Is there a need for HALL debouncing

[Harry H. Arends]

Hi there,
 
Is there a need for a piece of debouncing check on slow moving HALL sensors
connected to a MCU?
 
Harry
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[Harold Hallikainen-2]

> Hi there,
>
> Is there a need for a piece of debouncing check on slow moving HALL
> sensors
> connected to a MCU?
>
> Harry

I don't think so, but the input should be a Schmitt input to reject
electrical noise. The output of the Hall effect sensor will directly
follow the magnetic field and not suffer from bounce like mechanical
contacts.

Harold

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[Alan Smith-10]

Course not, its a solid state switch, nothing bouncing like a mechanical contact.  On the other hand, you might consider multiple polls to ensure the sensor is active and your not just reading noise on the line (depending on how you have it designed).


--- On Thu, 8/14/08, Harry H. Arends <h.arends@...> wrote:

> From: Harry H. Arends <h.arends@...>
> Subject: [PIC] Is there a need for HALL debouncing
> To: "'Microcontroller discussion list - Public.'" <piclist@...>
> Date: Thursday, August 14, 2008, 5:46 AM
> Hi there,
>  
> Is there a need for a piece of debouncing check on slow
> moving HALL sensors
> connected to a MCU?
>  
> Harry
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[Brendan Gillatt]

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Harry H. Arends wrote:
> Hi there,
>  
> Is there a need for a piece of debouncing check on slow moving HALL sensors
> connected to a MCU?
>  
> Harry

Depends how slow--if the output swings _really_ slow you might have some
problems if the device is rotated through the Earth's field or a piece of
machinery is moved past it. It would be best to put it through a Schmitt
trigger to be safe.

- --
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
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[Spehro Pefhany]

Quoting "Harry H. Arends" <h.arends@...>:

> Hi there,
>
> Is there a need for a piece of debouncing check on slow moving HALL sensors
> connected to a MCU?
>
> Harry

Probably not. Read the data sheet. If there is built-in hysteresis,  
then no (unless you want to reject noise glitches, which is not really  
debouncing). For example, the  Panasonic DN6849 has minimum 7 (typical  
10) mT of hysteresis.

If there is no built-in hysteresis then mechanical vibration or perhaps
power supply noise could cause a sort of 'chattering'.

Best regards,
Spehro Pefhany
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[Olin Lathrop]

Harry H. Arends wrote:
> Is there a need for a piece of debouncing check on slow moving HALL
> sensors connected to a MCU?

Any signal can have noise.  It would be a good idea to use hysteresis at
least a bit greater than the noise.


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[Apptech]

> Harry H. Arends wrote:
>> Is there a need for a piece of debouncing check on slow
>> moving HALL
>> sensors connected to a MCU?

Olin escrit
> Any signal can have noise.  It would be a good idea to use
> hysteresis at
> least a bit greater than the noise.

What he says.

While in many cases the hysteresis which is often included
in Hall sensors will be enough, the result will vary with
application. Depending on how the magnetic field variation
is produced you may get "noise" or may not. If you are
monitoring a rotating shaft (or similar object) then
inertia/momentum/natural cussedness will mean that you
probably only see 'vibration' bucking the trend and most
Hall sensors will probably work OK. If you are monitoring
something like a door / safety interlock etc then you may be
more at risk. But even with a rotating shaft "vibration" may
be due to eg bearings beginning to fail or an out of balance
load, so what works this week as an RPM monitor may work
next month as a bearing failure or out of balance load
detector.

Also, as Olin suggests, noise may be injected electrically
in some manner - circuit impedance, inductive pickup and
signal strength (and more) will help affect how great the
resultant noise is. "Relay turning off / inductive kick"
transients, which love to ride in on all sorts of other
signals may find a magnetically coupled path for this one.

Looking at the signal on a scope and comparing the observed
variations with the sensor's hysteresis spec  (or measured
hysteresis characteristic in the absence of a spec) will
give you a rough feel for what is or isn't required. Failure
in service in equipment 4000 km away will give you a finer
feel :-).


        Russell




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