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Power on Reset

  • Thread starter Benjamin David Lunt
  • Start date
B

Benjamin David Lunt

Jan 1, 1970
0
Hi guys,

I am new to electronic design and have been building a few
small devices. I am currently studying the TUSB2046B 4-port
hub controller.

One of the pins is the RESET pin and from what I have read,
this pin needs to be held high, as soon as power is asserted,
for 100uS to 1ms, then held low, maybe using a Schmitt Trigger.

However, I haven't been able to find any documentation on how
one would go about making this POR circuit.

Would someone here please point me to a schematic or circuit
diagram of a POR circuit that would be used for the TUSB2046B?

Is there a chip or device, maybe a TO-92 case device that already
exists to do this?

Anyway, any help would be appreciated.

Thank you,
Ben

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B

Benjamin David Lunt

Jan 1, 1970
0
John Larkin said:
Look at the MAX809 series. 809 is low on reset. Several people
second-source this.

Most por inputs are low at powerup, high to run. The TUSB part appears
to be low for reset.

John Larkin Highland Technology, Inc

John,

Thank you for the response. This help greatly.

May I verify that I got it correctly? I believe that if
I use an MAX809, giving 5v0 at Vcc, Grounding the GND pin,
and sending the RESET pin to the RESET pin of the TUSB2046B
chip, also putting a 100k resistor on this RESET between
the MAX809 and TUSB2046B to hold reset low due to any
leakage that may take place, as with Figure 2, Page 6, at
http://datasheets.maximintegrated.com/en/ds/MAX803-MAX810Z.pdf.

On the "Selector Guide" Table on page 5 of that same document,
do I want a low reset threshold, MAX809Z, or a high threshold,
MAX809L?

Thanks again for your help,
Ben

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D

Don Lancaster

Jan 1, 1970
0
Look at the MAX809 series. 809 is low on reset. Several people
second-source this.

Most por inputs are low at powerup, high to run. The TUSB part appears
to be low for reset.

And the problem with a resistor and a capacitor is....?



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
R

rickman

Jan 1, 1970
0
And the problem with a resistor and a capacitor is....?


Really? You need to ask this? What there is a power glitch and the
power supply voltage dips below proper values, but not to ground? An RC
circuit will typically hold up and let the device be corrupted. This is
why many customer service folks tell you to unplug a device for a number
of seconds, to let crappy reset circuit completely discharge.

A proper reset chip is a good thing to use in any product that has to
work reliably.

Rick
 
B

Benjamin David Lunt

Jan 1, 1970
0
John Larkin said:
The TUSB needs a 3.3 volt supply, no?

If you run the TUSB gadget from 3.3 volts, you could use the 809T
(3.08 volts reset threshold) or S (2.93) maybe. S might be good.

Got it, thank you. I appreciate your help, I have learned a bit
about Power-on Reset circuits today, thank you.

Ben

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To reply by email, please remove the zzzzzz's

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G

garyr

Jan 1, 1970
0
Benjamin David Lunt said:
Hi guys,

I am new to electronic design and have been building a few
small devices. I am currently studying the TUSB2046B 4-port
hub controller.

One of the pins is the RESET pin and from what I have read,
this pin needs to be held high, as soon as power is asserted,
for 100uS to 1ms, then held low, maybe using a Schmitt Trigger.

However, I haven't been able to find any documentation on how
one would go about making this POR circuit.

Would someone here please point me to a schematic or circuit
diagram of a POR circuit that would be used for the TUSB2046B?

Is there a chip or device, maybe a TO-92 case device that already
exists to do this?

Anyway, any help would be appreciated.

Thank you,
Ben

--
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Forever Young Software
http://www.fysnet.net/index.htm
http://www.fysnet.net/The_Universal_Serial_Bus.htm
To reply by email, please remove the zzzzzz's

Batteries not included, some assembly required.
How about a TPS77833 regulator, it has a POR output.
 
J

Jamie

Jan 1, 1970
0
rickman said:
Really? You need to ask this? What there is a power glitch and the
power supply voltage dips below proper values, but not to ground? An RC
circuit will typically hold up and let the device be corrupted. This is
why many customer service folks tell you to unplug a device for a number
of seconds, to let crappy reset circuit completely discharge.

A proper reset chip is a good thing to use in any product that has to
work reliably.

Rick






_/
+-----------+-o/ o+--------+----------+
| | .-.
| | | | 5k
| | | |
+ .1u --- '-'
--- 3.3V --- +----------+ RESET PIN
- + |
| | ||-+
| | ||<- 2N7000
=== +------+||-+
GND .-. +
| | |
470 | | |
'-' |
| |
GND ===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

That works most of the time ;)

Selection of a mosfet with lower Vgs(th) would be nicer.

Jamie
 
J

Jamie

Jan 1, 1970
0
John said:
Soft edges, brownout hangups, uncertain thresholds.
Only if the chip does not have a hysteresis input. Most do.

You simply need to calculate the raise time to make sure the hysteresis
window falls with in the spec's of the reset time.

But, it never hurts to add a little speed I guess and set a time gap
externally with a square pulse.

Jamie
 
M

miso

Jan 1, 1970
0
Really? You need to ask this? What there is a power glitch and the
power supply voltage dips below proper values, but not to ground? An RC
circuit will typically hold up and let the device be corrupted. This is
why many customer service folks tell you to unplug a device for a number
of seconds, to let crappy reset circuit completely discharge.

A proper reset chip is a good thing to use in any product that has to
work reliably.

Rick

Products that don't POR cleanly are a total pain in the ass. However,
POR is a common place to skimp.

POR is really a big deal if you have some part of the circuit that can
draw significant power without control circuitry functioning. A SMPS is
a prime example, where you could be "charging" the inductor while
powering up. This alone is one reason why SMPS controller chips are far
better to put in a product than those roll your own power supplies,
which usually skimp or POR and/or UVL.
 
M

miso

Jan 1, 1970
0
Only if the chip does not have a hysteresis input. Most do.

You simply need to calculate the raise time to make sure the hysteresis
window falls with in the spec's of the reset time.

But, it never hurts to add a little speed I guess and set a time gap
externally with a square pulse.

Jamie

It is possible that the power supply can itself can be rising slowly, or
even dropping as parts of the system turns on and load the supply.
Typically chips under evaluation (pre-production) have to handle any
voltage from 0 to abs max without exhibiting unusual behavior.

I've gone as far in designs to include POR, UVL, and a watch dog timer
should the product be clocked by one-shots. Pre-production evaluation
should include shorting any two pins together, to ground, or to the
supply voltage, without causing the chip to go haywire. This is to avoid
the situation where the tech or design engineer poking around with a
scope probe.
 
S

Spehro Pefhany

Jan 1, 1970
0
And the problem with a resistor and a capacitor is....?

Well, because it only works "most" of the time, which is a bit like
having a spouse who is "mostly" faithful.

Das ist absolutely verboten for any application where decent
reliability is required, and, sadly, a typical newbie mistake.

The only application where I'd consider it is a battery-powered type
where the user will see it not working and will cycle power if it
doesn't start up right, and a bad startup won't kill something like
non-volatile memory.

I'm retrofitting this onto an $8K-per-unit product that will
occasionally start up with an output enabled that does some very, very
bad things. Typical sort of setup, a microcontroller controlling stuff
via a bunch of HC latches- the latch states are more-or-less random
after power has been off long enough, and the micro is counted on to
reset them during initialization- so on a less-than-clean startup
(eg. blip during power-up) you can get the outputs to stay on
continuously because they left out a 50-cent reset chip (one occurence
will pay for a truckload of chips). Shockingly bad design. Oh, and
unplug the PLCC micro (or let it fall out of the socket) and sometimes
you'll get outputs stuck on. I would have put both a proper hardware
reset circuit to both the micro and latches, AND an external watchdog
timer in place (both of which are in something like an ADM805). And
more, but that gets into system level stuff.

The OP should pick a chip that is guaranteed to assert "reset" for any
conditions under which bad things could happen, and is guaranteed not
to come out of reset until the chip can operate to spec (usually it's
a threshold with hysteresis and a time delay of some milliseconds for
clocks to spin up, PLLs to stabilize and that sort of thing). On-chip
reset circuits, when present, are often inadequate for the task. The
second criteria is usually well specified, but it's often up to the
engineer to define the first criteria-- for example at what worst-case
_minimum_ voltage could a non-volatile memory be corrupted or at what
worst-case minimum voltage (over temperature and unit-to-unit
variations and vibration etc.) could a relay pull in to turn on a
heater. Semi manufacturers will usually not tell you the guaranteed to
"not" operate conditions (relay manufacturers usually do).

Other than the xx805, I've often used Microchip's MCP10x series, which
is 3-pin supervisor- a precision voltage detector and time delay of a
few hundred msec. Simple, cheap, and works fine in many cases. There
are better choices if you need really low power consumption or other
features (eg. Seiko for low power). The MAX809 (eg. On-semi) that JL
pointed to is a really popular choice that's pretty good all-round,
3-pin, inexpensive, and currently my favorite.

Digikey lists about 35,000 different part numbers of supervisors so
obviously there's a recognized need! ;-)

--sp


Best regards,
Spehro Pefhany
 
S

Spehro Pefhany

Jan 1, 1970
0
_/
+-----------+-o/ o+--------+----------+
| | .-.
| | | | 5k
| | | |
+ .1u --- '-'
--- 3.3V --- +----------+ RESET PIN
- + |
| | ||-+
| | ||<- 2N7000
=== +------+||-+
GND .-. +
| | |
470 | | |
'-' |
| |
GND ===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

That works most of the time ;)

Selection of a mosfet with lower Vgs(th) would be nicer.

Jamie

Argh, terrible. No hysteresis, no time delay, and assert / release
reset levels are not controlled. Reset is never asserted if the supply
comes up slowly, a short blip down that disrupts the digital stuff may
not generate a reset pulse.

In fact, if the reset input is a ST, this might actually be worse than
an RC directly on the reset pin.

http://www.onsemi.com/pub_link/Collateral/MAX809S-D.PDF

About a dime in quantity. Check it out, if it works, spec it, sleep
well at night.


Best regards,
Spehro Pefhany
 
S

Spehro Pefhany

Jan 1, 1970
0
Only if the chip does not have a hysteresis input. Most do.

The switching levels of Schmitt trigger inputs are never specified in
such a way as to be useful. Not only are they uncontrolled (in terms
of guarantees of chip operation or benign non-operation) but they
change with power supply voltage. With all that unspecified flipping
back and forth they should be called Schmitt Romney triggers.
(ducking)

A bulletproof POR circuit does need an absolute voltage reference in
it.
You simply need to calculate the raise time to make sure the hysteresis
window falls with in the spec's of the reset time.

If you can guarantee that the power supply will always be either zero,
within spec or transitioning in a specified way between the two that's
true. For every other (almost every real) situation, you need a proper
POR circuit. Digital circuits can get into untold mischief in brownout
situations, for example.
But, it never hurts to add a little speed I guess and set a time gap
externally with a square pulse.

Jamie

Time + tightly specified absolute voltage levels for on/off.
 
S

Spehro Pefhany

Jan 1, 1970
0
If you used a Schmidtt-Obama trigger it would never come out of
reset, but it would appologize & blame America. AKA: The can't ever do
anything right gate.

He did supposedly generate a reset with Russia.
 
   If you used a Schmidtt-Obama trigger it would never come out of
reset, but it would appologize & blame America. AKA:  The can't ever do
anything right gate.

mean while the tea party gate would be desperately trying to short
the
supply hoping something explodes they can blame on the Schmidtt-Obama
trigger
;)

-Lasse
 
S

Spehro Pefhany

Jan 1, 1970
0
You've never seen anything IO designed, becuse it was all under NDA.
Crap is crap, and your circuit is crap. Try that with multiple ICs that
need reset on powerup. Then it becomes a shit design.

It's even a sh*t design with just one IC that needs reliable reset
under all conditions.
 
M

miso

Jan 1, 1970
0
The switching levels of Schmitt trigger inputs are never specified in
such a way as to be useful. Not only are they uncontrolled (in terms
of guarantees of chip operation or benign non-operation) but they
change with power supply voltage. With all that unspecified flipping
back and forth they should be called Schmitt Romney triggers.
(ducking)

A bulletproof POR circuit does need an absolute voltage reference in
it.


If you can guarantee that the power supply will always be either zero,
within spec or transitioning in a specified way between the two that's
true. For every other (almost every real) situation, you need a proper
POR circuit. Digital circuits can get into untold mischief in brownout
situations, for example.


Time + tightly specified absolute voltage levels for on/off.

The bullet proof POR has crude and precise references. A bandgap may not
be awake yet, so usually you have a crude sensor that flips when there
is enough supply voltage to support the operation of the precise
reference. Then you compare the bandgap to a crude reference like a VTN,
just to make sure it is on its way to waking up, then use a one shot to
give the bandgap time to settle. Then the POR uses the precise reference.

There is often a crude lockout that insures the supply is a VTN at
least, if not a VTN plus VTP.

Timing circuits will have a parasitic diode from the supply rail to pull
the timing cap down to at least a diode drop from ground. This is to
insure the supply didn't get slammed to ground then suddenly brought
back up again. That is, you want to insure the timing cap has as little
residual charge on it as possible.
 
J

Jamie

Jan 1, 1970
0
Spehro said:
The switching levels of Schmitt trigger inputs are never specified in
such a way as to be useful. Not only are they uncontrolled (in terms
of guarantees of chip operation or benign non-operation) but they
change with power supply voltage. With all that unspecified flipping
back and forth they should be called Schmitt Romney triggers.
(ducking)

A bulletproof POR circuit does need an absolute voltage reference in
it.




If you can guarantee that the power supply will always be either zero,
within spec or transitioning in a specified way between the two that's
true. For every other (almost every real) situation, you need a proper
POR circuit. Digital circuits can get into untold mischief in brownout
situations, for example.




Time + tightly specified absolute voltage levels for on/off.

Then I must be the lucky one, I do PIC, AVR and now looking at some TI
stuff at work. All which uses a simple RC reset . They all reset
properly.

This is dealing with a supply that is there when switched on, like
a battery for example or ready operating source via a switch..

I just think some designs are over killed for such a simple approach
that works in most cases. It gets to the point where people just think
that if you had to do it once, then it means it's un holy to not ever do
it again.

And yes, I explore the options of a design, I just don't assume we
need to through in the kitchen sink to start with, just because that is
what we did last time and the time before that etc..

I get the idea that some designers are accustom to practices only
because it was done elsewhere but never really dig down deep to find out
why.

WHat ever..


Jamie
 
J

Jamie

Jan 1, 1970
0
Spehro said:
Argh, terrible. No hysteresis, no time delay, and assert / release
reset levels are not controlled. Reset is never asserted if the supply
comes up slowly, a short blip down that disrupts the digital stuff may
not generate a reset pulse.

In fact, if the reset input is a ST, this might actually be worse than
an RC directly on the reset pin.

http://www.onsemi.com/pub_link/Collateral/MAX809S-D.PDF

About a dime in quantity. Check it out, if it works, spec it, sleep
well at night.


Best regards,
Spehro Pefhany
I can't believe what you're saying..

I bet you haven't tried that type of circuit..

It goes to show how people open their mouth and insert..


That circuit is a classic used in many and I mean many of reset circuits.


I'm glad I don't take a good lot of you here seriously..


jamie
 
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