Microcontroller Principles

Many microcontrollers come equipped with a built-in PWM function, so that you do not have to code a custom PWM algorithm yourself. This fact points to the popularity of pulse-width modulation as a control scheme. Explain why PWM is so popular, and give a few practical examples of how it may be used.

Pulse-width modulation (PWM) is not only useful for generating an analog output with a microcontroller, but it is also useful for receiving an analog input through a pin that only handles on-off (high-low) digital voltage levels. The following circuit takes an analog voltage signal in to a comparator, generates PWM, then sends that PWM signal to the input of a microcontroller:

Pseudocode listing Declare Pin0 as an input

Declare Last_Pin0 as a boolean variable

Declare Time_High as an integer variable

Declare Time_Low as an integer variable

Declare Duty_Cycle as a floating-point variable

Set Time_High and Time_Low both to zero

LOOP

Set Last_Pin0 equal to Pin0

If Pin0 is HIGH, increment Time_High by one

If Pin0 is LOW, increment Time_Low by one

If Last_Pin0 is not equal to Pin0, go to SUBROUTINE

ENDLOOP

SUBROUTINE

Set Duty_Cycle equal to (Time_High / (Time_High Time_Low))

Set Time_High and Time_Low both to zero

Return to calling loop

ENDSUBROUTINE

Explain how this program works. Hint: the Last_Pin0 boolean variable is used to detect when the state of Pin0 has changed from 0 to 1 or from 1 to 0.

Digital computers communicate with external devices through ports: sets of terminals usually arranged in groups of 4, 8, 16, or more (4 bits = 1 nybble, 8 bits = 1 byte, 16 bits = 2 bytes). These terminals may be set to high or low logic states by writing a program for the computer that sends a numerical value to the port. For example, here is an illustration of a microcontroller being instructed to send the hexadecimal number F3 to port A and 2C to port B:

Suppose we wished to use the upper four bits of port A (pins 7, 6, 5, and 4) to drive the coils of a stepper motor in this eight-step sequence:

Step 1:

0001

Step 2:

0011

Step 3:

0010

Step 4:

0110

Step 5:

0100

Step 6:

1100

Step 7:

1000

Step 8:

1001

As each pin goes high, it drives a power MOSFET on, which sends current through that respective coil of the stepper motor. By following a “shift” sequence as shown, the motor will rotate a small amount for each cycle.

Write the necessary sequence of numbers to be sent to port A to generate this specific order of bit shifts, in hexadecimal. Leave the lower four bit of port A all in the low logic state.