closing and
   opening action that seldom results in a clean
   electrical transition. The connection makes and
   breaks several, perhaps even hundreds, of
   times before the final switch state settles.
Need to de-bounce switches
The consequences of uncorrected switch
 bounce can range from being just
 annoying to catastrophic. For example,
 imagine advancing the TV channel, but
 instead of getting the next channel, the
 selection skips one or two. This is a
 situation a designer should strive to
  use of a shift register

Whenever the switch is open, the high input on D propagates to Qn
after n clock cycles. If it closes even momentarily, the output goes
low and stays low until there has been a period of n cycles without a
switch closure
software debounce
   Consider a simple push button application. In
    this case, some event should occur when a
    button is pushed. Ideally the invoked event will
    occur immediately, and only once for each
    button push. The system should be also ready
    to respond to a repeat button push as soon as
    possible after the button is released. This
    presents an apparent dilemma.
   How is the difference between switch bounce
    and repeated button pushes determined?
brute force approach
   Assume that the bounce period is less than 10
   If the switch input level is stable for longer than 10
    milliseconds, then bouncing has stopped and the input
    level represents the pushed or released switch state.
   The Brute Force method only cares about a button-push
    event because this is what invokes the action. It
    recognizes the switch release state as the stable state
    and everything else is considered unstable.
   When the switch becomes unstable, the action is invoked
    permitting nothing to happen until the switch returns to
    the released stable state.
Program Design
To go any further we
   need to know how
1. read input bits
2. set the LEDs (Light
   Emiting Diodes).
3. Control the timer
The Micro controller development kit we are using

                                this is the microcontroller
This is the chip we are using PIC12F675

         power                             ground

pins 2 ..7 provide 6 general purpose programmable
input output lines
The switch is connected to pin 4
When you push it the input goes
towards ground, in digital terms
it reads as 0
LED driving
   The board has 8 LEDs
   It has only 4 input output pins available
    to drive them
   This would imply that it could drive only 4
   How does it manage it ?
   Use of a diode matrix
Each LED is switched on by activating one pin high and another pin low
If pin 2 = 5v and pin 3 = 0 v, then LED D1 will light
if pin 2 = 0v and pin 3 = 5v then LED D0 will light etc

technique used for controlling indicator lights on stereos, dashboards etc
multiple LEDs on at once?
   How could we get LED D0 and D1 on at
    the same time?
   Electrically this is impossible.
   We can fool the human eye though, by
    rapidly flashing the two LEDs thousands
    of times a second.
   It then looks as though both are on all
    the time.
Tristate Logic
   Normal logic uses the convention that
    high voltage is 1 and low voltage is 0.
   You will all recall that in addition to high
    an low outputs, a pin can be TRISTATE
   Tristate is needed in order to drive the
    LED matrix.
If we are trying to light LED D0, then clearly only pins 2 and
3 must be active.
we want 2 low and 3 high
if pin 5 was active we would get another light on
   if pin 5 was low, then d2 would be on as well
   if pin 5 was high then d5 would be on as well

Thus pins 5 and 6 must be set to be tristate
A timer is basically a counter operated by
  the processor clock. It can count down a
  fixed number of clock cycles.
Almost all micro controllers have built in
PIC timers
    3 timers
1.   Watchdog timer used to wake from
2.   8 bit TIMER0
3.   16 bit Timer 1
   This is in register 1, it counts instruction
   Its rate of counting is controlled by the
    bottom 3 bits of the option register, which is
    register 81
Option value         count rate
n               1/2n+1
6                1/128
the maximum option value is 7
prescaler logic
10 ms
   We want to wait 10 ms.
   Since the instruction cycle of the chip is
    200ns, this amounts to 50,000
   50000= c350 hex
   If we set the prescaler to divide by 256,
    then when the timer reaches c4 hex we
    have waited just over 10ms
   Set the option register
   Set the timer register.
   Problem the option register is register
    81hex, we can only address registers 0
    to 7Fhex with normal instructions
Register banks
   Processor has two banks of registers,
    bank0 and bank1.
   Bank0 for normal use, bank1 contains
    special control registers.
   Which one you access is controlled by
    the status register
    status reg ( reg 3)

   C =carry flag
   z = zero flag
   PD,TO give cause of timeout interrupts
   RP0 selects the register bank
to select bank 1 use instruction
BSF 3,5 ; set bit 5 of reg 3
to select bank 0 use instruction
BCF 3,5     ; clear bit 5 of reg 3
intcon reg (reg 11)

    if this bit is set
                         this bit is set when the timer0
    an interrupt will
                         wraps round past 255
    be generated on
    wrap round of
    timer 0
    keep it 0 for now
using the t01f flag
   This flag is set when the timer wraps
   we want to count C4=196 cycles
   if we initialise the timer to 256-196=60
   then it will wrap round at the right time
timer 1
   This is a 16 bit timer that counts one
    step each instruction. Its low byte is in
    reg 0Eh and the high byte in register 0Fh
   It is controlled by register 10h ( T1CON)
   To switch it on set bit 0 of T1CON
   When it wraps round it sets bit 0 of
    register 0Ch
   This is much simpler to use for large
timer init routine

