Sensor interface ICs bridge the physical world and digital systems. A temperature sensor in an engine management unit, a pressure transducer in a hydraulic line, and an accelerometer in a vehicle safety system all require a means of communicating their measurements to a processing unit. But sensors don’t speak the language of computers natively. Raw analogue signals (such as voltage fluctuations, current changes, and frequency variations) must be conditioned, interpreted, and transmitted according to a custom protocol that both the sensor interface IC and the host system understand.
This is where standardised sensor interface protocols matter. Over decades of development in automotive, industrial, and aerospace sectors, several standards have emerged as industry workhorses: Pulse Width Modulation (PWM), Single Edge Nibble Transmission (SENT), Serial Peripheral Interface (SPI), and Inter-Integrated Circuit (I2C). Each has evolved to solve specific engineering challenges. Understanding their strengths, limitations, and architectural trade-offs is essential for anyone designing sensor systems.
As Ross Turnbull, Director of Business Development at Swindon Silicon Systems, puts it:
“A raw sensor signal represents the measured parameter at its most fundamental level. At this stage, the signal is extremely vulnerable. It must be amplified, filtered and calibrated before it can be reliably used. While many of these conditioning functions can be implemented using discrete components, a sensor interface ASIC integrates amplification, filtering, compensation, digitisation and diagnostics into a single, optimised chip. This ensures every stage of the signal chain is tightly controlled, consistent and tailored to the specific sensor technology.”
This article explains what these standards are, how they differ, and how they shape the architecture of sensor interface ICs.
Why sensor interface standards matter
Before sensor protocols were standardised, embedding sensors in systems meant designing signal chains for each application. This created several problems: supplier incompatibility, complex integration workflows, higher cost, and longer time-to-market.
Standardised protocols solved these problems by establishing agreed-upon methods for transmitting sensor data. A standard protocol means:
- Compatibility. A host microcontroller from one vendor can accept data from a sensor interface IC from another vendor, as long as both implement the same standard.
- Simplification. The application engineer doesn’t need to decode proprietary signal formats. The protocol is documented and predictable.
- Economies of scale. High-volume standard interfaces drive down costs through competition and manufacturing maturity.
- Design reuse. Reference designs and driver libraries are abundant for standard protocols, reducing development risk.
However, standardisation also imposes constraints. A standard protocol is optimised for a range of applications, but not for any single application. If your system has unusual environmental constraints (extreme temperature, high electromagnetic interference), unusual signal characteristics (very low voltage, very high frequency), or unusual latency requirements (real-time critical feedback), a standard interface may impose compromises. In these cases, an ASIC design that optimises the signal chain specifically for your application can unlock performance that standard solutions cannot achieve.
Overview of sensor interface standards
PWM: Pulse Width Modulation
PWM is the simplest and oldest standard discussed here. The sensor measurement is encoded in the width of a square-wave pulse.
How it works: The sensor interface IC generates a fixed-frequency square wave signal. The duty cycle (the ratio of time the signal spends high to the total period) is proportional to the measured value. For example: a pressure sensor might output 10% duty cycle for 0 bar, 50% duty cycle for 5 bar, and 90% duty cycle for 10 bar. The host microcontroller measures the pulse width and converts it back to a physical quantity.
Advantages of Pulse Width Modulation:
- Extremely simple to implement. Requires only a timer and a GPIO pin on the host side.
- Robust to electrical noise. A square wave is easy to detect even in noisy environments.
- No clock synchronisation needed. The host can measure the pulse width asynchronously.
- Very low power consumption in the interface IC.
- Widely supported in legacy systems and microcontrollers.
Limitations of PWM:
- Low resolution. Measurement resolution is limited by the timer resolution of the host microcontroller. Achieving 12-bit resolution requires careful design.
- No built-in diagnostics. There is no mechanism for the sensor IC to signal a fault or out-of-range condition.
- Vulnerable to signal slew-rate errors. If the rising or falling edge of the pulse is slow or distorted, timing errors accumulate.
- Single-ended signal. PWM typically uses a single signal line.
Applications: Throttle position sensors (TPS), mass air flow sensors (MAF) in older automotive systems, simple industrial limit switches and position transducers.
Typical resolution: 8–12 bits.
Bandwidth: Low. Typically 0–10 Hz for pressure and temperature measurements.
SENT: Single Edge Nibble Transmission
SENT emerged in the 2000s as an enhancement to PWM. Defined in the SAE J2716 specification, it addresses the need for higher resolution and built-in diagnostics whilst maintaining simplicity and robustness.
How it works: SENT encodes data as a series of pulses on a single wire. Each “nibble” (4-bit word) is represented by a pulse width. A SENT message consists of a sync pulse, followed by nibbles representing the measurement, checksum, and optional counter and CRC (cyclic redundancy check) fields. The message repeats at a fixed rate (typically 10–100 Hz).
