2 SPI Communication Issues and Root Cause

Communication issues can occur for a variety of reasons. Noise from other circuits can be the cause, or other signals can couple to existing signals, causing malfunctions. This application note describes a case where a normally functioning AFE4xxx device malfunctions when the RF signal is turned on. Because EVBs are designed to eliminate interference and verify optimal device operation, it is difficult to identify issues that can arise in the complex system structure. Therefore, quickly and accurately identifying the cause and providing designs to customers when issues arise in the field is crucial.

Figure 2-1 shows normal SPI communication. Signals are exchanged periodically, indicating no issues on any pins. However, when AFE4xxx is in abnormal state, it shows behavior such as BG not functioning properly, inconsistent and no data is received.

Several tests confirmed that RF signals were affecting AFE4xxx SPI communication and attempted various designs to minimize redesigning effort and resolve the issue. External electromagnetic noise (EMI) tends to be more severe in high-frequency bands due to the antenna effect. Therefore, the user tried decreasing the SPI speed (frequency) compared to the previous setting, but the same issue occurred.

Therefore, when applying an RF signal directly to the SCLK line and verifying device operation, results were obtained similar to those observed in the field. This revealed that the RF signal coupled to the SCLK line affected the operating decisions. Figure 2-2 shows the test schematic for coupling noise effect of a RF signal applied to the SCLK line using a signal generator, and Figure 2-3 is a zoomed-in view of the rising edge. Due to glitches (noise) on the rising edge, the signal does not rise in a single low-to-high transition, but rather, after an additional cycle at the midpoint, rises. This noise introduced into the SCLK line caused the device to malfunction.

Figure 2-4 shows the impact of a glitch occurring on the rising edge. Glitches at the VIH and VIL levels cause malfunctions in the internal clock of the AFE. While a noise-free signal must rise evenly as the signal rises, the glitch causes a drop from high to low on the rising edge.