Serial Peripheral Interface (SPI) is widely used in embedded systems to communicate with external Flash memory, displays, ADCs, sensors, and other peripherals. Its simple architecture, full-duplex communication, and support for high data rates make it a popular choice in microcontroller-based designs.
Configuring an SPI peripheral is usually straightforward. Select the clock frequency, configure the clock polarity and phase, assign the GPIO pins, and start transferring data. Yet, when an SPI interface works at a low clock frequency but fails at a higher one, the problem may not be in the firmware at all.
The real problem may be on the wires.
As SPI clock frequencies increase, signal rise time, propagation delay, output drive strength, PCB capacitance, and voltage-level compatibility become increasingly important. A buffer that works perfectly in a low-speed control circuit may introduce unacceptable delays when used on an SPI clock or data line.
One common source of difficulty is the use of open-drain buffers with external pull-up resistors.
Unlike push-pull outputs, which actively drive both logic HIGH and LOW levels, open-drain outputs actively pull the signal LOW and rely on an external pull-up resistor to bring it HIGH. This difference changes the way the signal transitions between logic levels and can significantly affect SPI timing.
In this article, we will examine how open-drain buffers work, why pull-up resistors influence signal rise time, how to calculate the RC charging behavior, how to select a suitable resistor, and how to identify signal-integrity problems using an oscilloscope.
Understanding Open-Drain Buffers:
An open-drain output uses a transistor to pull the output toward ground. When the transistor is switched off, the output enters a high-impedance state, allowing an external pull-up resistor to establish the HIGH level.
The Texas Instruments SN74LVC07A is a good example of an open-drain buffer. It can help strengthen signals and connect devices that operate at different logic voltages. However, its output needs an external pull-up resistor to produce a logic-high signal.
The resistor and the circuit’s total capacitance affect how quickly the signal rises, which can become important at higher speeds. Before using the buffer, make sure its voltage and current limits are suitable for the connected devices.
How does an open-drain output work?
An open-drain output operates in two states.
- LOW (logic 0): The internal output transistor turns ON and pulls the signal toward ground.
- HIGH (released): The internal output transistor turns OFF, placing the output in a high-impedance state. An external pull-up resistor brings the signal toward the pull-up voltage.
The important point is that the output transistor actively pulls the signal LOW but does not actively drive it HIGH.
When the output is released, the pull-up resistor supplies the current needed to charge the capacitance connected to the signal line. The resulting voltage rises gradually rather than changing instantaneously.
This behavior is perfectly acceptable in many low-speed applications. However, in a high-speed SPI interface, the time required for the signal to reach a valid HIGH level can become a significant timing constraint.
Open-drain vs push-pull outputs:
The difference between these output structures is fundamental to understanding their behavior in SPI circuits.
| Characteristic | Open-Drain Output | Push-Pull Output |
|---|---|---|
| Logic LOW | Actively pulls the output LOW. | Actively drives the output LOW. |
| Logic HIGH | Relies on an external pull-up resistor. | Actively drives the output HIGH. |
| Rising-Edge Behavior | Depends strongly on pull-up resistance and total line capacitance. | Depends on driver strength and load capacitance. |
| LOW-State Current | Current flows through the pull-up resistor when the output is LOW. | Current depends on the load and output-driver characteristics. |
| Shared-Line Operation | Allows multiple devices to share a line when the circuit is designed appropriately. | Requires careful coordination to prevent output contention. |
| Typical SPI Application | Used in specialized designs with suitable timing and pull-up selection. | Generally preferred for conventional SPI communication. |
For most conventional SPI interfaces, push-pull buffers or suitable push-pull voltage-level translators are a better starting point because they actively drive both logic levels.
Open-drain outputs are not inherently unsuitable for every SPI application. Their suitability depends on the required data rate, electrical loading, input thresholds, and available timing margin.
SPI Signal-Specific Design Considerations:
An SPI interface typically uses four signals: SCK, MOSI, MISO, and CS. Each signal has a defined direction and specific electrical and timing requirements that must be considered when selecting a buffer or voltage-level translator.
The following table summarizes the direction of each signal and the key design considerations for reliable SPI communication.
| SPI Signal | Direction | Key Design Considerations |
|---|---|---|
| SCK (Serial Clock) | Controller → Peripheral | Clock transitions must meet the peripheral’s timing requirements. Slow rising edges can delay logic-threshold crossings and reduce timing margins. |
| MOSI (Master Out, Slave In) | Controller → Peripheral | Data must reach a valid logic level within the peripheral’s setup and hold time requirements. |
| MISO (Master In, Slave Out) | Peripheral → Controller | The return path must satisfy the controller’s input timing requirements and provide sufficient time for reliable data sampling. |
| CS (Chip Select) | Controller → Peripheral | The peripheral must recognize valid active and inactive logic levels, including during startup and reset. Chip-select setup and hold times must also be satisfied. |
In a conventional four-wire SPI interface, SCK, MOSI, and CS travel from the controller to the peripheral, while MISO travels in the opposite direction. Therefore, a complete interface requires three forward signal paths and one return signal path.
