6. Hardware

Question 1

What is the power supply for modern mCs?

Answer 1

They don’t care too much about the precise voltage, nor do they draw much current, so batteries are ideal. Most current designs work at 3V, which you can easily get from 2 cells in series (2xAA for example).

Question 2

What are Vss and Vdd?

Answer 2

Vss - ground - negative

Vdd - supply - positive

Question 3

Why is it good practise to connect a decoupling capacitor and where do we connect it? What is the usual value for the capacitor?

Answer 3

Because all digital circuits produce electrical noise as they switch. The capacitor ‘shorts this out’ and prevents it affecting other components. Connect between pins for Vss and Vdd as close to the mC as possible. Usually about 10^(-7) F.

Question 4

What are LEDs?

Answer 4

Light-emitting diodes are the simplest type of output that can be driven by a mC. They are diodes (p-n junctions in a semiconductor) that emit light.

Question 5

What is an important feature of LEDs?

Answer 5

They are diodes, which means they pass current in only one direction. Almost no current flows if they are connected the other way around.

Question 6

Can you draw an LED showing it connected the correct way and label the anode and cathode? Can you then draw it the wrong way around showing what happens?

Answer 6

DRAW DIAGRAM.

Question 7

What colours of LED can you get?

Answer 7

For many years they were red, yellow or green but now blue and white LEDs are available.

Question 8

Can you draw an LED and label the parts?

Answer 8

DRAW DIAGRAM

Question 9

How do we connect the LED if we want it to illuminate when the logical value of the output is high (1) - active high.

Answer 9

The LED should light when the pin is at Vdd (supply, positive, logic 1) but not when the pin is at Vss (ground, negative, logic 0). We therefore connect the LED between the pin and Vss. Conventional current flows from the pin to Vss. DRAW DIAGRAM.

Question 10

How do we connect the LED to illuminate when the logical value of the output is low (0) - active low?

Answer 10

The LED should light when the pin is at Vss (ground, negative, logic 0) but not when the pin is at Vdd (supply, positive, logic 1). We therefore connect the LED between the pin and Vdd. Concntional current flows from Vdd to the pin. DRAW DIAGRAM.

Question 11

Can you draw operation of active high led in detail? (showing the pins inside the mC?)

Answer 11

DRAW DIAGRAM

Question 12

Why is a resistor usually needed in series with LEDs?

Answer 12

To limit the current through the LED.

Question 13

What current can each pin provide/ receive?

Answer 13

Provide (source) - 3mA or 15mA or receive (sink) - 6mA or 9mA, depending on the pin.

Question 14

What is the total drive current?

Answer 14

All pins combined limited to 130mA. This is plenty for a small LED, but clearly won’t drive the motor or heater in a washing machine.

Question 15

What does each I/O line embed?

Answer 15

An ODT (On-Die Termination).

Question 16

What does ODT consist of?

Answer 16

An internal series resistor termination scheme for impedance matching between the driver output and the PCB track impedance preventing signal reflection. DRAW DIAGRAM.

Question 17

Why is an on/off switch or pushbutton input not connected as Vdd for logic 1 and Vss for logic 0? How would this look?

Answer 17

1. It needs an expensive switch (3 terminals instead of 2)
2. It needs 2 or 3 wires
3. What happens while the switch is being operated?

Question 18

Why must digital circuits never be left with their inputs floating?

Answer 18

This leads to unpredictable behaviour and possibly severe problems.

Question 19

What is the correct way to connect a switch?

Answer 19

DRAW DIAGRAM

Question 20

What are the advantages of the correct way to connect a switch circuit?

Answer 20

1. Needs only a simple switch or pushbutton
2. Only one wire to the switch is usually needed, because a ground connection can usually be found nearby.
3. The input is always connected to something.

Question 21

How does the correct switch connection circuit work?

Answer 21

DRAW DIAGRAMS

Question 22

What is switch bounce?

Answer 22

The contacts of a switch are mechanical. When the switch is operated they do not open or close cleanly but vibrate for a while. This causes the contacts to open and close several times, each time the switch is operated.

Question 23

Can you draw the graphs for ideal input signal and input signal with bounce?

Answer 23

DRAW DIAGRAM