Should Reset pin be tied high with resistor?

Don M · 2014-03-22 09:24

I'm making a board that will have the familiar 4 pin header to plug in the Prop Plug (only for programming). Is it a good idea to tie the Reset pin high through a 10K resistor to try and avoid any inadvertent resets?

LoopyByteloose · 2014-03-22 09:43

I suspect the answer would need to know something about what context you want to use the Propeller in.

If you want to create a board for developement work, the reset it handy. You might provide it and also provide a second circuit to tie it high for situations where it seems to be triggered for mysterious reasons.

If you are building a board as a final project, you may just want to tie it high if the environment is noisy (like an automotive application) and if it is easy to do a 'hard reset' by powering down and then powering up again.

Parallax left it optional as there are reasons to do both.

++++++
At times, the Forum gets postings asserting that the Reset on the Propeller is no good. But the reality is that the situation is causing sudden resets, the Propeller is not the problem.

There are even chips made to control reset sensitivity in problem situations. But I don't think it is necessary to use them. If you don't have any reset problems, having the reset button is handy if your program locks up due to a code error. You can easily start over and see that the problem repeats itself at the same point in the program.

Don M · 2014-03-22 10:35

It's a board as a final project. The header pins are only used for programming after manufacture and for terminal connection (Prop Plug) during any troubleshooting/debugging.

Yes it can be powered down/up for reset if necessary. No reset button provided or necessary.

The environment is not necessarily noisey and pins will be covered from human touch.

I'll make room for a resistor...

Was just wondering if anyone else uses one for the above reasons.

Thanks.

Duane C. Johnson · 2014-03-22 13:53

Hi Don M;

In general a 10K resister to +3.3V works well.
In addition I like a 10nF capacitor from the reset pin to +3.3V.
This allows one to still have the reset pin operational, not that you need to use it.

Duane J

Tracy Allen · 2014-03-22 15:52

The Prop internally has a 5kΩ resistor pullup from rstn to +3.3V. So 10kΩ more in parallel on the outside won't make much difference. I don't think an external pullup is necessary if the wiring over to the prop plug header is short. Maybe a capacitor like Duane mentioned, no more than 10nF.

An aside, the 5kΩ internal pullup is connected not directly to Vdd, but to the output of the internal brownout circuit, which is normally high. If the power supply voltage falls below brownout and BOE is tied low, the external reset pin pulls low (through 5kΩ). That will form a voltage divider with any external pullup resistor you have attached to rstn.

msrobots · 2014-03-22 16:02

In his spin propelleroader @Chip mentioned to pull rst high LOW with 1Mohm [edit] to prevent starting[/edit].
see post below. @kuroneko got me there... sorry.

Enjoy!

Mike

kuroneko · 2014-03-22 16:16

msrobots wrote: »

In his spin propelleroader @Chip mentioned to pull rst high with 1Mohm.

You mean this bit?

''This object drives the other Propeller's RESn line, so it is recommended that
''the other Propeller's BOEn pin be tied high and that its RESn pin be pulled
''to [COLOR="#FF0000"]VSS[/COLOR] with a 1M resistor to keep it on ice until showtime.

Lawson · 2014-03-22 16:59

My experience is that the reset line only needs a pull-up if the brownout detector is disabled. Otherwise you're fine without one except for noisy situations.

Marty

Phil Pilgrim (PhiPi) · 2014-03-22 20:36

Tracy Allen wrote:

An aside, the 5kΩ internal pullup is connected not directly to Vdd, but to the output of the internal brownout circuit, which is normally high. If the power supply voltage falls below brownout and BOE is tied low, the external reset pin pulls low (through 5kΩ).

This could be a problem with a 10nF cap on /RST, since it entails a 50us time constant, during part of which Vdd could be too low for normal operation before the reset actually engages. A ponderable degree of misbehavior could take place during that short interim.

-Phil

msrobots · 2014-03-22 23:43

@kuroneko.

autsch, kalt erwischt.

sorry - pulled LOW. to prevent running before the loading prop.

will correct above post

Enjoy!

Mike

kuroneko · 2014-03-22 23:46

msrobots wrote: »

autsch, kalt erwischt.

Kann schon mal vorkommen ... man wird ja nicht j

Tracy Allen · 2014-03-23 23:55

Phil, to clarify, the low level from brownout detection feeds back to the resn pin, but I don't think that is the pathway that causes the reset. There is a more direct path, and the output to resn may just be informational.

Here is the thread where I ran some experiments to verify the brownout pullup/pulldown action. You had suggested potential logic circuits in post #23. I think circuit #3 there is closest to observed behavior.

Also this earlier thread.

Peter Jakacki · 2014-03-24 01:37

Tracy Allen wrote: »

Phil, to clarify, the low level from brownout detection feeds back to the resn pin, but I don't think that is the pathway that causes the reset. There is a more direct path, and the output to resn may just be informational.

