In the new design I'm working with, the STM32L475 device uses SPI interface in order to communicate with a transceiver. This transceiver is used in other designs without problems and it must act as SPI-master, so STM32L4 SPI must work in slave mode.
Most of the times, SPI interface is working fine, but sometimes we get communication errors. We have checked the SPI frames and we have noticed some bits displacements when a failure occurs. This is quite strange, because SPI bus is working a very slow rate: 250Kbit/s.
In what regards SPI tracks on PCB: SPI lines are quite short and they are very close to STM32. They go from the STM32 to H1-AgroBee connector (look below): the place where the transceiver is connected. SPI lines have not any shunt capacitors: they only have serial resistors (now they are 0 Ohm) in order to filter spikes. We have been doing some testing by adding some shunt capacitors, but behaviour is the same.
Please, find here below top and bottom layers of this PCB where the SPI area is rounded.
Do you have any idea why we are getting this behaviour with SPI?