Polling Method in Embedded Programming

Polling operation in embedded programming refers to actively sampling the status of an external device. Polling is most often used in terms of input/output (I/O), and it is the process where the embedded system waits for an external device to check for its readiness or status. For example, we have a simple microcontroller device with one push button and one LED. When we press the button, LED will be lit. If we press the button again, then the LED will be off. The pseudocode code would be like:

loop:
if button pressed, toggle LED
repeat

In this program you can see that the microcontroller always checks the status of the button and then decides the LED condition.

A real-life example about polling method is you are working on homework in your room, and waiting for UPS to deliver an important package. The polling method is exactly like you often (may be every 5 minutes) go to the front door and open the door to check whether your package has come or not.

Polling Cycle

Polling basically means that the embedded system will check the all the status for each device at set intervals, which is called polling cycle. For example, check the switch state every 5 ms, detect the obstacle every 20 ms, and read the room temperature every 5 sec. The optimal polling cycle will vary according to several factors, including the desired speed of response and the overhead (e.g. processor time and bandwidth) of the polling.

Implementation of Polling Method in a Single Task

For a simple embedded system, it is usually designed to execute multiple jobs in a single task, such as in the main() function. There are two types of polling methods can be used in the code, as shown below. Assume that the latency time on each job can be ignored.

Same Polling Cycle
Different Polling Cycles

Same Polling Cycle
Different Polling Cycles

Same Polling Cycle

MultiJobs SamePollingCyc
Figure 1: Multi-Jobs with Same Polling Cycle

It is easy to implement If all the jobs have the same polling cycle. The following code uses a simple delay function for polling interval.

loop:
    execute Job_A
    execute Job_B
    execute Job_C
    delay for 20 ms
repeat

Or, the following code uses SysTick to get more accurate time interval:

loop:
    get the SysTick value to time1
    execute Job_A
    execute Job_B
    execute Job_C
    loop until ( time1 - SysTick) greater then or equal to 20 ms
repeat

Different Polling Cycles

MultiJobs DiffPollingCyc
Figure 2: Multi-Jobs in Different Polling Cycle

Most of embedded systems execute multi-jobs in different polling cycles. For example, job A needs to be executed every 20 ms, job B needs 8 ms, and job C is 2 ms. Assume that the latency time on each job can be ignored and minimum delay is set to 1 ms. A simple implementation using delay function is as follows:

initialize delayms to 0
loop:
    if delayms mod 20 equal to 0, then execute Job_A
    if delayms mod 8 equal to 0, then execute Job_B
    if delayms mod 2 equal to 0, then execute Job_C
    delay for 1 ms
    increase delayms
    if delayms equal LowestCommonMultiple, then reset delayms to 0
repeat

If the system requires a more accurate time interval, the SysTic can be used in the code:

initialize delayms to 0
loop:
    get the SysTick value to time1
    if delayms mod 20 equal to 0, then execute Job_A
    if delayms mod 8 equal to 0, then execute Job_B
    if delayms mod 2 equal to 0, then execute Job_C
    loop until (time1 - SysTick) greater then or equal to 1 ms
    increase delayms
    if delayms equal to LowestCommonMultiple, then reset delayms to 0
repeat