I did some SPI software configuration for my DSP board. Then I soldered several missing 0 ohm resistors onto the board so I can tapping the SPI signal from pin headers. Then I measured the SPI Chip Select and SPI Clock signal. To my surprise, there are very large noise in both signals. And the noise is there regardless whether there are SPI transfers.
For the Clock signal, the noise is a roughly regular sawtooth shape with a frequency of 20MHz and 1V amplitude, when it should be a constant voltage of 3.3V.
What might be the cause of this noise and how to remove it?
(Edit: I found the noise coming from another chip driving SPI clock line with 20MHz clock. That chip is not using SPI protocol. Instead it's using other duplexing functions so the clock is always there. After disconnecting that chip from contaminating SPI, the signal becomes clear. Thanks for the help of you all, especially @gl3829, @FakeName.)