Why are you still using 8 bit MCUs?

[nctnico]

If you get way more interrupts than anticipated your application won't work anyway. Even with short ISRs and offloading the work to the main task. The events need to be handled somewhere. Like I typed before: there is X time to execute a Y amount of instructions.

Anyway, I can't recall ever using GPIO interrupts. Stuff on GPIO is usually slow so I simply poll it and while I'm at it I do the filtering in software.

[dannyf]

Stuff on GPIO is usually slow

Yes, GPIO instructions can be as slow as 1 instruction / clock cycle.

Many other peripherals run much faster than that, I am sure.

[geraldjhg]

hi
because a 8 bit mcu powered from a 3.7 lipo thats gets charged from a 5v usb socket
and uses low power is FUN

[cloudscapes]

Was PCB design mentioned? Probably.

8bit MCU. Drop DIP or SOIC package onto PCB. Single-sided is fine, even home-etched. It'll work without decoupling caps, even. It'll just work.

32bit MCU. Extremely difficult on home-etched boards. Probably need copper fills, vias and caps all over unless you clock it really low. 32bit PCB design considerations are at least an order of magnitude more complicated than 8bit, unless you stick with one of the low-speed low-pin ARM DIPs.

I'd say 8bit is an invaluable learning tool in part because it is much more forgiving.

[nctnico]

Ebay is swamped with ready-to-go ARM boards you can drop onto a breadboard.

[westfw]

ready-to-go ARM boards you can drop onto a breadboard.

But then you're no longer price-competitive with the 8bit chip-only solution.
At least; not obviously.

[WarSim]

32bit MCU. Extremely difficult on home-etched boards. Probably need copper fills, vias and caps all over unless you clock it really low. 32bit PCB design considerations are at least an order of magnitude more complicated than 8bit, unless you stick with one of the low-speed low-pin ARM DIPs.

LOL. You amuse me. 

[mariush]

32bit MCU. Extremely difficult on home-etched boards. Probably need copper fills, vias and caps all over unless you clock it really low. 32bit PCB design considerations are at least an order of magnitude more complicated than 8bit, unless you stick with one of the low-speed low-pin ARM DIPs.

I'd like to know what you're smoking. You can get 32 bit cpus in DIP packages, but there are chips in surface mount packages that are easy to solder, like SOIC20 or TQFP that are easy to solder..

For example
http://www.digikey.com/product-detail/en/PIC32MX110F016B-I%2FSP/PIC32MX110F016B-I%2FSP-ND/2802113
http://www.digikey.com/product-detail/en/LPC812M101JD20FP/568-10435-5-ND/4368883

[c4757p]

32bit MCU. Extremely difficult on home-etched boards. Probably need copper fills, vias and caps all over unless you clock it really low. 32bit PCB design considerations are at least an order of magnitude more complicated than 8bit, unless you stick with one of the low-speed low-pin ARM DIPs.

The last PIC32 I used might as well have been a PIC18F or an ATmega for how stupidly simple it was.

[cloudscapes]

I must have been lucky/unlucky, then, because I started with 8bit mcus and they were incredibly easy to design PCBs for, while I had so many problems getting 32bit mcus to function properly on my own layouts. even started a thread about it here, last year:

https://www.eevblog.com/forum/projects/got-my-new-batch-of-faulty-pcbs-!/

Where I was kindly taught by members here that PCB design for higher speed parts is a bit more involved. if you guys think I'm crazy, maybe you should take it up with those who helped me in that thread?  here's an example reply:

https://www.eevblog.com/forum/projects/got-my-new-batch-of-faulty-pcbs-!/msg148192/#msg148192

was he wrong? because that advice (and others) certainly fixed my problems.

EDIT: actually, okay. I admit the problem I was having in that other thread was in regards to higher speed, not necessarily them being 32bit. when I said "32bit is harder to design pcbs for" I kind of meants high-speed 32bit. I should have elaborated.

[WarSim]

I must have been lucky/unlucky, then, because I started with 8bit mcus and they were incredibly easy to design PCBs for, while I had so many problems getting 32bit mcus to function properly on my own layouts. even started a thread about it here, last year:

https://www.eevblog.com/forum/projects/got-my-new-batch-of-faulty-pcbs-!/

Where I was kindly taught by members here that PCB design for higher speed parts is a bit more involved. if you guys think I'm crazy, maybe you should take it up with those who helped me in that thread?  here's an example reply:

https://www.eevblog.com/forum/projects/got-my-new-batch-of-faulty-pcbs-!/msg148192/#msg148192

was he wrong? because that advice (and others) certainly fixed my problems.

EDIT: actually, okay. I admit the problem I was having in that other thread was in regards to higher speed, not necessarily them being 32bit. when I said "32bit is harder to design pcbs for" I kind of meants high-speed 32bit. I should have elaborated.

Re-edit:  What you actually want to say, so you are saying something actually correct is. 

Any IC with with higher exposed frequencies that require special attention to PCB design are more difficult to design for. 

With your statement that you happened upon this consideration for the first time with 32bit MCUs, would indirectly support you experiences. 

If anyone has difficulty maintaining signal integrity under 80MHz, they should read more and learn more.  Above 80MHz you should use signal safe practices as required.  400MHz ball park range is when the designs should specifically be focusing on signal integrity.  Multi GHz is about when everything is about good RF design.  Of course all of this is ballpark, there are many other factors that will effect these generalities. 

