Developing a universal module

Probably will be nice to develop serial input board with extended logic to get not only SCS, SCK, MOSI, MISO signals, but also TXEN, RXEN, PAEN signals, Power On/Off and more then two (unlimited???) number of modules.

I think that properly developed extension board can be used not only for RF modules but also for various other modules, for example sound board (sound warnings, telemetry and so on).

Probably will be better to develop general purpose serial input extension board, and then connect to it various modules - RF modules, pots module, switch module, sound module and so on.

Would that be fast and precise enough to control the timing critique RF modules while serving other peripheral devices as well?

I think yes, if it will be connected to SCK, MOSI and MISO lines.

@rbe: I don't know much about the MEGA8, but doesn't it have enough I/O lines to directly control the enable lines rather than needing a switch for it?

That is pretty cool actually. I have forgotten most of what I know about the protocol transfer, but I assume the Atmega can keep up with the timing? such that a slight delay on the enable line won't impact bandwidth?
I thought the spi ran at 4MHz. Can an Atmel chip run fast enough to do an edge-detect, and set a pin set in less than 1 cycle?

I've started looking at a universal module design again. The Taranis is really limited as far as available ports to solder modules to, so something like this will be necessary to support anything other than the Frsky protocol. Following victzh's lead, my plan is to use a 4-wire SPI protocol from the Taranis, and use a small micro-controller to act as a programmable switch to select which pins to activate.

I'd like to build a small board that can attach ~4 modules with an MCU and as few parts as possible to keep costs low and assembly easy.

I'd like to use a 14pin MCU with internal oscillator, which would leave 9 wires available for I/O (2supply, 3 SPI (no need for MISO)) which should allow driving >4 modules.

I was planning on using something like a pic16f1825 since I have them on hand, and they are cheap, but they cannot be programmed without a reasonably costly programmer which negates any cost-savings.

I think an ATtiny24a may be a better option as they're cheap on ebay, programmers can be had for $5 or so, and it should be possible to program directly with a Deviation Tx I think (no programmer needed)

I considered arduino (something like a micro pro), but 3.3V variants are still rare, and it would be larger than using a 14pin-PDIP. It is also overkill for just implementing a digital switch.

So I've ordered some ATTiny chips to play with. We'll see how it goes.

I ordered some attiny84a (just to make sure I'm not code-constrained during testing). For something as simple as what I intend, the attiny24a should be fine.

I don't need performance from the attiny. I can use a slow spi from the STM32 to control it (so a bit-banged interface is likely fine on the attiny if USI doesn't work for me), and I'll use an interrupt to actually toggle the I/O CSN pins.

Basically I plan touse it as a pure digital switch.
The STM32 will sends (via SPI with CSN High) something like:
set pins 1,3,5 high
set pins 2,4,6 low
set pins 8 to follow CSN
set pins 9 to follow !CSN

then just run the existing protocol code on stm32.
the attiny will watch for changes in CSN and flip the relevant output pins

This way the attiny is just a dumb switch and doesn't know anything about how the modules are hooked up. It should allow us the maximum flexibility without needing to reprogram the attiny if more transceivers are added in the future.

Have you done any work on layout for modules in the Devo7e? I am trying to decide between a board that you can solder modules to (i.e. module pin-layout is pre-wired making it almost fool-proof) vs just leaving a bunch of headers which can be wired to different modules. The former would make it easier for folks with limited skills to get it working, but requires planning of how to place the modules, limits the module choice to those with the right pin-out, and will be more expensive as the board will be larger. The 2nd would let me build a really small board, but requires more work to properly solder. I am leaning towards actually soldering the modules to the board as I think having an easy to build kit is worth it, and it means the user doesn't need to worry about securing the modules in the Tx, but it means I need a layout that can fit inside the 7e. I'll probably leave pin headers too in case different modules are chosen.

why would there be much propogation. An ATTiny operating at 8MHz should be able to repeat CSN within 1 to 2us . That is plenty fast enough for our need