Facing a trouble to use SPI mode. To solve the problem could you please answer to following questions?
Q1. Does communication speed need to be lower than 400kHz to move to SPI mode after Power up? Q2. At what timing does communication speed need to be changed to use SPI mode in MHz order? Is immediately after pulling CLATCH_B low three times right timing?Q3. In case that I2C start condition (a high-to-low transition on SDA/COUT while SCL/CCLK remains high) is satisfied accidentally after Power Up, is it impossible to move to SPI mode?
Q1: No, it can be at the max SPI speed.
Q2: You can use full speed SPI "immediately" after pulling the CLATCH line low three times. Now there will be a small delay for the internal multiplexers to switch over. I do not think that was characterized so I do not have that data. In looking at the other setup times I would expect this to be around 50ns or close to that. So I would wait a short time if possible before going forward with full speed communication.
Q3: No, the CLATCH pin is always active and waiting for three transitions. These transitions can come at any time during operation and even at a really low frequency. So this is why this pin needs to be pulled up if I2C is being used. The communications hardware is different for the I2C and the SPI so the state of the I2C state machine does not matter for the SPI engine. Once it is switched over to SPI mode the I2C section is no longer active.