Bare-metal Embedded Systems

Let's start off with learning how to program the uC from the beginning .

1. Adding Button support using board BSP API.

In order to use the button on the board which is labelled as 'user', i found it in the user manual. And the schematic of it was as follows.

· When the button is pressed, PA0 is connected to ground.

· If not, it is by-default connected to Vdd.

· Question: Why this cap in between?

Let's understand the concept of Key Debouncing

1. Switch Debouncing

1.1 What is Switch Bouncing

As we press the button, the pin connected to it should read 0V immediately. In real scenario, it does not! It bounces between 5V and 0V upto some time and then goes to constant 0V. This problem can not be neglected. Why this happens?

WHY?

As we push the button, like any other physical collision (Conservation of Momentum), the button bounces back a little and does so for a little amount of time. This is analogous to the an example of releasing the ball that we are holding in our hands. It goes to stationary state after bouncing a little.

This debouncing occurs for some milliseconds, which should not be a problem, isn't it? NO, our microproccessor work on MHz or Ghz, which is around 1 million operations in one millisecond.

1.2 Implementing Switch Debouncing using IC7400 (NAND Gate)

One hardware solution for Switch debouncing for a SPDT (Single Pole Double Throw) switch is using SR(Bar) latch {SR Latch is made up using NOR Gates, and SR(Bar) is made of NAND Gates}. As follows :-

Types of Switches

We are looking into the SPDT in this case

Floating Point Problem

Discussed Below

1.2.1 Floating Point Problem

When we release the button, the output floats (not 5V or 0V) .
If we directly connect the output to ground, this makes a short circuit when the button is pressed.

Solution for Floating Point Problem :

Pull-up or Pull down resistors!
When button is not pressed. Output is at 5V approx due to heavy resistance.
When the button is released, there is no short circuit between 5V and GND. Output is connected to GND and the higher the pull-up resistance magnitude, lower the current, the circuit is more low-power.

Actual Output contains bouncing. We will use a debouncing circuit to remove this.

1.2.2 Leveraging the 1,1 Condition of SR(Bar) latch

Debouncing Circuit

Current State of input is 0

Initial State

0,1

Second State

As we press the switch, it moves to a state where output becomes 1,0

Bouncing

Even if the bouncing occurs, state 1,1 remembers 1,0 state and our output remains Bouncing Free

Stable State

This remembers the 1,0 even in the bouncing state

Bouncing Free State

Yellow being the output and Red being the input

This completes the section for switch debouncing using SR(Bar) latch. Can the debounce circuit be more simpler and cost-effective?

1.3 Switch Debouncing using Diode and RC Circuit

1.4 Switch Debouncing for SPST Switches

The cap charges and discharges slowly that removes the high freq components from the circuit which in our case is noise due to Bouncing.

The choice of R1,R2 and C values is a matter of great concern over here. As we can see, the charging circuit equivalent resistance is R1+R2 while for discharging it is R2. Charging Time constant is the time taken to charge the capacitance which in our case is (R1+R2.) Similarly discharging time constant is time taken to discharge the capacitor which in our case is (R2)*C.

To avoid the problem of not being able to discharge is solved by taking R1>>R2, which ensures rapid discharge. As the frequency of switching in our case is not too high, 100 ms time constant for discharging should suffice. With R1=1k ohms, we get the cap value to be 10uF. Let's compare by putting the yellow probe to switch and red probe to the capacitor.

Hysteresis is a very interesting concept in Basic Electronics. Here, from a real life example, if we were to make an oven that maintains 200 temperature. In case 1, we'll end up switching rapidly while in case 2, switching would be lesser.

The hysteresis loop is what that solves our problem here. When the Vin to schmitt trigger is 0, the Vout is high (this is an inverting schmitt trigger), as we carry along, the vout will remain high till the time Vin becomes Vt+. Then it becomes low and remains there till the time we decrease the voltage to Vt-. The quantity Vt+ - Vt- is called Hysteresis Voltage Vh . This makes the output remains const for a longer range which removes the slope due to capacitance. Trade off being just a little delay.

Schmitt Trigger

Expected Signal

Output

Displaying noisy button on red probe and Schmitt output on yellow probe

1.5 Switch Debouncing using SOFTWARE

Well if you are in your hostel room at night and can't go to the labs to issue the required components to remove the bouncing effect. A simple cap between input and output should solve the purpose mostly. We can definitely use software also.
Also, if your product is about to be produced in mass numbers, adding hardware is gonna cost more. Here i have attached two links. First, shows debouncing and another does not. :) Try it out
1. https://github.com/manvendra88/Embedded-C-basic-codes/blob/master/toggling_Two_LEDs_witha_button.c
2. https://github.com/manvendra88/Embedded-C-basic-codes/blob/master/removing_debouncing_through_Software.c

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