Digital Signals

LED output

Clone the repository s1.1 this contains a basic blinky program.

Build the program and upload it into the FRDM-K64F. Once done you will need to press the reset button.

resources

1. Refer to the LED diagram in the FRDM K64F Userguide section 10, page 14 for pin connections.
2. Summary of important pin names
3. An overview of the board and the mbed library

The start point is (s1.1):

#include <mbed.h>

int main() {

    DigitalOut red(PTB22);

    while(1) {
      red.write(0);
      wait(0.5);
      red.write(1);
      wait(0.5);
    }
}

1. Using the FRDM K64F Userguide and/or the FRDM K64F Schematic, identify the pins that drive the Green and Blue LEDs on the board. Green LED pin PTE26 (aka LED_GREEN), Blue LED pin PTB21 (aka LED_BLUE)
2. Modify the program to manage the Green and Blue LEDs in addition to the Red LED. solution 1.1
3. Modify the flashing pattern to make use of all three LEDs. solution 1.1

Push Button input

Clone the repository s1.2.

This is the base level LED driver from s1.1 with the addition of a single push button (One of the two on the base board.)

#include <mbed.h>

#include <stdbool.h>

int main() {
    bool flashing = false;

    DigitalOut red(PTB22);
    DigitalIn  btn(PTC6);

    while(1) {
        if( btn.read() ) flashing = true;
            else         flashing = false;

        if(flashing){
            red.write(0);
            wait(0.5);
            red.write(1);
            wait(0.5);
        }else{
            red.write(1);
        }
    }
}

1. Locate the buttons (switches SW2 and SW3) on the FRDM K64F Schematic.
2. Using the schematic and observing the behaviour of the program what can be deduced about:
   • What value is read when the button is
     • pressed?  0 
     • not pressed?  1 
   • Why is this the case?

   The Pins are connected to 3.3V via a pull-up resistor, so the pin reads a logic 1 when the button in not pressed. Pressing the button creates a direct connection to ground (0V) so the pin reads a logic 0

3. Add a wrapper function that evaluates as bool ispressed( /*button*/ ) that returns the correct state of the button.
   solution 1.2

Application Shield IO

The Application Shield has a second RGB-LED and a 5-way joystick like switch.

1. Refering to the Application Shield Schematic,
   • Identify the pins that drive the shield RGB-LED
     • Red is on D5 which is PTA2
     • Green is on D9 which is PTC4
     • Blue is on D8 which is PTA0
2. Extend the capabilities of the LED wrapper functions to include the Shield’s LEDs. solution 1.3

1. Find the 5-way switch on the schematic
   • Is it wired the same way with respect to the ground and 3.3V signals?
     No. The switches are wired the opposite way round.
   • What will the impact be on the implementation of the ispressed() function?
     Not nice, I need to invert some pin readings and not others. Solution look at the truth table for (a xor true)
2. extend the capability of the ispressed() function to include the application shield.
   solution 1.4

exercises

• Flash an LED at given rate
  1. 1Hz
  2. 10Hz
  3. 45 flashes per minute

• Traffic light sequence: Using Blue instead of Amber:
  • create a sequence of led on and offs that replicate a traffic light sequence. What timings do you use for each phase?
  • using the RGB LED on the base and the shield create a junction with two traffic lights:
    • what are the allowed combinations of lights?
    • how would you ensure correct operation?

• Maritime navigation lights have characteristic sequences of flashes, (see https://en.wikipedia.org/wiki/Light_characteristic), How would you create the following patterns
  • a flashing light has rate of 37.5 flashes per minute, the light is off for three times longer than it is on
  • A light flashes (as above) showing 3 flashes, every 6 seconds.

• safety switch (two hand start) is a common feature of machinery. To start flashing both switches (buttons) must be pressed together.

• flash at variable rates: a countdown timer on a camera flashes initially at 1 flash per second, it flashes faster over 10 seconds, showing a continuous light just before the camera and flash fires.

extension

explore plain C bare-metal code.

1. gate clock signals
2. configure pin
3. set gpio state
4. set,clear,toggle