Lab 4 – Data Logger with Timer Interrupts
Contents
1. Lab 4 Data Logger with Timer Interrupts Overview Using interrupts allows for microprocessors and microcontrollers to handle events as they happen without wasting time always looking for them polling In this lab we will be revising lab 3 to use interrupts for the timer Prelab No Prelab this week but it suggested you read pages 16 18 68 69 110 116 and 198 202 of the AT9O0USB datasheet and in your own words describe what each of the following registers or bits does TIMSKO and OCIEOA Procedure 1 Start with your code from lab 3 In this lab we will be converting our code to interrupt driven timing Interrupts are a useful tool for handling things that occur at random as far as the processor is concerned times Prior to this lab you have been using polling to look at the status of various functions in the microcontroller Using interrupt will create cleaner code that gives more functionality to the design Timer elapses and state Running Sample ADCs S received on UART s received on UART Transmit Data Figure 1 New Interrupt State Machine When an interrupt is enabled it will cause an Interrupt Service Routine ISR to occur An ISR is like a function call that can happen at any time You will need to replace the function of your checkTIMER with an ISR We did an example of this in class so look at your class notes Usually it is important to keep ISRs as short as possib2. le since it is possible for many ISRs to happen in quick succession Since you are only enabling the single Timer ISR this is not as important of a risk To point you in the correct direction you can move the majority of your state machine into the timer ISR and check for serial data in the main loop This usually can work well but it can also lead to trouble if your code is doing something that is time sensitive The first step is to implement the Timer interrupt and ensure that it does what you expect After you know it is working demonstrate it to your TA 10 Page 5 Now that the timer interrupt is functional integrate your lab 3 code into the interrupt and demonstrate it to your TA Hints e Any variables that are manipulated in both the main function AND the timer interrupt should be declared a global AND as volatile Setting them to be volatile tells the compiler that they may change values at random times when an interrupt happens If you don t do this they will not work e Be careful you need to still sample only twice per second So be careful that when the interrupt happens you correctly decide if this time is the time to sample Also be careful that whatever you do with in the interrupt is fast enough that it i completed before the next interrupt occurs Demonstrate and Submit code Show your code and program to your TA They will watch your system run for 30 seconds and count the number of samples i
3. n 30 seconds it should be 60 1 They will come back and look at your graphed output of the samples you took in their presence Study Questions 1 Interms of programming and in your own words define the following two terms Event and Interrupt 2 Read or reread pages 16 18 68 69 110 116 and 198 202 of the AT9OUSB datasheet and in your own words describe what each of the following registers or bits does a TIMSKO b OCIEOA 3 Using Google look up the Interrupt Vectors for AVR GCC What are the exact names of the vectors for UART1 for data received data sent and USART ready for more data Also what is the exact name of the interrupt vector for TimerCounterO Compare Match A A good place to start is http www nongnu org avr libc user manual grou 4 What is the general syntax for defining an interrupt service routine Lab Summary Task Prelab Timer interrupt works Final code Works Study Questions avr __interrupts html Completed 3pts 3pts Apts 11 Page
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