SPI Pull-up Resistors: Do You Need Them?

Summary

In this video, Zach Peterson explores the necessity of using pull-up resistors in SPI interfaces. He discusses common misconceptions and varying recommendations, particularly for chip select (CS) lines. While pull-up resistors generally don’t impact the functionality or switching characteristics of SPI signals, they may help ensure that the CS line remains in a known state to prevent unintended activation or data corruption. The video also covers the impact of power sequencing and the use of internal versus external pull-ups in different configurations. Ultimately, pull-ups may be useful for noise immunity and ensuring stable operation, but they’re not always essential.

Takeaways

• 😀 Pull-up resistors are commonly recommended for SPI interfaces, but it's important to understand when they are actually necessary.
• 😀 The guidance around using pull-up resistors on SPI pins varies widely, with recommendations differing between chip select (CS), MOSI, MISO, and clock pins.
• 😀 Pull-up resistors on SPI lines are often suggested to avoid memory corruption in certain scenarios, especially when the chip select (CS) line is left floating.
• 😀 Pull-up resistors may not impact the functionality of an SPI interface in most cases, as they don’t affect the switching of CMOS buffers in push-pull configuration.
• 😀 Pull-up resistors can be beneficial in preventing data corruption when dealing with memory chips, particularly during startup when lines might be in an indeterminate state.
• 😀 Some chips, especially those with configurable GPIOs, include internal pull-up resistors, with values ranging from 100K ohms or higher.
• 😀 For SPI buses using configurable or open-drain outputs, an external pull-up resistor may be required to ensure proper voltage levels and current limiting.
• 😀 Power sequencing can sometimes eliminate the need for pull-up resistors, especially when ensuring that power rails are brought up in the correct order to prevent issues.
• 😀 Using pull-up resistors to ensure proper logic levels on the chip select (CS) line can prevent erroneous behavior, but power sequencing is an alternative solution.
• 😀 Overall, pull-up resistors may not always be necessary if the SPI bus is correctly designed with proper power sequencing and signal integrity management.

Q & A

What is the main topic discussed in this video?

The main topic of the video is whether pull-up resistors are necessary on SPI interface lines and under what circumstances they should be used.

Why is there confusion about using pull-up resistors on SPI buses?

There is confusion because the recommendations vary widely depending on the source, with some suggesting pull-ups on all lines and others on just specific ones like the chip select or MISO pin. The guidance is often inconsistent, especially when considering different types of SPI peripherals and configurations.

What real-world examples are shown in the video to illustrate the use of pull-up resistors on SPI lines?

The video shows examples from Vincent Nguyen's motor control design, where a pull-up is applied to the MISO line, and from Sohab Zohidi's FPGA development board, where pull-ups are applied only to the chip select line in one instance, and all lines in another.

What happens inside an SPI bus when pull-up resistors are added?

Inside an SPI bus, adding a pull-up resistor does not significantly affect the switching behavior of the signal lines because the small on-state resistance of the PMOS or NMOS transistors dominates. The pull-up resistor doesn’t impact the signal integrity unless the lines are in an open-drain configuration.

What is the effect of a pull-up resistor on the CMOS inverter in an SPI bus?

A pull-up resistor doesn’t impact the switching behavior of the CMOS inverter. This is because the small resistance of the PMOS or NMOS transistor is much lower than the resistance of the pull-up resistor, making the pull-up negligible in the switching process.

What is the purpose of a pull-up resistor on the chip select (CS) line in SPI communication?

A pull-up resistor on the CS line ensures that the chip select is held in a specific logic state (high), preventing unintended activation of the SPI bus due to floating states, which could cause data corruption in certain peripherals, such as memory chips.

Do you need to use pull-up resistors on all SPI lines, including clock and data lines?

No, pull-up resistors are typically not required on the clock or data lines unless the specific SPI bus setup involves open-drain configuration or special chip requirements. In most cases, pull-ups are only necessary on the chip select line to prevent floating states.

When is it advisable to use pull-up resistors on SPI lines?

It is advisable to use pull-up resistors on SPI lines in cases where you want to ensure that the lines are not left floating, such as on the chip select line for preventing data corruption in memory chips. In general, the need for pull-ups depends on the configuration and the specific chip requirements.

What role does power sequencing play in the need for pull-up resistors on SPI lines?

Power sequencing can eliminate the need for pull-up resistors on SPI lines, particularly the chip select line. By ensuring that power rails are brought up in the correct order, the chip can be turned off before the SPI bus is initialized, preventing unintended activation or data corruption.

What is the significance of internal versus external pull-up resistors in SPI bus designs?

Internal pull-up resistors, often included in the chip, are typically much higher in value (e.g., 100 KΩ) compared to external pull-ups (e.g., 1 KΩ), and are primarily used in open-drain configurations or for configurable pins. External pull-up resistors are typically more effective in controlling the signal when required outside of specific chip configurations.