Embedded Systems and Software

Transcription

1 Embedded Systems and Software Potpourri & Notes on Lab 2 Artist's concept of Mars Exploration Rover. Courtesy NASA Lab 2 Notes Slide 1

2 The AVR Assembler We use the AVRASM2 assembler that comes with AVR studio Assembler NOT compiler The Assembler performs the conversion from English mnemonics to opcodes that are programmed into the microcontroller C compilers generate assembly language mnemonics that are then assembled by an assembler Much confusion arises when one does not have a conceptual understanding of the different assembly phases Lab 2 Notes Slide 2

3 The assembler s text substitution allows one to use symbols rather than numbers.equ PORTB = 0x18.equ DDRB = 0x17.include "tn45def.inc".include "tn45def.inc".cseg sbi DDRB,1 loop: sbi PORTB,1 cbi PORTB,1 rjmp loop Phase 1: text substitution sbi 0x17,1 These are program addresses loop: sbi 0x17, cbi 0x18, cbi 0x18, rjmp 0x01 Phase 2: machine instructions, Lab 2 Notes Slide 3

4 The assembler s text substitution allows one to use symbols rather than numbers.include "tn45def.inc".cseg sbi DDRB,1 loop: sbi PORTB,1 cbi PORTB,1 rjmp loop.include "tn45def.inc".cseg sbi DDRB,1 sbi DDRB,2 loop: sbi PORTB,1 cbi PORTB,1 rjmp loop sbi 0x17,1 loop: sbi 0x17, cbi 0x18, cbi 0x18, rjmp 0x sbi 0x17, sbi 0x17,2 loop: sbi 0x17, cbi 0x18, cbi 0x18, rjmp 0x02 Lab 2 Notes Slide 4

5 Assembler Directives Assembler directives instruct the assembler what to do The directives are not translated directly into opcodes. Instead, they are used to adjust the location of the program in memory, define macros, initialize memory and so on..include "tn45def.inc".cseg sbi DDRB,1 This does not generate opcodes. Rather it direct the assembler to include replace this line with the contents of tn45def.inc loop: sbi PORTB,1 cbi PORTB,1 rjmp loop Lab 2 Notes Slide 5

6 We have seen this directive several times thus far Lab 2 Notes Slide 6

7 .INCLUDE Include another file ASM Directive The INCLUDE directive tells the Assembler to start reading from a specified file. The Assembler then assembles the specified file until end of file (EOF) or an EXIT directive is encountered. An included file may itself contain INCLUDE directives. Contents of iodefs.asm Include this in another AM program, and one can use symbols such as sreg rather than 0x3f: Lab 2 Notes Slide 7

8 .EXIT - Exit this file ASM Directive The EXIT directive tells the Assembler to stop assembling the file. Normally, the Assembler runs until end of file (EOF). If an EXIT directive appears in an included file, the Assembler continues from the line following the INCLUDE directive in the file containing the INCLUDE directive. Lab 2 Notes Slide 8

9 Assembler Functions This returns the low byte ldi r19,low(0x07f3) ldi r20,high(0x07f3) This returns the high byte Text substitution & apply functions ldi r19,0xf3 ldi r20,0x07 Generate opcodes Lab 2 Notes Slide 9

10 Assembler Functions LOW and HIGH are often used, others less frequently, and we will not cover these in this course Lab 2 Notes Slide 10

11 Assembler Preprocessor The AVRASM2 preprocessor is modeled after the C preprocessor, with some exceptions (see AVRASM2 documentation). #define #if #pragma Operators: #elif #ifdef #undef # (stringification #else #ifndef #warning ## (concatenation) #endif #include # (empty directive) #error #message.equ c1 = 0b equ c2 = 0b #define DEBUG #ifdef DEBUG ldi r18,low(c1 c2) rcall delay #else ldi r18,low(c1^c2) #endif As with the C preprocessor, the AVRASM2 preprocessor allows conditional processing. For example, one can build a special debug version as shown in the snippet. Lab 2 Notes Slide 11

12 Microcontroller Fuses AVR ATtiny45 8-Pin Microcontroller Except for GND and VCC all other pins can perform at least 4 functions PB5 can be hardware RESET or an ADC input PB3 & PB4 can be ADC inputs or where crystal for external oscillator goes How does one configure controller for the external environment? Lab 2 Notes Slide 12

13 Microcontroller Configuration/Fuses AVR ATtiny45 8-Pin Microcontroller Except for GND and VCC all other pins can perform at least 4 functions PB5 can be hardware RESET or an ADC input PB3 & PB4 can be ADC inputs or where crystal for external oscillator goes How does one configure controller for the external environment? Lab 2 Notes Slide 13

14 Microcontroller Configuration/Fuses The Configuration Fuses (Configuration Bits) are the settings that configure a microcontroller for the external environment it is expecting to find Typical settings include Oscillator Type, REST pin usage Code Protection Brown-Out and Watchdog Timer usage Low Voltage Programming Different microcontrollers, and members within a device family has its own fuses Lab 2 Notes Slide 14

15 Enables use of WDT This chooses if the clock is divided by 8 or not, start-up time, Lab 2 Notes Slide 15

16 Divide the clock by 8 Use internal RC clock Lab 2 Notes Slide 16

17 Switch Bounce and Debouncing Bounce ~ 5 ms Lab 2 Notes Slide 17

18 Switch Bounce and Debouncing When the switch closes, the capacitor discharges through Rb Voltage Question: what is the fall time? Make sure you can calculate this. t Lab 2 Notes Slide 18

19 Software Debounce One idea. Sample n times at regular intervals, say 10 ms apart. Count how many times the switch is zero. In this is larger than the number of times the switch is high, consider the switch pressed. char ispressed(void) { char ones=0, zeroes=0, i; } for(i=0;i<=10-1;i++){ if(pina&0x01){ // read pin == 1 ones++; } else { // read pin == 0 zeroes++; } _delay_ms(10); } return (ones > zeroes); Lab 2 Notes Slide 19

20 Lab 2 Notes Slide 20