Can you please explain why we need a dummy SPI after switching to SPI protocol? and What is the minimum delay time needed after dummy SPI read?
BMP280 supports both I2C and SPI interface but has no PS (protocol select) pin.
It starts with the I2C interface enabled. If there is a raising edge on the CSB line, it will switch to the SPI protocol. There should be at least a 140us delay after the first SPI dummy read operation to make sure the bus switch was successful.