SPI protocol trigger is unstable, what's going on?

The triggering modes of the ZDS2022 oscilloscope include normal triggering and automatic triggering. If automatic triggering is used when testing SPI signals, the waveform may not be displayed stably.

1. Normal trigger

In normal trigger mode, the waveform is displayed only when the trigger condition is met, and the original waveform display is kept when the trigger condition is not met, and waits for the next trigger. In other words, in this mode, the waveform will be sampled only when there is a trigger event that meets the conditions.

2. Automatic trigger

In the automatic trigger mode, the waveform will be displayed regardless of whether the trigger conditions are met. That is, in this mode, after waiting for a period of time, if no trigger event that meets the conditions occurs, it will be forced to trigger and sample the waveform. Since the trigger position is random, it often leads to unstable waveform display.

1.1.2SPI signal clock

In the SPI protocol trigger mode, the rate and continuity of the SPI signal clock will affect the trigger, resulting in unstable waveform display.

1. SPI signal clock rate

If the SPI signal clock rate is relatively high (reaching tens of MHz), the protocol trigger of the ZDS2022 oscilloscope may not be triggered normally, resulting in unstable waveform display, but it will not affect the normal decoding of the SPI signal.

2. SPI signal clock continuity

Because the ZDS2022 oscilloscope has only two channels and only measures the CLK signal and MISO signal, but no CS signal, the protocol trigger of the SPI signal must rely on the idle time of the clock signal to identify the start bit. If the clock signal is continuous, it will affect the trigger and the touch code.