Serial Peripheral Interface Riddle

For those of us who have no idea what the thing is your describing, could you explain what the thing is meant to do.?

and a circuit diagram of your circuit, also is your circuit working correctly, and if so is all you need an explanation of how it works or why it works ?

sorry not be of more use but i cant understand the question the way you have formatted it. It may be just me.

No worries. I figured that anyone who would have an off the cuff answer would have a pretty sharp working understanding of what I'm describing and opted for brevity and detail.

Basically, I have a working circuit that I am repurposing. Consisting of an 8-bit microcontroller, a few buttons, a programming interface, and a BH1418 FM transmitter chip. The uC and transmitter communicate using a common serial standard (SPI, based on some reading today isn't really a standard at all). I have no access, nor is there a physical way to access for this controller, the firmware in the chip.

The problem is that the connection wiring between the two chips is odd. Odd enough to maybe not even be able to use "SPI".

Sorry to be exclusive.

Can you cut the track between the MISO pin and the transmitters' CLK input and put a scope on the MISO pin? If you see a clock pulse, it will confirm it's not using SPI but just bitbashing port pins to drive the FM module. The occasional two bytes you're seeing are probably to set the PLL frequency.

Just out of interest, how will you 'repurpose' the circuit if you can't access the uC.

I see what you mean!

What is the slave?

Thanks for everyone's feedback.

So a test set up to see what the uC is doing would take a while, which is why I asked first. I think thats what I will have to end up doing though. I will have to borrow a friends iPod to do some handshaking to get the thing to turn on.

I can very much access and program the uC. I just can't read the flash, otherwise I could just reverse engineer the code to see if/how they are using the SPI special function registers. As a security feature, there is simply not provisions in the hardware to read the flash.

The slave is a Rohm BH1418F FM transmitter.

So.... no clear answer is a good enough result right now. Thanks for the thought.

I think the code is toggling between setting the SPI port from being a master to being a slave. In this manner you get the ability to drive two output lines for one chip select. They must be relying on the storage state of a previoiusly set MOSI used in master mode to stay put while the port is confiugred to switch to SPI slave, which turns the MISO into an output driving the FM chip.

I would guess that a SPI CLK is much too fast for the FM part so that you need to bit bang at SW speed a clock signal by using the slave mode drive out of the uC.