[AlfBaz]

EDIT: actually, okay. I admit the problem I was having in that other thread was in regards to higher speed, not necessarily them being 32bit. when I said "32bit is harder to design pcbs for" I kind of meants high-speed 32bit. I should have elaborated.

meh

Most of the high speed stuff happens inside the chip. conceivably the highest speed stuff would be the osc at around the 10-20MHz mark internally the pll ramps it up to whatever. Last time I looked max rise/fall times for any pin on a PIC32 was 10ns, hardly an issue unless your board is 1x1 meter

[AndyC_772]

Bear in mind that, when considering signal integrity, it's always the edge rate that matters, not the clock frequency. It's not something that's taught particularly well, or emphasized frequently and clearly enough.

It may well be the case that an output driving 10 MHz has slower edges than one which drives 100 MHz, but not necessarily so. If an I/O pin has a rise / fall time of, say, 2ns, it could easily require the same SI treatment whether it's carrying a 100 MHz clock or a reset signal that's only active once.

On the other hand, if a device runs at 10 MHz, chances are its I/O pin drivers will be designed to have slower rise / fall times, so it'll be easier to breadboard with and will be generally more forgiving of a bad PCB layout for that reason.

Check the edge rate, not the switching frequency.

[Rufus]

Bear in mind that, when considering signal integrity, it's always the edge rate that matters, not the clock frequency. It's not something that's taught particularly well, or emphasized frequently and clearly enough.

Edge rate - would be the rate at which edges occur under any sensible interpretation of the english language. Not really helping clarity is it?

[mrflibble]

Edge rate - would be the rate at which edges occur under any sensible interpretation of the english language. Not really helping clarity is it?

With edge rate he no doubt means slew rate. And lets not get into "sensible interpretation of the english language" because, yes just for the fun of it I checked the dictionary definition of "slew". Natural languages and their usage are not exactly a bastion of logic.

[alm]

Edge rate is a perfectly standard term for slew rate. See this Altera appnote for example.

[Rufus]

Edge rate is a perfectly standard term for slew rate. See this Altera appnote for example.

A lot of people being wrong doesn't make something right.

Slew isn't right either since the required frequency components then also depend on the size of the signal.

Rise and fall times are what we should be talking about.

[AlfBaz]

Given my feeble technical background I have yet to come to terms with this.

Having seen and mucked around with BW=0.35/Tr it would seem that rise times have some obvious importance but here's where it's' difficult. If we have a rise time of 10ns on two waveforms where one goes from 0V to 1V and another from 0V to 10V, the gradient of the second will be much steeper and would have major impact on impedance since I=C(dV/dt) and since a lot if integrity issues are born from impedance mismatches then surely the rate of rise is important

[Rufus]

If we have a rise time of 10ns on two waveforms where one goes from 0V to 1V and another from 0V to 10V, the gradient of the second will be much steeper and would have major impact on impedance since I=C(dV/dt) and since a lot if integrity issues are born from impedance mismatches then surely the rate of rise is important

A 10ns rise time square wave of 1v and 1000v amplitude have exactly the same frequency spectrum, they travel down a transmission line at the same speed, and the same proportion of voltage will be reflected by transmission line impedance mismatches. The impedance of ideal passive components does not change with applied voltage.

[nctnico]

Any math to back that up?

[c4757p]

Having seen and mucked around with BW=0.35/Tr it would seem that rise times have some obvious importance but here's where it's' difficult. If we have a rise time of 10ns on two waveforms where one goes from 0V to 1V and another from 0V to 10V, the gradient of the second will be much steeper and would have major impact on impedance since I=C(dV/dt) and since a lot if integrity issues are born from impedance mismatches then surely the rate of rise is important

Impedance isn't the amount of current conducted, it's the amount of current conducted per volt.

[AlfBaz]

Thanks for the feedback guys

A 10ns rise time square wave of 1v and 1000v amplitude have exactly the same frequency spectrum

Ok I struggled to see that at first but then it dawned on me that to recreate the square waves by superpositioning the relevant odd harmonic frequencies, the amplitudes of those harmonics would be proportional to the square wave voltages, thereby producing the steeper edges but still using the same set of frequencies... am I close?

they travel down a transmission line at the same speed, and the same proportion of voltage will be reflected by transmission line impedance mismatches.

This makes sense. I forgot that one of the reason we have signal integrity issues is because the voltage creates an initial current determined by how much of the path is "seen" due to the relatively slow speed of electricity.

The impedance of ideal passive components does not change with applied voltage.

Impedance isn't the amount of current conducted, it's the amount of current conducted per volt.

So the impedance remains the same because I is soley dependant on dV as dt remains the same?

[c4757p]

Yes.

Z = v/i

Capacitor: i = C dv/dt

Z = v / (C dv/dt)

Z': Replace v by 2v; dv/dt becomes 2dv/dt:

Z' = (2v) / (2C dv/dt) = 2/2 v/(C dv/dt) = v/(C dv/dt) = Z

So doubling the input voltage does not change the impedance.

Edit: Sorry, mistake.

[jahonen]

Any math to back that up?

Perhaps this: The Fourier transform is a linear operator, so F{A*x(t)} === A*F{x(t)}.

Regards,
Janne

[mrflibble]

Perhaps this: The Fourier transform is a linear operator, so F{A*x(t)} === A*F{x(t)}.

If the system is linear time invariant, then problem solved.