Advantages of Single Edge Nibble Transmission:
- Higher resolution than PWM. Standard SENT supports 12-bit or better resolution.
- Built-in diagnostics. CRC and counter fields allow the receiver to detect transmission errors and data loss.
- Simultaneous measurement and error detection.
- Single wire, asynchronous. No clock synchronisation required.
- Excellent noise immunity.
- Low-cost IC implementation.
Limitations of SENT:
- Slower data rate than digital protocols. SENT messages repeat at 10–100 Hz typically.
- Still single-ended.
- Requires a precise oscillator on the sensor IC side and a precise timer on the host side.
Applications: Modern automotive pressure sensors, position sensors, advanced throttle bodies, brake pressure sensors.
Typical resolution: 12–16 bits.
Bandwidth: 10–100 Hz (configurable).
SPI: Serial Peripheral Interface
SPI is a synchronous digital protocol that is common in embedded systems. It operates over multiple signal lines and uses a shared clock signal between the master (typically a microcontroller) and slave (the sensor IC).
How it works: SPI uses four signal lines: clock (SCLK), chip select (CS), master-out-slave-in (MOSI), and master-in-slave-out (MISO). The master generates the clock and initiates communication by pulling CS low. Data is exchanged simultaneously on MOSI and MISO, synchronised by rising or falling clock edges.
Advantages of Serial Peripheral Interface:
- High speed. SPI clock rates often reach 10 MHz or higher.
- Full-duplex communication. Data can flow in both directions simultaneously.
- Simple protocol. No handshaking; communication is deterministic and easy to implement.
- Multiple slave support.
- Very widely supported. SPI is available on the majority of microcontrollers.
Limitations of SPI:
- Requires multiple signal lines (minimum 3, typically 4).
- Requires both MOSI and MISO lines, even if bidirectional signalling isn’t needed.
- Requires a shared clock. The master must be present and actively clocking.
- Requires chip-select line(s). Adding more sensors complicates the PCB (printed circuit board) layout.
- Vulnerable to crosstalk on high-frequency buses.
- Distance limitation. SPI is typically point-to-point over short distances (< 1 metre).
Applications: High-bandwidth sensors (accelerometers, gyroscopes, pressure sensors in high-speed systems), data logging systems, SD card interfaces, real-time control systems.
Typical resolution: 12 to 24 bits (or higher).
Bandwidth: kHz to MHz (up to 50 MHz in some variants).
I2C: Inter-Integrated Circuit
I2C is a synchronous, multi-master, multi-slave protocol designed for communication between ICs on a single board or over short distances. It uses only two signal lines: serial clock (SCL) and serial data (SDA), both of which are open-drain and pulled high by resistors.
How it works: Any device on the I2C bus can initiate communication if the bus is idle. A master sends a start condition, then the slave address (7 or 10 bits), then a read/write bit. The addressed slave acknowledges. Data follows, typically in 8-bit bytes, with acknowledge bits between bytes. A stop condition signals the end of the transaction.
Advantages of Inter-Integrated Circuit:
- Minimal wiring. Only two signals (SCL and SDA) are required.
- Multi-master and multi-slave support.
- Open-drain design enables arbitration and error recovery.
- Standard protocol across a vast ecosystem.
- Wide range of slave addresses. The 7-bit address space allows up to 128 unique devices.
Limitations of I2C:
- Slower than SPI. Standard I2C runs at 100 kHz, 400 kHz, or up to 1 MHz.
- Open-drain design adds complexity. Pull-up resistors must be carefully sized.
- Distance limitation. Practical I2C buses span a few metres at most.
- Clock stretching can introduce unpredictable latency.
- Electrical noise susceptibility. Open-drain signalling is higher impedance than CMOS (complementary metal-oxide-semiconductor) push-pull.
- No collision resolution for data content.
Applications: Automotive body electronics (lighting, climate control, seat adjustment), industrial Internet of Things sensors, consumer electronics, environmental monitoring systems.
Typical resolution: 8–24 bits.
Bandwidth: 100 kHz to 1 MHz.
Signal chain architecture: from sensor to digital output
All four of these standards sit atop a common foundation: the signal chain. Understanding the architecture of a sensor signal chain explains why each standard is suited to different applications.
A complete sensor signal chain comprises:
- Sensing element. The physical transducer that responds to the measured quantity (temperature, pressure, displacement, acceleration).
- Signal conditioning. Amplification, filtering, and offset correction to bring the sensor signal into a range suitable for digitisation.
- Analogue-to-digital conversion. The ADC converts the analogue signal to a digital representation.
- Digital processing and calibration. Linearisation, temperature compensation, and outlier rejection.
