Electronic – STM32F2xx: Data from SPI Slave to Master

spistm32

I have a STM32F2xx connected to a FTDI UM232H using SPI, STM32F2xx will be configured as SLAVE and FTDI will be configured as MASTER

My scenario is this, I am getting data in STM32F2xx, and when I received data I would like to send it to my FTDI using SPI

But, how can a slave SPI send to Master??? since to initiate the communication, it needs the SCLK, which is controlled by the master

So how can I do that?

Or my master needs to constantly read the SPI Slave??? (like polling method) or is there any better way to do ti??

My current configuration for my SPI is:

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

Best Answer

The master always controls the clock. In the most basic SPI implementation, the master clocks 8 times and the slave outputs a single byte. If the slave has more output, the master must ask for it.

It is possible to have an external interrupt that the slave triggers when it has data for the master, but again the master controls the clock and must clock the data out of the slave. But this only works if the master supports such an interrupt, which the FT232H does not.

Related Solutions

Electronic – arduino – Making two Arduinos talk over SPI

I took a quick look at the Arduino SPI library and couldn't find the attachInterrupt() function. I assume that you're using an extended library of some sort and I'll make the assumption that it works although I would like to see how it's implemented underneath.

A couple things that I saw:

This compiles? if (pos < sizeof buf) - it probably needs to be if (pos < sizeof(buf))
In the sample code on the forum, he has a key line right here that you're missing in your slave code:
```
// turn on interrupts
SPCR |= _BV(SPIE);
```

Without this your receive interrupt probably won't fire. I also don't see where global interrupts are being enabled although I have no idea if Arduino takes care of that for you under the hood. I also haven't bothered to look up what any of those registers mean, I'm trying my best to go by memory of AVR's SPI registers.

Electronic – Understanding SPI

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.

Best Answer

Related Solutions

Electronic – arduino – Making two Arduinos talk over SPI

Electronic – Understanding SPI

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