Understanding SPI

Question

I have some problems understanding SPI:

I am currently using STMF2xx to connect with FTDI UM232H, I will configure my STMF2xx into slave and FTDI into master.

But, I am having problem getting the data in my STM32 from the FTDI, but when I check my signal, I could see my MOSI (FTDI to STM32) signal is correct!

May I know what could be wrong?

But, isn't that my MISO line should be getting the same data as MOSI but is bit shifted? (am I right? isn't that SPI works like a shift register, whenever sends data into MOSI, the hardware will automatically output to MISO?)

My second question is, does it matter if my MOSI line is always LOW, but MISO line is always HIGH? Or do I need to setup that MOSI and MISO must be low/high initial, or MOSI high, MISO low?

My current configuration for my SPI is:

1. 2 line duplex
2. 1 Mhz clock
3. MSB
4. 8bit
5. Phase = Low
6. Polarity = Positive
7. Hardware: 4 lines connection, CS, SCLK, MOSI, MISO

Answer 1

But, isn't that my MISO line should be getting the same data as MOSI but is bit shifted? (am I right? isn't that SPI works like a shift register, whenever sends data into MOSI, the hardware will automatically output to MISO?)

No. The slave should clock out whatever it has put into the output register. If there is a 0 there, then 0 is clocked out.

My second question is, does it matter if my MOSI line is always LOW, but MISO line is always HIGH? Or do I need to setup that MOSI and MISO must be low/high initial, or MOSI high, MISO low?

Irrelevant. The level only matters when sampled at the appropriate phase and polarity of the clock.

Make sure that the SPI peripheral is enabled in slave mode and that MISO is configured as an output.

Answer 2

There are several ways interrupt driven SPI can fail to work.

Trivially, the MOSI and MISO connections may be wrong. I am assuming that is not the case.
AFAICT the FT232H might drive signals to 3.63V, which might be an issue the STM32F, but IIRC shouldn't damage it. I am assuming this is okay.
The STM32F should be able to drive FT232H inputs, which the recognise an input high at 2V. So I am assuming this is okay.

The STM32F SPI might not be correctly configured:

• clock for SPI operation
• their are four clock vs edge configurations which must match FTDI
• STM32F pins must be configured for the correct alt function
• IIRC STM32F pins still need to be configured correctly for output too

Interrupt failure:

• Incorrect interrupt vector
• Too low priority, and getting blocked

So try to simplify.

Are you using a 'working' example from ST?

Try to receive data by polling the SPI. Removing issues about interrupts, and just toggle a pin each time data is received. You can watch that pin on your 'scope.

If this doesn't work, try simple 'bit bashing', and receive data by polling the clock and data pin directly. You might only achieve a few Mbit/s, but that would check things are working.