Electronics > FPGA

FPGA design: bus size vs die size

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ataradov:

--- Quote from: tom66 on November 21, 2022, 11:07:56 am ---Is tri-state logic used in microcontroller buses at all? (I am talking of buses that do not exit the processor; things like I2C excluded.)

--- End quote ---
There is not tri-state logic, but there are pass transistors that are used as switches, which greatly simplifies the design.

dmills:
Given that the usual implementation of a flipflop uses transmission gates, an odd sort of tristate bus inside a processor is possibly reasonable.

SiliconWizard:
Bus multiplexing is certainly something that is suboptimal in FPGAs compared to synthesis on silicon.

NorthGuy:

--- Quote from: tom66 on November 21, 2022, 11:07:56 am ---Is tri-state logic used in microcontroller buses at all? (I am talking of buses that do not exit the processor; things like I2C excluded.)

--- End quote ---

There are many things sitting on the bus. One of them is driving the bus (high or low). Others don't - they're in high impedance state and have no influence on the bus. FPGA cannot simply disconnect a driver and hence needs a mux.

I2C devices have only 2 states - driving low and high impedance.

T3sl4co1l:
I can't speak for any particular chip, but many embedded CPUs read an "open" data bus same as previous data -- i.e. evidence of a tristate bus with enough capacitance that the level doesn't leak away between transactions.

This is relevant in a number of game console emulators, where open-bus logic has been found necessary to replicate certain obscure glitches in programs on those platforms.

And, while most of those use external memory, I believe some are wholly integrated as well??



--- Quote from: ataradov on November 17, 2022, 10:34:30 pm ---
--- Quote from: josuah on November 17, 2022, 09:22:55 pm ---Fascinating. Peripherals would then be 8-bit bus, with multiplexing in the rare case of 16/32-bit fetches/writes from a 32-bit CPU core.

--- End quote ---
No, on a good 32-bit MCU peripherals will use 32-bit registers. And byte and half word access would be prohibited or results undefined. 

--- End quote ---

What about x86? -- Oh wait, you said "good"... :-DD

Tim

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