Open collector to MCU input pin

ussername · Jan 25, 2014

Hello,
as I'm reading the documentation of certain GPS module, I found these lines:

If an external reset is desired, the signal must be either open collector or open drain without
any form of pullup. Do not pull this line high with either a pullup or a driven logic one. When
this line is pulled low, the module will immediately drop into hibernate mode with some loss
of data. When the external reset is released, the module will go through its normal power up
sequence provided the VDD supply is within specifications.

I see, when the transistor turns on, the signal state is low, and the reset of the module occurs. I don't understand, why there cannot be a pull-up resistor. And so, the signal won't be floating, while the transistor is off?

alexan_e · Jan 25, 2014

The manufacturer says don't use a pull-up or force a high state, I don't understand why there a need for an explanation in order to follow what the manufacturer suggests

bigdogguru · Jan 25, 2014

I would also like to add, posting a single quote out of context from the datasheet makes it difficult to determine the reasons.

It is not clear, from the quote alone, the specific purpose of this "line."

You would be much better served to always include a link to the specific datasheet from which the quote originates.

Or at the very least specify the manufacturer and model number of the device to which it refers.

BigDog

ussername · Jan 25, 2014

alexan_e said:
I don't understand why there a need for an explanation in order to follow what the manufacturer suggests

Good advice, though there also might be a reason for them to write it (maybe for fun, or maybe due to some MCU structure, I'm not aware of...). I wouldn't bother writing there if I wasn't interested in that cause...

- - - Updated - - -

bigdogguru said:
I would also like to add, posting a single quote out of context from the datasheet makes it difficult to determine the reasons.

It is not clear, from the quote alone, the specific purpose of this "line."

You would be much better served to always include a link to the specific datasheet from which the quote originates.

Or at the very least specify the manufacturer and model number of the device to which it refers.

BigDog

Sure, I only tried whether this problem isn't generaly well-known there.

The modul is TELIT JUPITER SE880 (docs). It's the first PDF (Hardware_User_Guide), page 30.

ussername · Jan 25, 2014

bigdogguru said:
I would also like to add, posting a single quote out of context from the datasheet makes it difficult to determine the reasons.

It is not clear, from the quote alone, the specific purpose of this "line."

You would be much better served to always include a link to the specific datasheet from which the quote originates.

Or at the very least specify the manufacturer and model number of the device to which it refers.

BigDog

Sure, I only tried whether this problem isn't generally well-known there.

The modul is TELIT JUPITER SE880 (docs). It's the first PDF (Hardware_User_Guide), page 30.

alexan_e · Jan 25, 2014

I don't believe a manufacture gives the drive specs of an input for fun, but in any case you can provide a link to a couple of datasheets, maybe there is an internal diagram that explains it

- - - Updated - - -

In page 13

it says

The JF2 NRESET pin is normally connected to a 0.33uF ceramic capacitor. The JF2 will
generate an internal reset as appropriate. No external reset signal needs to be applied to the
JF2.
If an external reset is desired, the signal must be either open collector or open drain without
any form of pullup. Do not pull this line high with either a pullup or a driven logic one. When
this line is pulled low, the JF2 will immediately drop into hibernate mode with some loss of
data.

So the manufacturer gives that direction because an external pullup or strong drive will interfere with the internal reset circuit

ussername · Jan 25, 2014

alexan_e said:
So the manufacturer gives that direction because an external pullup or strong drive will interfere with the internal reset circuit

Sorry, I'm a noob, can you refer to some similar schematics circuit or source info where there is obvious the interference?

alexan_e · Jan 25, 2014

I don't have a reference.

The value of a pullup must be high enough for the driver to be able to pull it down.
The manufactures has already included such a resistor internally (it seems so) and has an appropriate internal pull down switch , if you add another external pullup then the internal switch will have to sink more current and it may not be able to.

FvM · Jan 25, 2014

I don't have a reference.

The value of a pullup must be high enough for the driver to be able to pull it down.
The manufactures has already included such a resistor internally (it seems so) and has an appropriate internal pull down switch , if you add another external pullup then the internal switch will have to sink more current and it may not be able to.

There may be also additional reasons why an external pullup is not permitted. E.g. an internal pull-up is achieving a required reset delay with the existing circuit during power-on.

Good advice, though there also might be a reason for them to write it (maybe for fun, or maybe due to some MCU structure, I'm not aware of...). I wouldn't bother writing there if I wasn't interested in that cause...

You may want to question any requirement for each pin specified in the hardware manual. But what is it good for? Unless you have a serious reason to ignore a specification (and are able determine the consequences for devive operation), it's either waste of time or risking failure in operation.

Open collector to MCU input pin

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