Final Exam: Attempt review

Started on	Saturday, 9 May 2020, 12:53 PM
State	Finished
Completed on	Saturday, 9 May 2020, 2:52 PM
Time taken	1 hour 59 mins

Question 1

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Question text

What is the minimum number of bits that an Analogue to Digital Converter with a range of 0-5.0V should have to accurately measure a signal at a resolution of at least 5.0mV?

Answer:

Question 2

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Question text

Following is an assembly program written for ATmega 2560 microcontroller to reverse a string using the stack.


.equ string_length = 12                           
.def zero = r17                                   
                                                  
.dseg                                             
.org 0x380                                         
reversed_string:                                  
	.byte string_length+1                         
xvar:                                  			  
	.byte 1			                        
                                                  
.cseg                                             
.org 0x0000                                       
jmp main                                         
                                                  
.org 0x0072                                       
string:                                           
	.db "ha ha ha ha!",0                        
                                                  
main:                                             
	;init stack pointer                           
	ldi yl,low(0x1811)                              
	ldi yh,high(0x1811)                             
	out spl,yl                                    
	out sph,yh                                    
                                                  
	;set xvar           						  
	ldi zl,low(xvar)    						  
	ldi zh,high(xvar)   						  
	ldi r16, -1         						  
	st z, r16           						  
	                    						  
	;string address in program mem                
	ldi zl,low(string<<1)                         
	ldi zh,high(string<<1)                        
                                                  
	;init                                         
	clr xl                                        
	clr xh                                        
	clr zero                                      
                                                  
;loop for reading from program mem and pushing    
loop:                                             
	lpm r16,z+                                    
	cpi r16,0                                     
	breq storeback                                
	push r16                                      
	adiw xh:xl,1                                  
	rjmp loop                                     
                                                  
;loop for popping and storing to data mem         
storeback:                                        
	ldi zl, low(reversed_string)                  
	ldi zh, high(reversed_string)                 
                                                  
loop2:                                            
	pop r16                                       
	st z+,r16                                     
	subi xl,1                                     
	sbc xh,zero                                   
	cp xl,zero                                    
	cpc xh,zero                                   
	breq end                                      
	rjmp loop2                                    
	                                              
;store nullchar	                                  
end:                                              
	st z+,zero                                    
                                                  
halt:                                             
	rjmp halt

What is maximum length of the string (string_length in the program) that can be reversed, without the stack overwriting the memory location of xvar?

Answer:

Question 3

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Marked out of 3.00

Question text

Consider two single precision floating point numbers x and y in IEEE 754 format, where x = 0xC0C80000 and y = 0x40D00000. What is the hexadecimal value of x-y?

Answer:

Question 4

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Question text

The following table contains partial information regarding the TCCR0B register, which is used for configuring timer0 in ATMega2560.

Assume the user wants to configure timer0 such that approximately 244 timer0 interrupts would occur per second. The following code fragment was written by a designer who decided to calculate the value later and instead used XXXXXXXX for the bits to be used. Which value of XXXXXXXX would correctly configure timer0 so that it would have the desired property? You may assume the clock speed of ATMega2560 is 16 MHz and the size of the counter for timer0 is 8-bits. Assume X=0 for all bits that are unknown.

in r16, TCCR0B
ldi r16, 0bXXXXXXXX
out TCCR0B, r16

Answer:

Question 5

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Question text

Which of the following programs will never go out of “AAA” segment?

i)

AAA:

ldi r16, 10

ldi r17, 10

cp r16, r17

jmp BBB

rjmp AAA

…

BBB: …

ii)

AAA:

ldi r16, 11

ldi r17, 10

cp r16, r17

brge BBB

rjmp AAA

…

BBB: …

iii)

AAA:

ldi r16, 10

ldi r17, 10

cp r16, r17

brge BBB

rjmp AAA

…

BBB: …

Choose your answer from the following: i, ii, iii, i and ii, i and iii, all, none

Answer:

Question 6

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Question text

Following is an assembly program written for ATmega 2560 microcontroller.


.macro delay 
	clr r16 
loop: 
	inc r16 
	nop  
	nop  
	cpi r16, @0 
	brne loop 
.endmacro 
 
delay 27 
delay 153 
delay 74 
delay 237

What is minimum number of bytes required in the program memory to store this program?

