Get Connected: How to extend an SPI bus through a differential interface

Welcome back to the Get Connected blog series here on Analog Wire. In my previous Get Connected post, we examined using a general-purpose serializer/deserializer (SERDES) to aggregate multiple data inputs from different sources for high-speed transmission in short-reach or long-haul applications. In this post, I’ll look at extending a serial peripheral interface (SPI) bus through a differential interface, which can be useful when designing systems that support remote temperature or pressure sensors, for instance.

In SPI applications, the master and slave are relatively close to each other, and the signals typically never travel off the printed circuit board (PCB). SPI signals are single-ended, transistor-to-transistor logic (TTL)-like signals that can run up to 100Mbps depending on the application. An SPI bus consists of four signals: system clock (SCLK), master out slave in (MOSI), master in slave out (MISO) and chip select (CS). The master provides the SCLK, MOSI and CS signals, while the slave provides the MISO signal. Figure 1 shows the bus architecture of a standard SPI bus.

Figure 1: SPI bus

What if you need to send your SPI signals off-board from your microcontroller or digital signal processor (DSP) to a remote board that contains an analog-to-digital converter (ADC), a digital-to-analog converter (DAC) or another device? This can be challenging for several reasons. Signal integrity becomes a big concern due to reflections caused by unterminated signal lines. The characteristic impedance of the transmission media and termination impedance will differ substantially, causing an impedance mismatch on the bus. The result will be a standing wave of energy that radiates from end to end on the bus, causing communication errors. Electromagnetic interference (EMI) is also a concern as the high-frequency portion of the SPI signal radiates outward, allowing the signal to couple onto adjacent signals.

There is a simple solution to this problem, however: differential signaling. Differential transceivers like the SN65LVDT41 and the SN65LVDT14 take the SPI signals and convert them to low-voltage differential signaling (LVDS). LVDS works well in SPI applications due to its noise immunity and bandwidth. A previous Get Connected blog post reviewed the fundamentals and benefits of LVDS; you can find it here.

The architectures of the SN65LVDT41 and the SN65LVDT14 allow for the entire SPI bus to be translated to LVDS: four transceivers in one direction for MOSI, SCLK and CS and one transceiver in the opposite direction for MISO. The LVDS chipset also has the added benefit of built-in termination, making implementation simple and reducing component count in applications where board space is at a premium. Figure 2 shows the makeup of an extended SPI bus architecture using the aforementioned chipset. Shielded twisted pair (STP) CAT5 cable is not a requirement for such an implementation, but it is rather a nice to have given its ease of implementation.

Figure 2: Extended SPI bus

Figures 3, 4 and 5 show the performance of the SN65LVDT41 and SN65LVDT14 transmitters at 100Mbps across multiple lengths of CAT5 cable. The receivers in the SN65LVDT41 and SN65LVDT14 support a 200mV input threshold tolerance, which is easily met by the transmitters at these distances and speeds.

Figure 3: 8-meter CAT5 100Mbps TX waveform

Figure 4: 15-meter CAT5 100Mbps TX waveform

Figure 5: 25-meter CAT5 100Mbps TX waveform

For answers to common questions on solving interface design challenges in your application-specific solutions, check out the TI E2E™ Industrial Interface Community to read Search posts from engineers already using TI interface products, or create a new thread to address your specific application. If you’re not connected, you can get connected with TI’s broad interface portfolio that spans and links together a wide range of interface standards and applications.

Please watch for my next post in the Get Connected series, where I’ll discuss a multipoint LVDS (MLVDS) device with extended ESD performance that meets the International Electrotechnical Commission (IEC) 61000-4-2 specification. In the meantime, read about extending SPI and McBSP with differential interface products in this app note.

Leave your comments in the section below if you’d like to hear more about anything discussed in this post, or if there is an interface topic you'd like to see us tackle in the future. And be sure to check out the full Get Connected series.