Electronics > FPGA

ice40 HX, A 5v tolerant FPGA ??

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--- Quote from: julian1 on November 22, 2022, 08:48:25 pm --- and hct for outputs

--- End quote ---

When I investigated for my own use cases, HCT looked nice as they can output 'real' 5V (most of my solutions output TTL-compatible 3V3, so don't have a lot of margins...), but they were way too slow - I saw numbers in the 30-40ns propagation delay, on a 10 MHz bus (NuBus) it's quite high and on a 25 MHz bus (fastest SBus) it's unusable.

The CB3T have a propagation delay < 1ns, while LVC and LVT are in the 5-6 ns delay so even going through them twice is OK on a 10 MHz bus.

Currently trying to figure out if CB3T would also make do to connect an Artix-7 directly on the 25-40+ MHz bus of a 68030...


--- Quote from: julian1 on November 19, 2022, 09:11:48 am ---Any suggestions for generic level convert inputs, 5V -> 3.3V ?

Is there something better than 74LVC14A?
(Hex inverting Schmitt-trigger with 5Vtolerant input)

--- End quote ---

That works, but is only unidirectional.  A more universal part is a switch based level shifter.  They don't buffer, only connect the two pins through FETs.  They are much faster between the two I/O pins, sub-ns.  They limit the voltage driven through the switch because the gate drive is limited and the FET automatically turns off. 

They have a logic level enable signal, that is usually tolerant, but that needs to be verified.  No need for a DIR control since they are bi-directional.  It also makes it easy to mix directions with the same chip without getting a messy layout.

I've used the SN74CBTD3384CPW, a 10 bit part, very available.


Yeah, for 5V TTL, the 74CBT[D] is the way to go (that includes MOST 5V systems from the 70's and 80's). Super fast and super cheap so there's very little timing slew. Even for some CMOS designs it might be enough to just add pull-ups on the high side and call it done.

While reading the datasheet, I noticed under "DC Electrical Characteristics" it says
--- Quote ---Some products are clamped to a diode when VIN is larger than VCCIO.
--- End quote ---
which likely refers to this phenomenon. Of course, it also says
--- Quote ---IOs can support a 200mV Overshoot above the Recommend Operating Conditions VCCIO (Max) and -200mV Undershoot below VIL (Min). Overshoot and Undershoot is permitted for 25% duty cycle but must not exceed 1.6ns.
--- End quote ---


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