Lab 7 – Timer/Counters

Lab 7 – Timer/Counters ECE 375
ECE 375: Computer Organization and
Assembly Language Programming
Lab 7 – Timer/Counters
SECTION OVERVIEW
Complete the following objectives:
• Learn how to configure and use the 8-bit Timer/Counters on the ATmega128
to generate pulse-width modulation (PWM) signals.
• Use the upper half-byte and lower half-byte (otherwise known as the upper
and lower nibbles) of an I/O port for two different tasks.
PRELAB
To complete this prelab, you may find it useful to look at the full ATmega128
datasheet. If you consult any online sources to help answer the prelab questions,
you must list them as references in your prelab.
1. List the correct sequence of AVR assembly instructions needed to store
the contents of registers R25:R24 into Timer/Counter1’s 16-bit register,
TCNT1. (You may assume that registers R25:R24 have already been initialized to contain some 16-bit value.)
2. List the correct sequence of AVR assembly instructions needed to load
the contents of Timer/Counter1’s 16-bit register, TCNT1, into registers
R25:R24.
3. Suppose Timer/Counter0 (an 8-bit timer) has been configured to operate
in Normal mode, and with no prescaling (i.e., clkT0 = clkI/O = 16 MHz).
The decimal value “128” has just been written into Timer/Counter0’s 8-bit
register, TCNT0. How long will it take for the TOV0 flag to become set?
Give your answer as an amount of time, not as a number of cycles.
BACKGROUND
Timer/Counters are one of the most versatile and commonly-used components of
a microcontroller, and have many uses in embedded applications. In basic terms,
a Timer/Counter is simply a register whose contents are regularly incremented
(or decremented) at a specified, configurable frequency. If you know the rate at
which this incrementing or decrementing is occurring, then you can compare the
contents of a Timer/Counter’s register at two different points in a program, and
calculate how much time has elapsed!
Together, the width (in bits) of a Timer/Counter and the frequency of the clock
supplied to the Timer/Counter determine its range (the largest interval that can
be measured) and resolution (the smallest interval that can be measured). The
ATmega128 microcontroller features two 8-bit Timer/Counters and two 16-bit
Timer/Counters, and they all have multiple clock frequencies available.
Beyond simply measuring an interval of time, Timer/Counters can also be
configured to take some action based on an observed interval, such as toggling
an I/O pin after a certain amount of time has passed, generating a waveform.
Depending on how the Timer/Counter is configured, changing this interval will
change either the duty cycle or the frequency of the waveform. When the duty
cycle is varied, this technique is known as pulse-width modulation (PWM).
To successfully complete this lab, you will need to have a solid understanding
of how to configure Timer/Counters for PWM, and also how to modify the PWM
duty cycle after initial configuration. For more details on how to make the ATmega128 microcontroller create PWM signals, please refer to Timer/Counter0’s
“Fast PWM Mode” subsection on pages 98-99 of the ATmega128 datasheet.
PROCEDURE
For this lab, you will write an assembly program that allows a user to modify the
speed of the TekBot by providing input via Port D. The speed itself will be modified by using both of the 8-bit Timer/Counters in Fast PWM mode, and driving
the right and left Motor Enable port pins with the PWM signals generated by
the Timer/Counters. By varying the duty cycle of the PWM waveforms being
generated, you will effectively be modifying the speed of the TekBot’s motors.
You are not required to implement any BumpBot behavior in this lab, so the
TekBot should just be configured to move forward indefinitely.
Some design decisions have been left up to you for this lab (such as how to
detect user input on the Port D pushbuttons), but you must adhere to the
following requirements for full credit:
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Lab 7 – Timer/Counters ECE 375
1. Your TekBot needs to have sixteen equidistant speed levels, with Speed
Level 0 being completely stopped, Speed Level 15 being full speed, and
Speed Levels 1 through 14 in between. Refer to Table 1 for more details.
Speed Level TekBot Speed
15 100% (255/255)
14 ∼93.3% (238/255)
13 ∼86.7% (221/255)
.
.
