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Figure 20.27 Simple circuit to demonstrate the ability to use TMR0 as a digital input that creates an interrupt request.
; ; ; ; ; ; ; ; ;
Hardware Notes: This application runs on a PIC16F84 executing at 4 MHz _MCLR is tied through a 10K Resistor to Vcc and PWRT is Enabled A 10K Pull-Up and a Momentary On Switch is Connected to RA4 A 220 Ohm Resistor and LED is connected between PORTB.0 and Vcc Myke Predko 99.12.28
TABLE 20.12 PART
TMR0Int BILL OF MATERIALS DESCRIPTION
PIC 0.1- F 10k Momentary-on switch 4-MHz LED 220 Misc.
PIC16F84 04/P 0.1- F tantalum 10 k , 1/4 W Breadboard-mountable momentary-on switch 4 MHz with built-in capacitors Red LED 220- , 1/4 W Breadboard, wiring, +5-V power supply
I/O WITH INTERRUPTS
+5 Volt P/S
Momentary On Switch
Vcc Gnd
LED 0.1 uF
The breadboard circuitry for TMR0Int.
LIST R=DEC INCLUDE p16f84.inc ; Registers CBLOCK 0x020 _w, _status ENDC
Context Register Save Values
__CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _PWRTE_ON PAGE Mainline of cylon 0
org nop movlw movwf goto org Int: bcf bcf
0x0FF PORTB Mainline 4 ; INTCON, T0IF PORTB, 0 ; ; TMR0 has Over owed - New Input Reset the Interrupt Flag Turn on the LED
PIC16F84
10 K 10 K
4 MHz
PIC MICROCONTROLLER APPLICATION BASICS
ret e Mainline: bsf bcf bcf movlw movwf clrf movlw movwf movlw movwf movlw movwf bsf Loop: goto Loop STATUS, RP0 PORTB, 0 STATUS, RP0 OPTION_REG FSR TMR0 ; ; ; Setup TMR0 Interrupt
Goto Bank 1 to set Port Direction ; Enable RB0 for Output Go back to Bank 0 ; Point to the Option Register
Reset the Timer
(1 << GIE) | (1 << T0IE) INTCON ; Enable Interrupts 0x0C0 INDF 0x0FF TMR0 INDF, T0CS ; ; ; ; Now, Make TMR0 Driven Externally Loop Here Let Interrupt Handler Work in the Background ; ; Make TMR0 Driven by the Instruction Clock
This code will seem somewhat conventional except for my passing the address of the OPTION register to FSR and updating it from there rather than the usual way of updating the register while bank 1 is being accessed. I used this code because the time required to set TMR0 and then jump to bank 1 would be much more dif cult to time than the code I used. In this example code, TMR0 is initially given the value of 0xFF and waits to be updated by the instruction clock. As I indicated earlier in this book, TMR0 has a twocycle clock counter (or synchronizer) that must have two inputs before it passes one on to TMR0. This synchronizer, like the prescaler, is reset any time TMR0 is written to. This means that it will over ow and request an interrupt two instructions after it is set. For this application, I want TMR0 to over ow after the rst external input, so to set up this condition, I want one instruction between the instruction when TMR0 is loaded with 0xFF and when I change its input source to RA4.
I/O WITH INTERRUPTS
My solution is to use the index register (FSR) to point to OPTION and to change the source (by resetting the T0CS bit) using the bcf instruction. When I have done this before, I have used the option instruction, and if I were implementing this code in a low-end PIC microcontroller, I would have to use the option instruction. The code here avoids using the not-recommended option for mid-range PIC microcontrollers. When using this code in low-end PIC microcontrollers, remember not to use the movf TMR0, f instruction and instead use the movf TMR0, w instruction. Both will set the STATUS register s zero ag, but the rst instruction will reload TMR0 with the current value, and if it is 0xFF, it will reset the two-cycle timer, which means that two valid interrupt inputs will be required to change the state. What s worse is that if TMR0 is polled before the two interrupt inputs are received, it will result in the TMR0 never resetting and incrementing for the interrupt source. This application is very focused on the single task of latching the LED connected to RB0 on when the input on RA4 goes low, and for this reason, I didn t bother with a complicated interrupt handler (in fact, I dispensed with context register saving and restoring). In a real application, the interrupt handler code probably would carry out the code for setting TMR0 as an interrupt source after acknowledging that the interrupt has been received.
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