Learning Microchip Lesson 1-2

Transcription

1 Learning Microchip Lesson 1-2 Last lesson introduced you to Microchips MPLAB software and our PicoFlow LT software to turn a LED on. Here you looked at installing the MPLAB software, writing simple OPCODE instructions, the Register Memory Map (with SFRs and GPRs), Numerical Values (in Hexadecimal, Decimal, and Binary), Compiling a HEX file, and programming the HEX file to your project using the PicoFlow LT software. Each lesson is designed to challenge your learning in some way. This lesson will introduce you to the GOTO instruction, Bit-Tests, and Delays. Here, you will also learn how to use a switch input and numerous ASM instructions. Contents Where to Start:... 2 Instruction Memory:... 2 Where to GOTO:... 3 Example... 3 Bit Test (Conditional GOTO)... 4 Example... 4 Delay (Conditional GOTO)... 5 Example... 5 Naming GPRs (cblock)... 6 Lastly... 7 Conclusion... 7 Test What You ve Learned... 7

2 Where to Start: By now you should have the following code in your *.asm file, compiled into a *.hex file, written to your device, and running your project. If not, please revisit Lesson 1-1. ASM CODE: #include <P16F505.INC> CONFIG _IntRC_OSC & _MCLRE_OFF & _WDT_OFF end MOVLW b' ' ; Binary value moved into the W register and OPTION ; Clear T0CS to turn off CLKIN in OPTION reg. MOVLW b' ' ; Binary value moved into the W register to TRIS PORTB ; select RB2, 1, 0 as Outputs in TRISB reg. MOVLW b' ' ; MOVWF PORTB ; Initialise the port (B) with all LEDs off BSF PORTB, 2 ; ; Turns On a LED at RB2 for PicoDice Instruction Memory: So far you have been writing your code into the *.asm file. This code is compiled into HEX code which is stored on the microcontroller device. But where is it stored? All microchip PICs have at least two types of memory: Instruction Memory and Data (Register) Memory. When we write the hex file to the microcontroller device, it is stored in the Instruction Memory. So on the PIC16F505 devices the maximum amount of asm code you can write is 1024 because each line of asm (not including blank space, comments or #include or CONFIG statements) converts directly into one line of Instruction Memory. (Yet in actuality, there are only 1023 lines on this device because the last line holds a MOVLW command to define the calibration value of the microcontroller.)

3 Where to GOTO: Now if all we could do was write code so as to modify peripheral register bits then we would have a problem: We would have to write the code so it went from Instruction line 0000h to line 03FFh without any deviations. But instead we can create deviations with a GOTO command, conditional deviations with a BTFSC (Bit Test File Skip if Clear) command, and conditional deviations with a DECFSZ (Decrement File Skip if Zero) command. The Program Counter (PC) keeps track of which instruction is next. The GOTO command modifies the PC, and so the code will effectively jump to that address in Instruction Memory. The GOTO command can be used in numerous ways. The easiest way is to use headings such as Main: or Init: (or a name of your choice followed by a colon) to indicate the start of each section of code instructions. Then you refer to these headings with the GOTO command: Example Main: BSF PORTB, 2 ; Turns On (to +ve) pin RB2 of the PORTB file GOTO Main ; Loops through the BSF command indefinitely Another way to use a GOTO command is with the $ symbol to indicate a relative position. The Above example could be stated simply as GOTO $-1 without the heading. The $ sign indicates the current instruction with + or value to indicate the relative position. You can also use the $ symbol alone (as indicated below). Main: BSF PORTB, 2 ; Turns On (to +ve) pin RB2 of the PORTB file GOTO $ ; Loops through the GOTO command indefinitely In the first example above it was a bit silly to repeat the BSF command over and over, because once a bit is set, it stays set. But with the GOTO $ command, the Bit is set only once and the code effectively stops after this instruction.

4 Bit Test (Conditional GOTO) GOTO is useful in combination with a BTFSC command (Bit Test File Skip if Clear). Putting these two commands together creates a conditional GOTO. This combination can be used like an IF statement as in other programming languages. For example: let s do a Bit Test on a Port that has been designated as an input, such as RC5 on the PicoLights project. With a switch connected to RC5 we will periodically test the input to trigger a LED. Example #include <P16F505.INC> CONFIG _IntRC_OSC & _MCLRE_OFF & _WDT_OFF Init: MOVLW b' ' ; Binary value moved into the W register and OPTION ; Clear T0CS to turn off CLKIN in OPTION reg. MOVLW b' ' ; Binary value moved into the W register to TRIS PORTC ; select RC2, 1, 0 as Outputs in TRISC reg. MOVLW b' ' ; MOVWF PORTC ; Initialise the port (C) with all LEDs off SetLED1: BSF PORTC, 2 ; Bit Set on pin RC2 Test: BTFSC PORTC, 5 ; Bit Test RC5 GOTO SetLED1 ; GOTO $-2 would also work here ClearLED1: BCF PORTC, 2 ; Bit Clear on pin RC2 GOTO Test ; end So this is how a Bit Test is used with a GOTO statement to make it a conditional GOTO. The BTFSC command says, IF input RC5 is Clear (Pressed) then Skip to the heading label ClearLED1, else IF input GP1 is Set (Un-Pressed) then GOTO the heading label SetLED1. The two loops above represent the resulting code deviation where RC5=Clear repeats the Red loop & RC5=Set repeats the Blue loop. This code turns the LED off when the switch is held down; this in turn grounds the pin to 0V. But when the switch is released, the 10kOhm Pull-Up resistor sets the pin to 4.5V un-pressed. Try this example with BTFSS (Skip if Set) & note the result.