.
.
.
.
2 ∼13.3% (34/255)
1 ∼6.7% (17/255)
0 0% (0/255)
Table 1: Equidistant Speed Levels
2. A user must be able to modify the speed in 4 different ways: (1) increase
speed by one level, (2) decrease speed by one level, (3) immediately increase
speed to the highest level (full speed), and (4) immediately decrease speed
to the lowest level (stopped).
3. A single button press on Port D must result in a single action; for example,
if the user presses the “decrease speed by one level” button, your program
should smoothly decrease the speed by just one level, not several levels in
rapid succession. Keep in mind, the correct way to meet this requirement
can depend on some of your previous design choices, especially regarding
how to detect user input.
4. The speed levels must not wrap around from 15 → 0 or 0 → 15. In other
words, if the user requests a speed increase and the TekBot is already at full
speed, the speed must not wrap around to a lower speed (i.e., overflow). In
a similar fashion, the TekBot must not go from stopped to a higher speed
(i.e., underflow) if a decrease is requested by the user.
5. In order to use both of the 8-bit Timer/Counters properly, you need to
check Timer/Counter Control Registers(TCCR0 & TCCR2) with Waveform
Generation Mode (WGM), Compare Output Mode (COM), and Clock Selection (CS). See 8-bit Timer/Counter Register Description and Alternate
Functions of Port B in ATmega128 Datasheet (103p and 73p).
6. To allow the user to visually assess the current speed level of the TekBot,
you must use the mega128 LEDs connected to the lower nibble (pins 3:0) of
Port B to display a 4-bit indication of the current speed level. For example,
you can use “0000” (all 4 LEDs off) to represent Speed Level 0, and “1111”
(all 4 LEDs on) to represent Speed Level 15. This 4-bit indication of speed
level must not interfere with the “Move Forward” motor control signals,
which are also being output to Port B at the upper nibble (pins 7:4).
STUDY QUESTIONS / REPORT
A full lab write-up is required for this lab. When writing your report, be sure
to include a summary that details what you did and why, and explains
any problems you may have encountered. Your write-up and code must be
submitted by the beginning of next week’s lab. Remember, NO LATE WORK
IS ACCEPTED.
Study Questions
1. In this lab, you used the Fast PWM mode of both 8-bit Timer/Counters,
which is only one of many possible ways to implement variable speed on a
TekBot. Suppose instead that you used just one of the 8-bit Timer/Counters
in Normal mode, and had it generate an interrupt for every overflow. In the
overflow ISR, you manually toggled both Motor Enable pins of the TekBot,
and wrote a new value into the Timer/Counter’s register. (If you used the
correct sequence of values, you would be manually performing PWM.) Give a
detailed assessment (in 1-2 paragraphs) of the advantages and disadvantages
of this new approach, in comparison to the PWM approach used in this lab.
2. The previous question outlined a way of using a single 8-bit Timer/Counter
in Normal mode to implement variable speed. How would you accomplish the same task (variable TekBot speed) using one or both of the 8-
bit Timer/Counters in CTC mode? Provide a rough-draft sketch of the
Timer/Counter-related parts of your design, using either a flow chart or
some pseudocode (but not actual assembly code).
CHALLENGE
To receive extra credit for this lab, you will need to implement a new feature that
supplements your regular Lab 7 code. Use one of the 16-bit Timer/Counters to
keep track of how many seconds have elapsed since the TekBot’s speed was last
modified, and continually display this count as an 8-bit binary number on
the LCD. Do not display any leading zeros – for example, 0 seconds should be
displayed as just ‘0’, and 8 seconds should be displayed as ‘1000’.
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Lab 7 – Timer/Counters ECE 375
Note: Using the LCD will affect the way the speed level is displayed on Port
B. This is expected; just make sure you are prepared to disable any LCD-related
parts of your code when you demonstrate the regular portion of this assignment.
For full extra credit, demonstrate the correct operation of this feature to your
TA, explain your design in the lab write-up, and be sure to also turn in your
challenge .asm file as part of the online submission process.
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