- Interface logic. Formats the processed measurement according to the chosen standard (PWM, SENT, SPI, I2C).
- Monitoring and diagnostics. Self-diagnostic circuits that monitor for faults: open circuits, shorts, out-of-range conditions, supply voltage faults.
In a standard sensor IC, each of these stages is usually optimised for a typical range of applications. In an ASIC design, however, each stage can be tailored to the specific needs of your application.
Compensation and calibration
Temperature compensation and calibration are critical elements of sensor interface IC design. Whilst signal conditioning and analogue-to-digital conversion provide the foundation for accurate measurement, it is compensation and calibration that unlock real-world accuracy across the full operating range of temperature, ageing, and manufacturing variation.
Why compensation is critical
Raw sensor measurements drift with temperature. A pressure sensor calibrated at 25 °C may shift by 1–2% per °C if its output is not compensated. A sensor designed for an automotive application spanning −40 to +125 °C could accumulate errors of 10–15% across that range without compensation. In safety-critical applications (brake systems, structural monitoring, medical devices), this is often not acceptable.
Compensation strategies
Analogue compensation: Uses resistor networks, temperature-sensing diodes, and precision voltage references to adjust signal gain and offset as temperature changes. It’s fast, and it requires no software, but offers limited flexibility and accuracy.
Digital compensation with look-up tables (LUTs): After digitisation, compensation is performed in software using calibration coefficients stored in non-volatile memory. Memory-efficient and accurate, but limits flexibility to predefined temperature points.
Polynomial compensation: The IC stores polynomial coefficients (typically 2nd or 3rd order) and computes the corrected measurement: corrected_value = raw_value × (1 + c1 × T + c2 × T² + …). It’s smooth, memory-efficient, and more accurate than LUTs.
Dual-sensor compensation: Some advanced sensor ICs integrate two measurement channels: one for the physical quantity and one for temperature. This is particularly effective in systems where ambient temperature changes rapidly.
How compensation is transmitted across protocols
The choice of compensation strategy is closely linked to the output protocol:
PWM and SENT: Compensation is performed on-die entirely before the output pulse is generated. The host microcontroller receives only the compensated measurement. It has no visibility into raw or intermediate values.
SPI and I2C: Digital protocols allow richer communication. A sensor IC can transmit both raw and compensated measurements, or raw measurement plus temperature. This allows the host to apply its own compensation if desired.
A standard sensor IC is designed for a broad range of applications. In comparison, an ASIC can be tailored to your application’s exact requirements.
Output format trade-offs: which standard for your application?
Choosing the right sensor interface standard is fundamentally a trade-off between complexity, cost, power, resolution, and bandwidth.
| Standard | When to Choose |
| PWM | Simplicity and cost are paramountHost has no SPI or I2C interface (legacy systems)Environmental ruggedness is criticalBandwidth is low (<10 Hz)Resolution requirement is moderate (8–12 bits) |
| SENT | Higher resolution than PWM (12–16 bits) without the complexity of digital protocolsBuilt-in error detection is essentialSingle-wire simplicity with diagnosticsBandwidth is low to moderate (10–100 Hz) |
| SPI | High bandwidth is required (kHz to MHz)High resolution (16–24 bits) is neededReal-time control loops demand minimal latencyHost has an SPI interfacePoint-to-point wiring is acceptable |
| I2C | Multiple sensors must share a common busMinimal wiring is critical (only two lines)Bandwidth can be moderate (100 kHz to 1 MHz)Host and all slaves support I2C |
In practice, many systems use a mix, as ASICs can add value to a vehicle in many ways. The car might use PWM for legacy throttle position sensing, SENT for modern pressure sensors, SPI for high-speed accelerometers, and I2C for environmental sensors in the cabin. The diversity reflects the evolution of automotive electronics over decades and the need to support both new platforms and legacy vehicle programmes.
Sensor interface standards: the foundation of modern systems
Standardised sensor interface protocols have become the backbone of modern control systems. PWM, SENT, SPI, and I2C each reflect decades of refinement, addressing the needs of different application domains.
Understanding their strengths and limitations is essential for system architects, but it is equally important to recognise that standards are enablers, not constraints. When your application demands performance, integration or supply certainty that standard protocols cannot provide, an ASIC design tailored to your needs can unlock a competitive advantage.
Whether you are designing next-generation automotive control systems, industrial automation platforms, or aerospace sensor networks, a discussion with an experienced sensor IC design partner can help you evaluate whether a standard interface IC will suffice or whether a tailored approach is warranted.
Swindon Silicon brings 40+ years of expertise in mixed-signal design and deep knowledge of sensor interfacing across automotive, industrial, and aerospace applications. If you would like to explore whether an ASIC could benefit your application, contact our experts.