Answer:

Question 7

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Marked out of 3.00

Question text

Match the most suitable instruction to the description given below:

The most suitable instruction to perform add immediate is	Answer 1
The most suitable instruction to jump over one instruction if a bit in a register is 0	Answer 2
The most suitable instruction to set particular bits in a register	Answer 3
Set the least significant bit in the status register	Answer 4
Set the most significant bit in the status register	Answer 5
Supposing carry was set to 0. Which instruction would be most suitable to subtract one register value from another?	Answer 6

Question 8

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Marked out of 2.00

Question text

A C program consists of six functions. Their call relations are shown as follows (the arguments and irrelevant C statements are omitted.):

int main(void)

{

…

if (x>0) func1(…);

else func2(…);

func3(…);

…

}

void func1(…)

{

…

func1(…);

…

}

void func3(…)

{

…

func3(…);

…

}

void func2(…)

{

…

if (y>0) func4(…);

else func5(…);

…

}

Both func1() and func3() are recursive functions. main() calls either func1() or func2() only once depending on the value of x. func1() calls itself 10 times for the actual parameters given in main(). func2() calls either func4() or func5() only once depending on the value of y. func3() calls itself 20 times for the actual parameters given in main(). Neither func4() nor func5() calls any function. The sizes of all stack frames are shown as follows:

main(): 70 bytes.

func1(): 100 bytes.

func2(): 570 bytes.

func3(): 40 bytes

func4(): 70 bytes

func5(): 50 bytes

Assume that no interrupt occurs during the execution of this program. How much stack space is needed to execute this program correctly in the worst case?

Answer:

Question 9

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Marked out of 3.00

Question text

Match the numbers to the following questions

In the JMP instruction, how many bits are there for determining the target address?	Answer 1
How many bits does an RCALL instruction take	Answer 2
A 15 bit program counter can execute a maximum of _________K memory locations?	Answer 3
How many different values can be loaded into an 8-bit register?	Answer 4
The largest value that can be loaded into a 8 bit register is 0x_________	Answer 5
To set the lower nibble register to 1, we must OR the contents of the register with 0x____	Answer 6

Question 10

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Question text

Q1. Multiply two decimal values: 231 and 178 using MUL instruction in AVR. What is the value in r1 (in decimal) after multiplication?

Answer:

Question 11

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Question text

How many times is this loop going to get executed?

LDI R20, 10

LDI R21, 44
now: DEC R21 
     BRNE now 
     OUT PORTB, R20

Answer:

Question 12

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Question text

Which of the following is correct about BRNE instruction in avr microcontrollers?

Select one:

a. it is used to compare two registers

b. it is used to compare two values

c. it is used to check the zero flag

d. it is used to jump to the given mentioned label when the zero flag accounts to 1

e. none of the above

Question 13

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Question text

In the above figure t_on= 5 ms and t_off = 6 ms. Calculate the duty cycle in percentage (to 2 decimal places).

Answer:

Question 14

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Question text

For an ATMega 2560 microcontroller which runs at a frequency of 8 MHz. If r17:r16 register pair stores the loop count, what is the number stored in r17:r16 for 6 ms timing delay?

-------------

loop:

subi r16, 1

sbci r17, 0

nop

brne loop ; branch takes two cycles.

Answer:

Question 15

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Question text

What is the relation between the target and the relative address?

Select one:

a. target address= PC address + relative address

b. target address= relation address*2

c. relative address= PC address + target address

d. all of the mentioned

e. none of the mentioned

Question 16

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Marked out of 1.00

Question text

What 8-bit binary value does the instruction “ldi r16, (10 << 4) + (4 << 1)” load into r16?

i) (200)₁₀

ii) (AF)₁₆

iii) (10101000)₂

Question 17

Complete

Marked out of 5.00

Question text

Following is an assembly program written for ATmega 2560 microcontroller.


.def n0=r16 
.def n1=r17 
.def n2=r18 
.def n3=r19 
.def count0=r20 
.def count1=r21 
.def count2=r22 
.def count3=r23 
.def pattern=r24 
.def zero=r25 
.def one=r26 
 
clr count0 
clr count1 
clr count2 
clr count3 
clr zero 
ldi one,1 
 
ldi n0, 0x45 
ldi n1, 0x43 
ldi n2, 0xFF 
ldi n3, 0xFF 
 
ldi pattern,0x33 
 
loop: 
	lsl pattern  
	brcc increment_count 
	inc pattern 
 
increment_count: 
	add count0,one 
	adc count1,zero 
	adc count2,zero 
	adc count3,zero 
	cp count0,n0 
	cpc count1,n1 
	cpc count2,n2 
	cpc count3,n3 
	brne loop

What is the value of the pattern (r24) in hexadecimal at the end of the program? (Enter the answer in form 0x00)

Answer:

Question 18

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Marked out of 30.00

Question text

A student wrote an assembly program for ATmega2560 microcontroller to detect push button presses and displays the count using LEDS. The microcontroller clock is running at 16 MHz. The specification of the program was as follows:

The push button is connected to PD0 (INT0) of PORTD.
Each push button press increments an 8-bit counter that resides in data memory.
The counter is displayed in binary using 8 LEDs connected to PORTC.
Button pressed are detected using falling edge external interrupts on INT0.
Software debouncing has been implemented using a flag stored in data memory and a delay implemented using the 16-bit timer 1.
- The flag is initially cleared.
- When the flag is cleared, a button press turns the flag on and turns on timer 1.
- Any interrupts will be ignored when the flag is turned on.
- When the timer overflows, the flag is reset.
- The delay for debouncing should be ~262 milliseconds.

The program assembles successfully, but it has 11 logical bugs which affects the intended functionality. Identify at least 10 bugs and explain how those bugs can be fixed in the table provided. Note that there are no bugs where instruction in/out has been interchanged with lds/sts.

1)	.include "m2560def.inc"
2)
3)	.dseg
4)	.org 0x200
5)
6)	count_m : .byte 1
7)	flag_m : .byte 1
8)
9)	.cseg
10)
11)	.org 0x0000
12)	rjmp RESET
13)	.org INT0addr
14)	rjmp EXT_INT0
15)	.org OVF1addr
16)	rjmp Timer1OVF
17)
18)	.org 0x0072
19)
20)	; interrupt subroutine to External interrupt request 0
21)	EXT_INT0:
22)	; prologue
23)	push r16
24)	in r16, SREG ;
25)	push r16
26)	push r17
27)
28)	;if dobouncing flag is set just return, else turn on debouncing flag
29)	lds r16, flag_m
30)	cpi r16, 1
31)	reti
32)	inc r16
33)	sts flag_m, r16
34)
35)	;enable timer and update the count
36)	ldi r17, 0b00000001
37)	sts TCCR1B, r17
38)	lds r17, count_m
39)	inc r17
40)	sts count_m, r17
41)	out DDRC,r17
42)
43)	return:
44)	;epilogue
45)	pop r16
46)	pop r17
47)	out SREG, r16
48)	pop r16
49)	reti
50)
51)	; interrupt subroutine to Timer1
52)	Timer1OVF:
53)	push r16
54)	in r16, SREG
55)	push r16
56)
57)	; reset debouncing flag and disable timer
58)	clr r16
59)	sts flag_m<<1, r16
60)	sts TCCR1B, r16
61)
62)	pop r16
63)	out SREG, r16
64)	pop r16
65)	ret
66)
67)	; main program
68)	RESET:
69)
70)	ldi r16, low(RAMEND)
71)	out SPH, r16
72)	ldi r16, high(RAMEND)
73)	out SPL, r16
74)	;set port directions and init variables
75)	cbi DDRD, 0
76)	sbi PORTD, 0
77)	ser r16
78)	in r16, DDRC
79)	clr r16
80)	sts count_m, r16
81)	out PORTC, r16
82)	sts flag_m, r16
83)
84)	; configure timer in normal mode
85)	sts TCCR1A, r16
86)	sts TCCR1B, r16
87)	sts TCNT1H, r16
88)	sts TCNT1L, r16
89)	ldi r16, 1<<TOIE1
90)	sts TIMSK1, r16
91)
92)
93)	;set INT0 as falling edge
94)	ldi r16, (1 << ISC01)
95)	sts EICRA, r16
96)	in r16, EIMSK
97)	ori r16, (0<<INT0)
98)	out EIMSK, r16
99)
100)	cli
101)	rjmp halt
102)
103)	halt:
104)	rjmp halt

line number	Bug explanation / fix
71	LOW(RAMEND) being stored in SPH. Effect: Bad memory accesses Fix: Store in SPL
73	HIGH(RAMEND) being stored in SPL. Effect: Bad memory accesses Fix: Store in SPH
76	Setting PORTD bit when it has been defined as an input bit Effect: Undesirable consequences Fix: Remove line
97	INT0 is being disabled instead of enabled Effect: Button interrupt won't be detected Fix: ori r16, (1<<INT0)
100	Global Interrupt Enable flag disabled. Effect: Push button press won't be detected Fix: Enable global interrupts with sei
41	DDRC used instead of PORTC Effect: Count won't be shown Fix: out PORTC, r17
45	Conflict registers are being popped in the wrong order Fix: pop r17; pop r16
59	Address reference for flag_m does not need to be left shifted Fix: sts flag_m, 16
31	Return from Interrupt is always performed Fix: Branch statement to go to the return label
56	There is no implementation of a 262ms delay Fix: Implement