A fixed-direction buffer can be used for an individual signal when its direction matches the required signal path. However, a single fixed-direction channel cannot independently support both directions. When selecting a multichannel buffer or translator, verify the available channel directions, input and output voltage compatibility, propagation delay, and signal transition characteristics.
Why SCK Deserves Special Attention:
The SPI clock determines when data is sampled. Consequently, clock-signal integrity is particularly important when selecting a buffer or voltage-level translator.
An open-drain output actively pulls the signal LOW but relies on an external pull-up resistor to produce a HIGH level. When the output transistor turns ON, the falling transition can be relatively fast. When the transistor turns OFF, the pull-up resistor charges the capacitance of the signal line, causing the rising transition to occur more gradually.
This difference can produce asymmetric rising and falling edges and distort the effective clock duty cycle. Depending on the SPI mode, receiver input thresholds, and signal timing, the resulting waveform can reduce the time available for reliable data sampling.
The approximate 10%–90% rise time for a simple RC network is
tr ≈ 2.2 × Rpull-up × Cbus
where:
- tr is the signal rise time.
- Rpull-up is the pull-up resistance.
- Cbus is the total signal-line capacitance, including device inputs, PCB traces, and other connected loads.
A lower pull-up resistance generally produces faster rising edges but increases the current flowing through the output transistor when the signal is LOW. The resistor value must therefore balance rise-time requirements against output-current limits and power consumption.
The acceptable rise time depends on the SPI clock frequency, receiver thresholds, clock polarity and phase, propagation delays, and the available timing margin. There is no single clock-frequency threshold at which open-drain signaling becomes unsuitable for every design.
Considerations for MOSI, MISO, and CS
Although SCK often receives the most attention, the other SPI signals also require careful timing analysis.
- MOSI: The data signal must reach a valid logic level before the peripheral’s sampling edge and remain valid for the required hold time.
- MISO: The return signal must propagate through the selected buffer or translator and reach a valid logic level at the controller before its sampling point.
- CS: The chip-select signal must establish the correct active or inactive state and satisfy the peripheral’s timing requirements around the SPI transaction. Its behavior during power-up and reset must also be considered.
Slow transitions or excessive propagation delays on any of these signals can compromise communication reliability.
Selecting a Buffer or Voltage-Level Translator:
For conventional SPI interfaces, push-pull buffers or suitable push-pull voltage-level translators are generally the preferred starting point. They actively drive both LOW and HIGH states, typically providing faster and more predictable transitions than open-drain outputs.
Open-drain devices can still be suitable for specific applications when their electrical characteristics and timing performance meet the system requirements. Before selecting a device, verify:
- Compatible input and output voltage ranges.
- Correct signal direction and channel count.
- Propagation delay and output transition times.
- Pull-up resistance and total bus capacitance, where applicable.
- Setup, hold, and sampling-time requirements at the intended SPI clock frequency.
The key design principle is to evaluate the complete signal path rather than the buffer alone. Reliable SPI communication depends on the combined effects of the controller, buffer or translator, interconnect, and peripheral timing requirements.
How Pull-Up Resistors Affect Signal Rise Time
A pull-up resistor establishes a HIGH level when an open-drain output is released. It also determines how quickly the signal voltage rises toward the pull-up supply.
When the output transistor turns OFF, the pull-up resistor charges the total capacitance of the signal line. This capacitance includes buffer output capacitance, receiver input capacitance, PCB traces, connectors, cables, and measurement probes.
For a first-order RC approximation, the signal rise time can be estimated using:
tr ≈ 2.2 × Rpull-up × Cbus
where, RPU is the pull-up resistance and CL is the total effective signal capacitance. This estimates the 10%–90% rise time.
A higher pull-up resistance or greater signal capacitance results in a slower rising edge. Although a lower resistance generally improves rise time, it increases the current flowing through the output transistor when the signal is LOW.
In SPI applications, the rise time must be sufficiently short for the receiving device to recognize a valid HIGH level within the required timing window. PCB layout, connected devices, and even oscilloscope probe capacitance can affect this performance.
Why Slow Rise Time Can Break High-Speed SPI:
Consider an SPI interface operating at 10 MHz. The clock period is:
TSCK =1 / fSCK = 110MHz = 100ns
The nominal half-cycle is 50 ns.
