Homework 03 Solution

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Description

Note: I strongly recommend that you utilize the ATmega128 datasheet when solving these questions.

Problem #1 [25 pts]

The AVR code below (with some information missing) is designed to initialize and service interrupts from three I/O devices (DevA, DevB, and DevC).

  1. There are 8 external interrupt pins (INT0-INT7) in AVR. Based on the code provided, which three interrupt pins are these I/O devices connected to and what is the immediate value needed in line 1?

  1. Which I/O device’s interrupt is triggered by a low level input?

  2. Assume that pins 7 and 6 on PORTC are connected to an additional output device (DevD). Fill in lines 2-3 such that the corresponding pins are specified as outputs. Do not configure any other pins on the port as outputs.

  1. Suppose DevA requires that no interrupts are detected while it is being serviced. Fill in lines 4-5 with the necessary code to clear any latched interrupts at the end of ISR_DevA.

  2. Consider the RCALL instruction on line 0. Would the program function correctly if this line was replaced with the following underlined text? CALL ISR_DevB Assume that no other lines of code are changed. Provide an explanation to support your answer.

.include “m128def.inc”

.def

mpr = r16

START:

$0000

.org

JMP

INIT

.org

$0006

RJMP

ISR_DevA

.org

$000C

(0)

RCALL

ISR_DevB

RETI

.org

$000E

RJMP

ISR_DevC

INIT:

mpr, 0b00110000

ldi

sts

EICRA, mpr

ldi

mpr, 0b00000100

out

EICRB, mpr

(1)

ldi

mpr,

out

EIMSK,

mpr

ldi

mpr, $00

out

DDRD, mpr

out

DDRE, mpr

(2)

(3)

sei

MAIN:

{ …

do something…

}

ISR_DevA:

(4)

(5)

RETI

ISR_DevB:

{…

RET

}

ISR_DevC:

{…

RETI

}

Problem #2 [25 pts]

Consider the following AVR assembly code that waits for 1 second using the 8-bit Timer/Counter0 with the system clock frequency of 16 MHz operating under Normal mode. Take the time to read through this code and determine how it is designed to function. Write and explain the instructions in lines (1-10) necessary to make this code work properly. More specifically,

  1. Fill in lines (1-2) with the necessary code to enable the interrupt on Timer/Counter0 overflow.

  2. Fill in lines (3-4) with the necessary code to load the value for delay that generates an interrupt after 5 ms.

  3. Fill in lines (5-6) with the necessary code to set the prescalar value and the mode of operation.

  4. Fill in line (7) with the necessary code to set counter.

  5. Fill in lines (8-9) with the necessary code to reload the value for delay.

  6. Fill in line (10) with the necessary code to decrement counter.

.include “m128def.inc

.def

mpr = r16

.def

counter = r17

$0000

.ORG

Initialize

RJMP

.ORG

$0020

; Timer/Counter0 overflow interrupt vector

JMP

Reload_counter

.ORG

$0046

; End of interrupt vectors

Initialize:

(1)

; Enable interrupt on Timer/Counter0 overflow

(2)

; Enable global interrupt

SEI

(3)

; Load the value for delay

;

(4)

; Set prescalar and mode

(5)

;

(6)

; Set counter

(7)

LOOP:

counter, 0

; Repeat until interrupted

CPI

BRNE

LOOP

Reload_counter:

; Save mpr

PUSH

mpr

(8)

; Load the value for delay

;

(9)

(10) ; Decrement counter

; Restore mpr

POP

mpr

RETI

Problem #3 [25 pts]

Consider the AVR code segment shown below (with some missing information) that configures Timer/Counter0 for Fast PWM operation, and modifies the Fast PWM duty cycle whenever a specific button on Port D is pressed.

  1. Fill in lines (1-2) with the instructions necessary to configure Timer/Counter0 for Fast PWM mode, non-inverting output, and a prescale value of 8.

  2. Based on the prescale value used in part (a), what is the frequency of the PWM signal (fPWM) being generated by Timer/Counter0? Assume the system clock frequency is 16 MHz.

  1. Fill in lines (3-4) to provide the compare value for Timer/Counter0 so that the initial duty cycle is 0%.

  2. What would be the value necessary for the variable step to increase the duty cycle by 10% each time the DUTY_STEP subroutine is executed? Ignore the case when/if the compare value overflows.

.include

“m128def.inc”

.def

mpr = r16

.def

temp = r17

.equ

step =

INIT:

  • stack pointer is initialized

  • I/O ports

ldi

mpr, 0b00010000

; set pin 4

(OC0) as output

out

DDRB, mpr

ldi

mpr, 0b00000000

; set pin 0

as input

out

DDRD, mpr

; enable pull-up resistor for pin 0

ldi

mpr, 0b00000001

out

PORTD, mpr

  • Timer/Counter0

  • Fast PWM mode, non-inverting, prescale = 8

(1)

(2)

  • Initial compare value for PWM output

(3)

(4)

MAIN:

PIND, 0

sbis

rcall

DUTY_STEP

rjmp

MAIN

DUTY_STEP:

mpr

push

push

temp

in

mpr, ________

; read the current PWM compare value

ldi

temp, step

; add step value to compare value

add

mpr, temp

out

________, mpr

; write new PWM compare value

pop

temp

pop

mpr

; return

ret

Problem #4 [25 pts]

Write a subroutine initUSART0 to configure the ATmega128 USART0 to operate as a transmitter that will send data every time a USART0 Data Register Empty interrupt occurs. The transmitter operates with the following settings:

  • 8 data bits, 1 stop bits, and odd parity

  • 3,000 Baud rate (note that you will have to choose the closest available rate)

  • Transmitter enabled

  • Normal asynchronous mode operation

  • Interrupt enabled

Assume the system clock is 16 MHz. The skeleton code is shown below:

.include “m128def.inc”

.def mpr = r16

.ORG $0000

RJMP initUSART0

.ORG $0026

RJMP SendData

.ORG $0046

initUSART0:

Your code goes here. The lines below provide hints as to what you should do…

  • Configure USART0 (Port E, pin 1) as output

  • Set baud rate to 3,000

  • Enable Transmitter and data register empty interrupt

  • Set asynchronous mode and frame format

  • Set the global interrupt enable bit

Main:

mpr, X+

; Send first data

ld

out

UDR0, mpr

Loop:

Loop

RJMP

SendData:

mpr, X+

; Send next data

ld

out

UDR0, mpr

reti


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