6 Serial Interfaces 1

Question 1

Where do we need to isolate systems?

Answer 1

• When we are connecting our embedded processor to other systems we sometimes need to consider electrical or galvanic isolation
• We often need to isolate high voltages from low voltages in order to protect parts of a circuit

Question 2

Describe what is meant by an Opto-isolater?

Answer 2

• Opto-isolators allow a signal to pass across a high voltage differential with total galvanic isolation (i.e no chance for current to flow over the gap!)
• Often available as an LED and photo-transistor in a single package
• Typical drive current is 20mA @2V, and gain < 1. Can be used to pass analogue signals, generally very good frequency response (fast!)

Question 3

Diagram for an optoisolator?

Answer 3

Question 4

Details of Transformer isolation?

Answer 4

Typically requires less current than an opto-isolator, no need to drive an LED just a small coil

Cannot pass DC components of signals! What about high frequency?

Requires the use of a DC balanced signal:

Question 5

Diagram of a Power Supply isolator?

Answer 5

Question 6

Explain what an power supply isolator does?

Answer 6

• These are called DC-DC converters
• They encapsulate all the functions needed to create an isolated DC supply
• Often feedback is included to reduce power consumption

Question 7

What are real switching issues?

Answer 7

• When a physical switch opens or closes, there is usually a short period of time during which the state changes rapidly back and forth (contact bounce)
• A typical way to solve this is to feed the smoothed input through a Schmitt trigger
• The Schmitt trigger has hysteresis – a tendency to stay in it’s current state

Question 8

What is a Schmitt trigger

Answer 8

A typical way to solve this is to feed the smoothed input through a Schmitt trigger

The Schmitt trigger has hysteresis – a tendency to stay in it’s current state

Question 9

Describe a basic serial interface?

Answer 9

• Serial interfaces connect in parallel to the bus
• The I/O side of the interface feeds from/to a shift register
• Depending on the interface, the unit may perform clock recovery to work out when to read the data bits
• Additional controls and connections can indicate receipt of data and errors

Question 10

Diagram of a basic serial Interface?

Answer 10

Question 11

Types of Serial interfaces?

Answer 11

Interfaces may be simplex, half-duplex or duplex

Question 12

How are devices normally laid out in a serial interface?

Answer 12

• The interface may be between more than one device but generally has one controller (master – slave)
• Additional controls and connections can indicate receipt of data and errors

Question 13

serial interfaces that are relevent?

Answer 13

• USB
• RS-232
• SPI
• I2C

Question 14

The types of synchronicity?

Answer 14

synchronous, asynchronous or isochronous

Question 15

Synchronicity: Synchronous: Explain?

Answer 15

Synchronous – Each bit of data is transferred relative to shared (common) clock reference

Question 16

Synchronicity: Synchronous: Good/Bad?

Answer 16

• Can be tricky to maintain required data rates, often uses FIFO buffers at either end of the link

Question 17

Synchronicity: Synchronous: Diagram?

Answer 17

Question 18

Synchronicity: Asynchronous: Explain?

Answer 18

Asynchronous – Data is transferred based on transitions, associated with start and stop conditions that allow for alignment of bytes

Question 19

Synchronicity: Asynchronous: Good/Bad?

Answer 19

Allows for variations in timing, quite easy to implement but not ideal for timing critical applications

Question 20

Synchronicity: Asynchronous: Diagram?

Answer 20

Question 21

Synchronicity: Isochronous: Explain?

Answer 21

Isochronous – Data is transmitted asynchronously but at regular predefined intervals

Question 22

Synchronicity: Isochronous: Good/Bad?

Answer 22

Used for high quality audio playback over USB at > 44.1 kHz 32-bit

Question 23

Synchronicity: Isochronous: Diagram?

Answer 23

Question 24

Advantages of Parrallel interfaces?

Answer 24

For many years parallel data interfaces were the standard for connecting devices together

Parallel interfaces can transfer all N bits in a single clock cycle, and so in theory should be much faster than serial interfaces that transfer 1 bit per clock cycle

Question 25

Comparing serial and parrallel interfaces?

Answer 25

However, in recent years, serial interfaces have totally dominated the peripheral interface market

Question 26

The issues with Parallel interfaces

Answer 26

• The maximum clock frequency for parallel transmission is fundamentally limited by design constraints such as signal transit time (skew) and cross talk
• All signals from the transmitter need to arrive at the receiver at the same time. The higher the frequency, the more tiny differences in arrival time matter. Hence the receiver has to wait until all signal lines are settled - lowering the transfer rate

Question 27

The problem with cross talk?

Answer 27

The higher the frequency, the more pronounced crosstalk gets and with it the higher the probability of a corrupted byte and the need to retransmit it

Question 28

How to reduce cross talk?

Answer 28

This is why large parallel buses often have multiple inserted ground lines, in an attempt to reduce crosstalk

Question 29

Cross talk interference diagram?

Answer 29

Question 30

How are parallel cables deigned?

Answer 30