Here is the thread where I ran some experiments to verify the brownout pullup/pulldown action. You had suggested potential logic circuits in post #23. I think circuit #3 there is closest to observed behavior.

Also this earlier thread.

Not having the reset go completely low is not really a problem if you happen to have a 10K pullup on the reset line for instance. It seems to have hysteresis and with a 3.3V rail the low threshold is 1.3V and it requires the voltage to go back up to 1.8V to come out of reset. So any capacitor across the reset should be taken into account along with the thresholds. As Phil mentioned a cap could be a potential problem in a brownout condition but in practical terms I can't really see it causing a problem.

EDIT: however it would not be wise to use a pullup of less than 10K it seems. I have yet to check the thresholds for when the supply rail is falling.

VonSzarvas · 2014-03-24 03:01

Peter Jakacki wrote: »

EDIT: however it would not be wise to use a pullup of less than 10K it seems. I have yet to check the thresholds for when the supply rail is falling.

Long time ago, Chip suggested 1K and 10nF, both from the resN pin to VDD.

Might be an old idea, or still valid reference. Sadly cannot find the post at the moment - certainly in these forums though...

LoopyByteloose · 2014-03-24 07:02

We all seem to think that Parallax can provide the perfect solution, while part values may not be as critical as some of us think.

Reset problems are mostly caused by outside influences. Because automobiles have a rather limited amount of circuitry with heavy loads (inductive and lighting) being switched on and off at random times, it may be where most of us would encounter a reset problem.

But as I said before, it is all about context and if you don't need or want the reset button --- you don't have to include it. Then a variety of solutions can be accommodated in your printed circuit board if you are fearful that one might not work.

If you look inside a lot of electronics these days, you will see empty pads for alternative solutions. It is economical to build a board that has all the possible fixed, and then to populate it with whatever proves to be the least expensive reliable alternative.

Why not do the same?

Duane C. Johnson · 2014-03-24 10:25

Hi Maxwin;

Maxwin wrote: »

Long time ago, Chip suggested 1K and 10nF, both from the resN pin to VDD.

Might be an old idea, or still valid reference. Sadly cannot find the post at the moment - certainly in these forums though...

The technical reason for the capacitor connected to V_DD so glitches on V_DD couple to the reset pin. This is a good thing because it is better to match the voltage on V_DD instead of lag behind, as would happen if the cap were connected to V_SS.

Duane J

Tracy Allen · 2014-03-24 12:43

I repeated the experiment from an earlier thread. Prop with BOE tied low (brownout enabled), running on RCslow with one pin outputting a 1Hz frequency. In the scope shots, the oscillation is yellow, green is the Vdd power supply that is sagging down from 3.3v to 2.5V and then back up, and the red line is the voltage on the rstn pin. Rstn is pulled high by its internal 5kΩ. The brownout reset happens when Vdd sags below ~2.7V, and that is reflected when the rstn pin goes low as an output. Now rstn is pulled to ground through the same internal 5kΩ. In the second scope shot I have added an external 1kΩ pullup, which interacts with the internal 5kΩ. You can see the voltage divider effect as the red line dips during brownout, but not all the way to ground as it did in the first scope shot where there is no external pullup. The 1kΩ pullup is quite strong against the 5kΩ internal pulldown, but still the reset happens and recovers normally. I take this as evidence that the actual reset due to brownout has a separate path within the Prop.

I put a short pulse at the top of the program, then the 1Hz oscillation starts.

Peter Jakacki · 2014-03-24 14:38

Tracy Allen wrote: »

I repeated the experiment from an earlier thread. Prop with BOE tied low (brownout enabled), running on RCslow with one pin outputting a 1Hz frequency. In the scope shots, the oscillation is yellow, green is the Vdd power supply that is sagging down from 3.3v to 2.5V and then back up, and the red line is the voltage on the rstn pin. Rstn is pulled high by its internal 5kΩ. The brownout reset happens when Vdd sags below ~2.7V, and that is reflected when the rstn pin goes low as an output. Now rstn is pulled to ground through the same internal 5kΩ. In the second scope shot I have added an external 1kΩ pullup, which interacts with the internal 5kΩ. You can see the voltage divider effect as the red line dips during brownout, but not all the way to ground as it did in the first scope shot where there is no external pullup. The 1kΩ pullup is quite strong against the 5kΩ internal pulldown, but still the reset happens and recovers normally. I take this as evidence that the actual reset due to brownout has a separate path within the Prop.

I put a short pulse at the top of the program, then the 1Hz oscillation starts.

Thanks for that Tracy, that makes more sense to run the reset internally and just output the status on the reset pin. So we can use a much smaller value resistor and a capacitor even without compromising the brown-out reset. The thresholds that I measured would still hold true but are only really useful when the supply is steady at 3.3V.

Should Reset pin be tied high with resistor?

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