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PSoC 5, 3 & 1 MCU

KeYp_290406
New Contributor II

The default mode is high impedance but this doesn't work well with a self powered device. If you pull the USB connector it can take a long time for the VBus line to drift low. It's a legal selection for the pin but don't want to cause any unwanted issues although I think an SIO is fairly hardy.

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Len_CONSULTRON
Honored Contributor II

KeYp,

It should work with virtually all USB ports.

If you're willing to use a PSoC5 comparator, you can detect when Vbus drops below xxx volts and launch a ISR to detect when the USB is disconnected and when Vbus rises above xxx volts you can launch an ISR to detect when the USB is connected.

This prevents the need to connect VBus to a ADC and for the ADC to scan/poll the VBus voltage.

Len

Len
"Engineering is an Art. The Art of Compromise."

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Len_CONSULTRON
Honored Contributor II

KeYp,

I don't see why you couldn't use a pull-down on a SIO line monitoring Vbus. 

Question: What is the hold-up cap value on this line?  Many Vbus lines recommend at least 1uF.  Therefore you RC factor is Rpull-down * Chold-up.

Consider this:  If the current load on Vbus downstream is 10mA @ 5V then the effective Rload of the circuit is 500 ohms already as a effective pull-down.

May I ask what you are trying to do with this circuit?

The PSoC5 has a low-voltage detect (LVD) circuit that is threshold settable.

Len

Len
"Engineering is an Art. The Art of Compromise."
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KeYp_290406
New Contributor II

I am monitoring the VBus to detect when the device is connected to USB. The device is self powered so it's possible for the USB cable to be plugged and unplugged while the device is operating. I noticed that in the default high impedance drive mode, the VBus line would take a long time to return to 0 when the USB disconnected. Setting the drive mode to pull-down seems to fix the problem without adding any external circuitry.

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Len_CONSULTRON
Honored Contributor II

KeYp,

It should work with virtually all USB ports.

If you're willing to use a PSoC5 comparator, you can detect when Vbus drops below xxx volts and launch a ISR to detect when the USB is disconnected and when Vbus rises above xxx volts you can launch an ISR to detect when the USB is connected.

This prevents the need to connect VBus to a ADC and for the ADC to scan/poll the VBus voltage.

Len

Len
"Engineering is an Art. The Art of Compromise."

View solution in original post

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KeYp_290406
New Contributor II

The USB component has a built-in feature for detecting VBus level. I think it just looks at the logic level of the pin you designate. They recommend using SIO since it can handle 5V.

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Len_CONSULTRON
Honored Contributor II

KeYp,

True.  Since you are just looking for a go/no_go signal.

Len

Len
"Engineering is an Art. The Art of Compromise."
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