Digital logic

Question 1

Define ‘Discrete logic’

Answer 1

> Physical logical chips are assembled on a single PCB to perform a function

> Often requires a physically large circuit board with lots of ICs.

Question 2

What are the 2 main benefits of using discrete logic?

Answer 2

> Discrete logic circuits are useful when simple logic circuits are needed.

> Discrete logic circuits are often cheaper.

Question 3

What are the different practical logic levels?

Answer 3

> V_IL: Max input voltage considered to be a LOW

> V_OL: Max output voltage asserted to be a LOW

> V_IH: Min input voltage considered to be a HIGH

> V_OH: Min output voltage asserted to be a HIGH

Question 4

What is the noise immunity?

Answer 4

> This is the difference between output thresholds

> V_IL - V_OL = V_Threshold

Question 5

What are the features of TTL?

Answer 5

> Power supply: 5V

> VIL = 0.8V

> VIH = 2V

> High power consumption

Question 6

What are the features of CMOS?

Answer 6

> Power supply: 1.8V ⇒ 5V

> V_IL = 1/3 × V_supply

> V_IH = 2/3 × V_supply

> Power consumption ∝ clock speed

> Susceptible to electro-static discharge ESD damage

Question 7

Define “propagation delay”

Answer 7

When inputs to a digital circuit are applied it takes a finite time for the output to respond.

Question 8

How can you physically combat propagation delay?

Answer 8

Differing lengths in the PCD traces cause the bits to arrive at different times.

Question 9

What is the problem with propagation delay?

Answer 9

It can limit the maximum speed that a digital circuit can operate at.

Question 10

Why do higher clock frequencies require more cooling?

Answer 10

Higher frequencies ⇒ more current flowing faster ⇒ because of discharging/charging of capacitances in the circuit

Question 11

What are the two types of IO current? What do they do?

Answer 11

> Sink Current: When current flows into an input/output

> Source Current: When current flow out of an input/output

Question 12

What are the current limits for IO?

Answer 12

> I_IL: The current sourced from an input when it is held LOW

> I_IH: The current sunk input an input when it is held HIGH

> I_OL: The max current that can be sunk into an output

> I_OH: The max current that can be sourced from an output held LOW

Question 13

What happens if the current limits for IO are exceeded?

Answer 13

Either damage will occur and/or the device may not maintain the correct levels of VOL or VOH.

Question 14

Define “Fan out limit”

Answer 14

The max number of inputs that a single output can be connected to whilst maintaining the correct levels of VOL or VOH.

Question 15

What are the different types of IO?

Answer 15

> Input only

> Output only

• Classic push-pull (totem pole)
• Open-collector / Open-drain

> Bi-directional

• Tri-state

Question 16

What is the circuit for the totem pole IO? What are the components?

Answer 16

[Picture42] [Picture43]

> Complementary pair of transistors (p-type/n-type MOSFET)

> Two TTL NPN transistors with a current limiting resistor

Question 17

How does the totem pole operate?

Answer 17

> At any time, one of the transistors is conducting.

> The output is either connected to high or low

Question 18

What are the advantages and disadvantages to the totem pole?

Answer 18

Advantages:

> Fast

> Can sink and source current

Disadvantage:

> Two or more outputs cannot be connected together to form a shared data bus.

Question 19

What is the circuit for the open-collector / open-drain?

Answer 19

[Picture 44]

Question 20

How does the open-collector / open-drain circuit operate?

Answer 20

> A pull up resistor forces the output HIGH

> A transistor forces the output LOW by connecting it to ground

> This allows multiple devices on the circuit to control the bus

Question 21

What are the advantages and disadvantages of the open-collector / open-drain circuit?

Answer 21

Advantage:

> Allows a bus connection because any digital device connected to the line can force it LOW.

Disadvantage:

> Slower. The reason for this is that due to the external pull up resistor, in order to change the state from LOW to HIGH, the line capacitance must be charged through the resistor creating an RC time constant.

> The circuit can only sink current.

Question 22

How should unused inputs for logic gates be connected? Why?

Answer 22

> To ground

> Failing to do this will result in erratic behaviour.

Question 23

What is the impedance of a microcontroller input?

Answer 23

Very high input impedance (>100kΩ)

Question 24

What is the rule about the value of pull up resistors?

Answer 24

Should not normally be more than 10% of the input impedance or voltage division will cause threshold errors

Question 25

What should happen to unused inputs on a microcontroller?

Answer 25

Not be connected to ground or supply

Question 26

What is the circuit for the outputs of a microcontroller?

Answer 26

[Picture45]

Question 27

What are 4 methods of shifting a 5V logic output to a 3.3V logic input?

Answer 27

> Potential divider

> Connecting the pull up resistor to a lower supply voltage assuming that the output is of the open collector variety.

> Direct connection

> Bi-direction level shifter IC

Question 28

What is the issue with using a potential divider to shift a 5V logic output to a 3.3V logic input?

Answer 28

The speed is reduced

Question 29

What are the considerations for direct connection when shifting a 5V logic output to a 3.3V logic input?

Answer 29

> Assuming that the low-voltage circuit can handle higher voltage connections

> As long 5V V_OL < 3.3V V_IH

> Noise margins may be reduced.

Question 30

What are the two methods of shifting a 3.3V logic output to a 5V logic input?

Answer 30

> Direct connection

• Providing that 3.3V V_OH > 5V V_IH

> Bi-direction level shifting IC

Question 31

How can a microcontroller connects to small loads?

Answer 31

> Direct connection because a microcontroller can source/sink a small amount of current.

> Connect one side of the load to ground and supply the current from an output activated HIGH

> Connect one side of the load to supply voltage and sink the current with an output activated LOW

Question 32

What are the problems with a microcontroller using direct connection to control small loads?

Answer 32

> It is dependent on the values of I_OH and I_OL

> Often I_OL > I_OH so its better to sink current.

> There is also often a limit for the sink/source current for the entire microcontroller

Question 33

How can a microcontroller connects to medium loads?

Answer 33

> A transistor is used as a switch operated in its saturated mode.

> Bi-polar transistor requires a current limiting resistor.

> MOSFET requires a pull-down resistor on the gate terminal.

Question 34

Draw the circuit for using a transistor controlled by a microcontroller to control a load?

Answer 34

[Picture46]

Question 35

What is an open-collector logic buffer?

Answer 35

> Allows a microcontroller to control medium loads

> Different than an op-amp buffer because it is not a linear amplifier

> Single input

> Single output

> Come as inverting or non-inverting types

> IC package

Question 36

How can a microcontroller connects to high loads?

Answer 36

> Using a relay

> Provides electrical isolation.

> Allows the use of much higher voltages and/or currents

Question 37

How does a relay work?

Answer 37

Uses an electro-magnet to physically open or close switch contacts (called an armature) which is against a return spring.

Question 38

Draw the circuit for a relay

Answer 38

[Picture47]

Question 39

Describe cold switching

Answer 39

A physical switch is used to control a load, but the load current does not flow through the switch. Instead a relay is controlled by the switch which in turn controls the load current.