I know this is bad to run things over the spec but...
Have anybody tried to run ATtiny1634 at 16-20MHz (max spec 12MHz at 5Vcc)?
Does it run at this frequency and is it stable enough for practical use?
Any temperature MCU can survive
I have a device designed around ATtiny2313/4313 which runs at 20MHz naturally,
but I may need to replace 4313 with a pin-to-pin compatible MCU in QFN20 package that has more FLASH memory.
Currently the only option I'm partially confident of is ATtiny1634, but it has lower speed spec.
It is a bit strange that the µC is not specified for 16 or 20 MHz. Other similar chips are. There is a good chance the µC can run faster than specified, espeically at moderate temperature. The temperature extremes are usually the conditions they fail first.
No. The attiny design is marginal at best. You are lucky if you can run it at full speed at the speficied supply voltage range.
Any temperature MCU can survive
Currently the only option I'm partially confident of is ATtiny1634, but it has lower speed spec.
Any temperature MCU can survive
I have a device designed around ATtiny2313/4313 which runs at 20MHz naturally,
but I may need to replace 4313 with a pin-to-pin compatible MCU in QFN20 package that has more FLASH memory.
Currently the only option I'm partially confident of is ATtiny1634, but it has lower speed spec.
ATTINY1634 isn't pin-to-pin compatibile with ATTINY4313.
ATTINY4313 has ports A,B and D, ATTINY1634 has ports A,B and C.
In QFN20 case there is ATTINY816/1616/3216 (20MHZ).
No. The attiny design is marginal at best. You are lucky if you can run it at full speed at the speficied supply voltage range.Marginal design? Why?
That's not a sensible engineering reply. What temperatures do you actually need ?
If you really do need extreme temperatures, then find a device spec'd to those. Tiny AVRs come in 20MHz and -40°C ~ 125°C (TA)
MCUs follow a predictable Temperature/Voltage/MHz curve, so try your choice, and maybe compare with a full spec'd 20MHz part like maybe ATTINY1626-MF or ATTINY3226-MF
I know this is bad to run things over the spec but...
Have anybody tried to run ATtiny1634 at 16-20MHz (max spec 12MHz at 5Vcc)?
Does it run at this frequency and is it stable enough for practical use?
ATTINY1634 isn't pin-to-pin compatibile with ATTINY4313.
ATTINY4313 has ports A,B and D, ATTINY1634 has ports A,B and C.
In QFN20 case there is ATTINY816/1616/3216 (20MHZ).Not only it's not pin compatible, it's not compatible because VCC pin is on opposite side. Not to say ATTINY816/1616/3216 are not less similar to 4313, are cheaper and don't need overclocking. Arbitrarily selecting ATTINY1634 seems strange.
And with all that said, I suppose switching to a more recent and more powerful MCU would be out of the question, instead of trying to beat this one hard until it barely meets your processing requirements?
If this is mainly a supply issue - which has unfortunately plagued us for over a year now, and still going strong - then I can understand. But as discussed on a regular basis, there's no real guarantee that picking any particular chip that looks available right now will make you safe. The ATtiny1634, for all I know, may become unobtainium in a couple months.
What's the quantity you would need per month/year? Is it going to be a product? Tons of MCUs out there would fit the short description you just gave. Now maximizing your chances of getting a steady supply is another story.
Back to original question, simply out of curiosity.
How fast the stability of operation (MTBF) will drop vs frequency vs supply voltage vs operating temperature?
Most MCUs have some sort of curves for MHz vs Vcc, and you can infer the temperature effects by comparing the same-die in differing temps specs.
You might think about this one:
https://www.microchip.com/en-us/product/PIC24F08KL301
This is a 16 bit architecture.