Daisy-Chained SPI Architecture

As SPI systems grow, designers often need to connect multiple peripheral devices to a single master.

The most common approach is to give each peripheral its own Chip Select signal. This works well for a small number of devices, but what happens when a system contains dozens of identical peripherals?

Adding a separate Chip Select line for every device increases PCB routing complexity, consumes valuable I/O pins, and makes hardware expansion more difficult.

SPI offers another solution known as daisy-chaining.

Instead of connecting every peripheral directly to the master, devices are connected in series, allowing data to flow through one device before reaching the next.

This architecture reduces the number of control signals while providing an efficient way to communicate with multiple devices.

Why Use a Daisy-Chain?

Imagine passing a message through a line of people.

The first person receives the message, passes it to the second person, who passes it to the third, and so on.

The message eventually reaches everyone, but it must travel through each person in sequence.

A daisy-chained SPI system works in much the same way.

Rather than the master communicating with each device independently, data moves through one device after another until it reaches the end of the chain.

This approach simplifies wiring while allowing multiple peripherals to share the same communication interface.

Understanding Daisy-Chained SPI

In a traditional SPI system, each peripheral has its own Chip Select line.

In a daisy-chain, the devices are connected together.

The master's MOSI output connects to the first device.

The output of the first device connects to the input of the second device.

The process continues until the final device returns data to the master's MISO input.

A simplified diagram is shown below.

               +-----------+

                |  Master   |

                +-----------+

                     |

                    MOSI

                     |

                     v

             +---------------+

             |   Device 1    |

             +---------------+

                     |

                     v

             +---------------+

             |   Device 2    |

             +---------------+

                     |

                     v

             +---------------+

             |   Device 3    |

             +---------------+

                     |

                    MISO

                     |

                     v

                +-----------+

                |  Master   |

                +-----------+

 

Notice that the data passes through every device before returning to the master.

Each device acts as part of a continuous communication path.

How Data Moves Through the Chain

Every device in the chain contains a shift register.

When the master generates clock pulses, each shift register shifts its data by one bit.

As one bit enters the first device, another bit leaves it and enters the second device.

The same process repeats for every device in the chain.

For example, consider three devices connected together.

1. The master begins transmitting data.

2. Device 1 receives the first bits.

3. As Device 1 shifts data out, Device 2 begins receiving it.

4. Device 2 passes data to Device 3.

5. Device 3 eventually returns data to the master.

Although all devices shift data simultaneously, information must travel through the entire chain before reaching its final destination.

Advantages of Daisy-Chained SPI

Daisy-chaining offers several practical benefits.

Reduced Wiring

One of the biggest advantages is fewer control signals.

Instead of assigning a separate Chip Select line to every peripheral, the entire chain typically shares a single Chip Select.

This simplifies PCB routing and reduces the number of I/O pins required on the master.

Improved Scalability

Adding another device is often straightforward.

The new peripheral is inserted into the existing chain without requiring additional Chip Select connections.

This makes system expansion easier.

Cleaner PCB Layout

Industrial systems may contain long rows of identical devices.

Using a daisy-chain reduces wiring congestion and can simplify board design.

Disadvantages of Daisy-Chained SPI

Although daisy-chaining simplifies hardware connections, it introduces several trade-offs.

Increased Latency

Data must pass through every preceding device before reaching the intended peripheral.

As more devices are added, communication takes longer.

This additional delay is known as latency.

Longer Transactions

The master must shift enough bits to move data through the entire chain.

For example, updating the last device requires transmitting data through every earlier device first.

Longer chains therefore require longer SPI transactions.

Single Point of Failure

If one device in the chain fails or becomes disconnected, communication with downstream devices may also fail.

Because every device depends on the previous one, a fault can affect the entire chain.

Understanding Latency

Latency is one of the most important considerations when designing a daisy-chained SPI system.

Imagine sending a package through several distribution centers.

Each center introduces a small delay before forwarding the package.

Similarly, each SPI device introduces an additional stage that data must pass through.

For a chain containing many devices, the total communication time increases.

This is usually acceptable for configuration data or slow control operations.

However, it may not be suitable for applications requiring immediate responses or very high-speed updates.

Engineers must balance wiring simplicity against communication delay when selecting an SPI architecture.

A Practical Example

Consider an industrial LED display made up of multiple display driver ICs.

Each driver controls a portion of the display.

Instead of connecting every driver directly to the controller, the devices are connected in a daisy-chain.

When new display data is transmitted:

1. The controller shifts data into the first driver.

2. Existing data moves to the next driver.

3. The process continues until every driver has received its new information.

4. Once all bits have been shifted into the chain, the display updates.

This approach allows large displays to be controlled using relatively few microcontroller pins.

Common Industrial Applications

Daisy-chained SPI is widely used in applications where many similar devices must be controlled together.

Common examples include:

• LED display driver ICs

• Industrial control modules

• Shift register expansion circuits

• Motor control systems

• Data acquisition equipment

• Factory automation systems

In these applications, reducing wiring complexity is often more important than minimizing communication latency.

Hardware Perspective

From a hardware standpoint, daisy-chained SPI relies heavily on shift registers.

Each device receives serial data, temporarily stores it, and shifts it to the next device as clock pulses arrive.

Because all devices shift data simultaneously, the communication process remains synchronized across the entire chain.

The master simply continues generating clock pulses until every required bit has moved through the system.

This makes daisy-chaining relatively easy to implement while supporting large numbers of peripherals.

Debugging Perspective

Several issues commonly appear in daisy-chained SPI systems.

Incorrect Device Order

If devices are connected in the wrong sequence, data reaches unintended peripherals.

Incomplete Clock Cycles

Stopping communication too early prevents data from reaching the final devices in the chain.

Faulty Intermediate Device

A failed device may interrupt communication with every downstream peripheral.

Data Misalignment

Incorrect data lengths can cause information intended for one device to shift into another.

When debugging daisy-chained systems, engineers often verify that the expected number of clock cycles has been generated and that every device is correctly connected in the chain.

Interview Questions

Basic

1. What is a daisy-chained SPI architecture?

2. How does data move through a daisy-chain?

3. Why are shift registers important in daisy-chained SPI?

Intermediate

1. What are the main advantages of a daisy-chained SPI system?

2. Why does latency increase as more devices are added?

3. How does daisy-chaining reduce the number of Chip Select signals?

Advanced

1. Why is a daisy-chained SPI architecture well suited for large LED displays?

2. What happens if an intermediate device fails in the chain?

3. What design trade-offs should an engineer consider before choosing a daisy-chained SPI architecture?

Key Takeaways

• Daisy-chained SPI connects multiple peripherals in a serial sequence rather than individually.

• Data passes through one device before reaching the next.

• Shift registers enable data movement throughout the chain.

• Daisy-chaining reduces wiring complexity and conserves I/O pins.

• The primary trade-off is increased latency as additional devices are added.

• A failure in one device can affect communication with downstream peripherals.

• Daisy-chained SPI is widely used in industrial systems, LED displays, motor controllers, and data acquisition equipment where simplified hardware connections are a priority.