 inittim         ;   initialise timer
   clrf TMR1L    ;   TMR1L defined as 0e
   movlw 03ch    ;
   movwf TMR1H   ;   set the timer to 3c00
   bsf T1CON,0   ;   switch it on
   bcf PIR1,0    ;   clear the finish flag
Sensing the switch
   Register 5 , also called GPIO is mapped
    to the i/o pins on the chip
   We can test the switch by monitoring
    GPIO bit 3, to which pin 4 ( switch input)
    is connected
Control of I/O
   GPIO is an 6-bit wide, bi-directional port.
   The corresponding data direction register is
    TRISIO. Setting a TRISIO bit (= 1) will make
    the corresponding GPIO pin an input (i.e., put
    the corresponding output driver in a tristate
   Clearing a TRISIO bit (= 0) will make the
    corresponding GPIO pin an output (i.e., put the
    contents of the output latch on the selected
   The exception is GP3, which is input only and
    its TRISIO bit will always read as ‘1’.
GPIO ( General Purpose I/O)
reading or writing GPIO
   Reading the GPIO register reads the
    status of the pins, whereas writing to it
    will write to the port latch.
   All write operations are read-modify-write
   Therefore, a write to a port implies that
    the port pins are read,this value is
    modified, and then written to the port
    data latch.
TRISIO( Tristate I/O)
How to Initialise GPIO
#define STATUS 3
#define RP0    5
bcf STATUS,RPO      ;Bank 0
clrf GPIO          ;Init GPIO
movlw 07h          ;Set GP<2:0> to
movwf CMCON        ;digital IO
bsf STATUS,RP0     ;Bank 1
clrf ANSEL         ;Disable analog input
movlw 0Ch          ;Set GP<3:2> as inputs
movwf TRISIO       ;and set GP<5:4,1:0>
                   ;as outputs
                GPIO register = 5
                        select bit 3

To test if the switch is on or off do the following
btfss 5,3                  ; test SWITCH input
goto SwitchDebounce        ; SWITCH was low - reset timer

If switch was low then bit 3 will be 0 and
btfss will not skip the next instruction
thus you will do the goto
If switch was high, then the bit 3 will be 1
and then the goto will be skipped.
   Header section of program.

list         p=12F675  ; list directive to
                       ;define processor
#include <> ; processor specific
                       ; variable definitions


 '__CONFIG' directive is used to embed configuration word within
.asm file. The labels following the directive are located in the
respective .inc file. See data sheet for additional information on
configuration word settings.
Define the constants to be used in this

#define Bank0            0x00
#define Bank1            0x80
#define SWITCH           GPIO,3
#define D0_1Tris         B'11001111'
#define D0On             B'00010000'
#define D0Off            B'00000000‘
#define Flags            0x20
#define LEDOn            Flags,0
We define Flags as register 20 it will be used to hold
working values for the program
Define entry point
   ORG 0x000 ; processor reset vector
   goto Init       ; go to beginning of prog
;Interrupt Vector
   ORG        0x004
   return           ; interrupt trap
                  ; - returns without re-enabling

   The ORG directive says where the instruction start in ROM.
        Address 0 is where the hardware starts running
     Address 4 is where the hardware goes on an interrupt
Initialise i/o
    call 0x3FF         ; retrieve factory calibration value
                      ; comment instruction if using
   BANKSEL Bank1
   movwf OSCCAL       ; update register with factory cal value
   movlw D0_1Tris     ; set direction so LEDs D0, D1 are
   movwf TRISIO       ; all others are inputs (high-z)
   clrf ANSEL         ; configure A/D I/O as digital
   banksel Bank0      ; change back to PORT memory bank
   movlw CM2 | CM1 | CM0 ; comparator digital I/O
   movwf CMCON
   clrf Flags         ; set initial LED state as off
   call ToggleLED     ; light initial LED
   goto Main
 Main program
 while not switch down do nothing;
 toggle LED
 wait for debounce

btfsc SWITCH            ; wait in loop until SWITCH closure sensed
goto Main              ; SWITCH closure grounds input pin
call ToggleLED         ; SWITCH closure sensed - toggle LED
call SwitchDebounce    ; wait for switch to release and settle
goto Main
Toggle LED subroutine
  btfss LEDOn        ; test flag of present LED condition
  goto TurnLedOn     ; the LED is presently off - go turn it on

   bcf LEDOn         ; clear flag to indicate LED is off
   movlw     D0Off   ; data for all LED outputs low
   movwf     GPIO    ; send data to GPIO port
   return            ; return to calling routine

   bsf LEDOn         ; set flag to indicate LED is on
   movlw     D0On    ; data to forward bias LED0
                     ; and reverse bias LED1
  movwf      GPIO    ; send data to GPIO port
  return             ; return to calling routine
Test state

  btfss      LEDOn
  goto       TurnLedOn
  goto       TurnLedOff
The first instruction skips the following instruction
  if the LEDOn flag is set.
Thus if it is not on, the goto TurnLedOn
  instruction is executed, if it is on, the mpu goes
  to TurnLedOff
Debounce routine
  call inittim
  btfss        5,3     ; test SWITCH input
  gotoSwitchDebounce    ; SWITCH was low - reset timer
  btfss PIR1,0         ; wrap round?
  gotoSD2              ; not counted to zero yet, continue
  return               ; full countdown
                       ; and no bounces – exit
  END                   ; directive 'end of program‘
                       ; this must terminate code

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