Assume the open-drain clock uses a 4.7 kΩ pull-up resistor and has a total signal capacitance of 30 pF. The approximate 10%–90% rise time is:
tr≈2.2RPUCL = 110MHz = 100ns
tr ≈ 2.2×4700Ω×30pF≈310ns = 110MHz = 100ns
The estimated rise time is significantly longer than the nominal half-cycle, indicating that the pull-up network may be unsuitable for the intended SPI timing requirements.
If the clock does not cross the receiver’s input-HIGH threshold in time, the receiving device may recognize the clock edge too late, reducing timing margins and potentially causing communication errors. The actual outcome depends on the input thresholds, SPI mode, clock waveform, and sampling requirements.
Selecting the Pull-Up Resistance:
Selecting a suitable pull-up resistor involves balancing two requirements: achieving an acceptable signal rise time and keeping the output’s LOW-state sink current within its electrical limits.
A lower resistance produces faster rising edges but increases LOW-state current. A higher resistance reduces current but slows the rising edge.
Calculating LOW-State Current
When the open-drain output pulls the signal LOW, the approximate current through the pull-up resistor is:
ILOW=VPU−VOL /RPU
where VPU is the pull-up voltage, VOL is the output LOW voltage, and RPU is the pull-up resistance.
For example, with a 3.3 V pull-up, a LOW-state voltage of 0.2 V, and a 4.7 kΩ resistor:
ILOW = 3.3−0.2 / 4700 ≈ 0.66mA
This is the approximate current supplied through the pull-up resistor. Verify that the buffer can sink this current while maintaining an acceptable LOW-state voltage.
Determining the Minimum Resistance from Current:
For a first-order RC network, the time required for the signal to reach the receiver’s minimum HIGH-input threshold, V IH , is:
tVIH=−RPUCLln(1− VIH/VPU)
If the signal must reach this threshold within an available time tavailable , the theoretical maximum pull-up resistance is:
RPU(max)=tavailable/CLln(1/ (1−VIH/VPU))
Determine the available time from the actual SPI timing requirements, accounting for the relevant clock edge, propagation delays, and sampling point. It should not automatically be assumed to equal the entire clock period or half-cycle.
Determining the Minimum Resistance from Current:
The output’s sink-current capability places a lower limit on the pull-up resistance:
RPU(min) ≥ VPU−VOL(max)/ILOW(max)
Here, VOL(max) is the maximum permitted LOW-state output voltage, and ILOW(max) is the allowable sink current under the selected operating conditions. This is a simplified design estimate. Verify the result against the buffer datasheet, particularly its guaranteed LOW-state voltage at the required sink current.
Establishing the Acceptable Resistance Range
The selected resistance must satisfy both constraints:
RPU(min) ≤ RPU ≤ RPU(max)
This range is valid only when both limits are calculated using compatible operating conditions and component specifications.
If the minimum resistance imposed by the current constraint exceeds the maximum resistance permitted by timing, no resistor value satisfies both requirements under the assumed conditions. Consider reducing signal capacitance or selecting a suitable push-pull buffer or voltage-level translator
Practical Resistor Selection:
| Pull-up Resistance | Rising-edge Behavior | LOW-state Current |
|---|---|---|
| Higher resistance | Slower rise | Lower current |
| Lower resistance | Faster rise | Higher current |
| Properly selected resistance | Meets timing requirements | Remains within electrical limits |
Do not select a pull-up resistor based solely on a rule of thumb. Calculate the expected signal rise time, verify the LOW-state sink current, and check both against the component datasheets and the complete SPI timing requirements.
Open-Drain or Push-Pull: Which Is Better for SPI?
For most SPI applications, push-pull buffers and suitable level translators are the preferred choice, especially when fast signal transitions are important.
A push-pull output actively drives the signal both HIGH and LOW. This allows the signal to change states quickly without relying on a pull-up resistor to produce the rising edge.
An open-drain output works differently. It actively pulls the signal LOW but relies on an external pull-up resistor to bring it HIGH. The rising edge depends on the pull-up resistance and the total capacitance of the signal line. As a result, the rising edge is generally slower than the falling edge.
When choosing a buffer or level translator for SPI, consider the propagation delay, output drive strength, voltage compatibility, and timing requirements of the connected devices.
Since SPI signals have defined directions, fixed-direction level translators are often a straightforward choice when their input and output directions match the connections. Automatic-direction translators require more careful selection because their direction-detection method and electrical characteristics may not suit every SPI interface.
Open-drain solutions can also work in SPI applications when the circuit is designed for them and the signal timing meets the requirements.
In most conventional SPI designs, push-pull is the better starting point. However, the final choice should depend on the datasheet specifications, circuit design, and required timing margins rather than a fixed SPI clock-speed limit.