5 Delay (Conditional GOTO) The DECFSZ (Decrement File Skip if Zero) command can help us create a delay by counting down from 255 to 0. It is similar to the Bit-Test because it is conditional upon an event, but whereas the Bit-Test was conditional upon a single bit, the DECFSZ is conditional upon a whole register (8 bits) and whether this has a value of zero. So if we put the DECFSZ in a loop it will continually subtract 1 from the register, but when it reaches zero the PC (Program Counter) jumps out of the loop. This effectively wastes some time but not very much, so we need to put a loop in a loop, and so on until we have a usable delay. Example #include <P12F508.INC> CONFIG _IntRC_OSC & _MCLRE_OFF & _WDT_OFF Init: MOVLW b' ' ; Binary value moved into the W register and OPTION ; Clear T0CS to turn off CLKIN in OPTION reg. MOVLW b' ' ; Binary value moved into the W register to TRIS PORTC ; select RC2, 1, 0 as Outputs in TRISC reg. MOVLW b' ' ; MOVWF PORTC ; Initialise the port (C) with all LEDs off Main: BSF PORTC, 2 ; Turns On (sets) pin RC2 of the PORTC register Test: BTFSC PORTC, 5 ; Bit Test RC5 GOTO Main ; GOTO $-2 would also work here ClearLED1: BCF PORTC, 2 ; Turns OFF (clears) pin RC2 of the PORTC register Delay: DECFSZ 0x08 ; 0x08 is the first GPR (General Purpose Register) GOTO Delay ; in Data Memory DECFSZ 0x09 ; Each DECFSZ represents a loop between itself GOTO Delay ; and the heading Delay so each loop is submerged DECFSZ 0x0A ; within the last GOTO Delay ; The time for this delay is about 52 seconds GOTO Test end

6 Program this example and you may be left wondering why the LED stays off initially even without pressing the switch. The reason is that the PIC16F505 requires a startup delay to balance the voltages correctly. To fix this we can create a start-up using DECFSZ loops. Add the following instructions before the Init: heading. Startup: DECFSZ 0x08 ; This Delay reuses the registers 0x08 and 0x09 GOTO Startup ; The time for this delay is about 0.2 sec DECFSZ 0x09 GOTO Startup The first delay we used is too long (about 52 sec) to be useful. So we can pre-load the last DECFSZ register (0x0A) with a smaller number (eg. 10) for a shorter delay. Add the following lines before the Delay: heading. Preload: CLRF 0x08 ; (Clear File) Reg. 0x08 & 0x09 will be.0 (.256 equiv.) CLRF 0x09 ; With a Decrement they will roll-over to.255 MOVLW.10 ; A preload of 10 (on the last register) will give us MOVWF 0x0A ; about 2 sec on this delay with three DECFSZ loops The CLRF instructions clear the files 0x08 & 0x09 to zero. The MOVLW (Move Literal to Working) instruction places the number 10 into the working register, and the MOVWF (Move Working to File) instruction places this number into reg. 0x0A. Naming GPRs (cblock) Instead of using the literal values to define the Data Memory registers, we can name the GPRs with words to describe what they are being used for. This makes the code easier to read. Just add the following lines after the CONFIG statement. cblock 0x08 ; cblock defines the register start DelayReg1 ; Add your register names & they will be assigned DelayReg2 ; Data Memory space up to 0x1F DelayReg3 ; The PIC16F505 has 24 General Purpose Registers in ; the first memory Bank 0 & 16 more in Banks 1-3 endc ; endc defines the end of the cblock After you have named your Data Memory registers within the cblock, you can use these names in your code instead of using literal values. For Example: Preload: CLRF DelayReg1 ; (Clear File) Reg. 0x08 & 0x09 will be.0 (.256 equiv.) CLRF DelayReg2 ; With a Decrement they will roll-over to.255 MOVLW.10 ; A preload of 10 (on the last register) will give us MOVWF DelayReg3 ; about 2 sec on this delay with three DECFSZ loops

7 Lastly Creating delays can be a bit cumbersome, so we ve created an easy to use tool for you. So to create your own delays quickly and easily, please refer to the Automatic Delay tool in the PIcoFlow_LT software. Conclusion In this tutorial, you have learnt about the Program Memory, the GOTO instruction and the Program Counter (PC). You have also learnt about the combination of a Bit- Test such as BTFSC with the GOTO instruction to make a Conditional GOTO the example given was a Bit-Test on a Port Input such as RC5 so as to use an external switch to deviate the code and light a LED on RC2. We also created our own Delays to exemplify the conditional GOTO with the DECFSZ; thereby determining the minimum time the LED would stay off and then used a cblock to name the GPR (Data Memory) registers. Test What You ve Learned Try creating the following programs and have them checked by your teacher. 1. Delays 2. Bit-Test a. LED1 on, Pause, LED2 on, Pause, LED3 on, Pause b. All LEDs off, Pause, Repeat from the start a. LED1 on, then wait for switch input, LED2 on, then wait for switch input b. LED3 on, then wait for switch input, Repeat from the start Contact Support: Web: Philip Tallents ptal@picokit.com Ph.: Fax: