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output_float and low signal input?
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hmmpic



Joined: 09 Mar 2010
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output_float and low signal input?
PostPosted: Thu Feb 24, 2022 11:16 am     Reply with quote

output_float and low signal input?


18F26K22 is connected to 3,8V->4,2V.
One pin is connected to one pin on a blackbox.

The pin on the blackbox is a reset pin. When floating there is 1,5V. When L(<0,4V) the blackbox is in reset mode.
On the PIC, i use the output_float to release the reset, and output_low to do the reset, NO output_high!
A more correct way was using a fet and driving H or L, or other thing in between. But in this case it is connected direct.

What is the situation when output_float is used, and the pin is connected to 1,5v. Normal TTL 0.8-2.0 is noise margin.

help?


Last edited by hmmpic on Thu Feb 24, 2022 11:37 am; edited 1 time in total
temtronic



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PostPosted: Thu Feb 24, 2022 11:32 am     Reply with quote

Do you know if the 'blackbox' HAS a pullup resistor on the '_reset' pin ?
hmmpic



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PostPosted: Thu Feb 24, 2022 11:34 am     Reply with quote

No
temtronic



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PostPosted: Thu Feb 24, 2022 1:24 pm     Reply with quote

hmm, no pullup or no, don't know.....
in any event, a floating pin will 'float' up to the VDD on the pullup resistor.
without a pullup resistor the pin though it could be almost anything.. static, EMI, RF could induce a voltage onto it
hmmpic



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PostPosted: Thu Feb 24, 2022 3:11 pm     Reply with quote

The BB output is from a radio module, no spec other then floating pin have 1,5v and <0,4v perform a reset...

I put a FET in.
Ttelmah



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PostPosted: Fri Feb 25, 2022 1:00 am     Reply with quote

As a comment, you don't tell us what pin or what PIC. Many modern PIC's
have built in programmable pullups on most of their pins. Even the older
ones usually have these on PortB. Use a pin with a built in pullup, and
enable this, and problem gone....
The FET is a tidier solution, but costs parts.
hmmpic



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PostPosted: Fri Feb 25, 2022 1:15 am     Reply with quote

The long story is.
Previous design have a diode between, and i just use output_high and output_low.
Problem in new design i the <0,4 to set it low, the diode have about 0,28V but the data sheet for the PIC 18F26K22 say uol MAX=0,6
I never seen so high low voltage, more <0,1. Then i have 0,1+0,3=0,4 maybe it ok, but in production is is not ok.
Therefore i removed the diode...

Pull up is not nice here. PIC is supplied with 4,2, pull up in pic is about 100K, so about >3,5V at the pin.
The blackbox say when floating there are 1,5v at the pin... Don't want it to break.

I hate to put a fet and 2 resistor...
Ttelmah



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PostPosted: Fri Feb 25, 2022 1:26 am     Reply with quote

Some devices use three states of pins like this. 'HIGH', 'FLOATING', and
'LOW'. However key is what does the device data sheet say about the
RESET pin?. Does it say that the pin has to be left floating?. It is quite
dangerous design if it must. The data saying that a floating pin will typically
float to 1.5v, really tells you nothing except what the pin floats to. It
doesn't tell you what the maximum voltage on the reset is, or what it
should be taken to when not being reset. Usually I'd expect there to be
an example circuit for the reset connection, or figures saying what voltages
it can go to, or a diagram showing the levels.
hmmpic



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PostPosted: Fri Feb 25, 2022 1:40 am     Reply with quote

Simcom LTE modem, using chipset from Qualcomm 9205 LTE Modem.
Simcom hardware datasheet is always so bad (china English), never know what is going on, and sometime totally wrong.

https://www.qualcomm.com/products/qualcomm-9205-lte-modem/documentation
Ttelmah



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PostPosted: Fri Feb 25, 2022 2:08 am     Reply with quote

If you pull their hardware design sheets for the chip, it does show this
pulled down by a transistor. However it also shows an external transorb on the
pin to prevent external RF from causing a problem. I'd suggest you follow
this reference design....

It actually says:
Quote:

The RESET pin has been pulled up to 1.8V with a resistor internally.
So it does not need to be pulled up externally. It is strongly recommended
to put an ESD protection diode close to the RESET pin. Please refer to the
following figure for the recommended reference circuit.
hmmpic



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PostPosted: Fri Feb 25, 2022 2:14 am     Reply with quote

And where do you found that gold?

Maybe a link?
Ttelmah



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PostPosted: Fri Feb 25, 2022 2:39 am     Reply with quote

I searched for 9205 LTE reference design on the Qualcom site.
newguy



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PostPosted: Fri Feb 25, 2022 6:37 am     Reply with quote

I've designed SIMCOM modems into a few products before and I have to agree with Ttelmah: find the hardware reference design document and you won't have issues.

I agree that their documentation isn't the best I've seen, but, taken as a whole, it is complete.
Ttelmah



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PostPosted: Fri Feb 25, 2022 8:23 am     Reply with quote

It's not available as a link. You have to fill in an NDA, amd they send it to you.
I just asked their tech department for it (since I am already registered with
them), and they sent it within a few minutes.
hmmpic



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PostPosted: Fri Feb 25, 2022 8:35 am     Reply with quote

All ok i have put the fet in.
In Simcom datasheet they say 1.5v and in other they say 1.8.
All this just because i want to do as simple as possible:-)
Thanks for the support:-)
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