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USB-UART bridge IC - powering VDDIO when VDD unpowered

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HwAoRrDk:
I have decided to go with using a pair of 74LVC1T45 in SOT23-6. There was enough room in the layout to squeeze them in, and the chips are pretty cheap (pennies each). :)

Both DIR pins connected directly to ground, giving B as input, A as output; on one IC, micro's TX in to B and A out to HT42B534's RX, with VCCB on 5V and VCCA on VDDIO (i.e. VBUS); the other with HT42B534 TX in to B and A out to micro's RX, with VCCB on VDDIO and VCCA on 5V rail.

When my device is powered but USB is unconnected, UART lines will be tri-stated but RX will be asserted high by an internal pull-up on the micro (and TX is actively driven). If for some reason the opposite scenario occurs - where USB is plugged in but device is otherwise un-powered - the HT42B534 UART pins will be floating, but that shouldn't be a concern as the user won't be able to do anything with the device. I suppose there is the possibility of garbage characters being transmitted over the serial link, but who cares? ;D

Ian.M:

--- Quote from: HwAoRrDk on November 08, 2018, 04:36:45 pm ---If for some reason the opposite scenario occurs - where USB is plugged in but device is otherwise un-powered - the HT42B534 UART pins will be floating, but that shouldn't be a concern as the user won't be able to do anything with the device. I suppose there is the possibility of garbage characters being transmitted over the serial link, but who cares? ;D

--- End quote ---
Windows may care!  You'd be surprised what it can mistake as a serial mouse at bootup or when enumerating new hardware.
Usually its GPS receivers that are mis-enumerated but other low baud rate random datastreams can also cause issues.

See https://what.thedailywtf.com/topic/17599/serial-mouse-still-in-windows-10-trigger-warning-disabled

If you can sneak in a pullup on TXD from your MCU on the bridge side of the buffer I strongly recommend fitting one.

Peabody:
The TI datasheet for the LVC chip says A&B go tristate if either Vcc supply pin is at "ground".  As opposed to floating, I assume.  If USB is unconnected, or your project supply is off, will the relevant Vcc be grounded?

I just wonder whether the flexibilty given to you by the separate VDDIO pin wouldn't allow you to connect the adapter directly to the MCU and avoid the need for two extra chips.  But I'm not really familiar with the Holtek, and maybe that won't work.

HwAoRrDk:

--- Quote from: Ian.M on November 08, 2018, 08:47:53 pm ---Windows may care!  You'd be surprised what it can mistake as a serial mouse at bootup or when enumerating new hardware.
Usually its GPS receivers that are mis-enumerated but other low baud rate random datastreams can also cause issues.

--- End quote ---

I had heard of that. Yes, kinda stupid. :) My device is highly unlikely to be plugged-in during bootup/restart, though - it's something you'll plug in only as-and-when-necessary to tweak settings via a serial console, not remain connected at all times. But, still, I can appreciate the risk of this problem occurring...


--- Quote from: Ian.M on November 08, 2018, 08:47:53 pm ---If you can sneak in a pullup on TXD from your MCU on the bridge side of the buffer I strongly recommend fitting one.

--- End quote ---

You know, looking again at the datasheet for the Holtek (for the millionth time), I have come to the conclusion that it must have internal pull-up resistors on the RX/TX pins, as evidenced by a section of the DC Characteristics table entitled 'I/O Pins Pull-high Resistance' (figure given is typical 30K @ 5V VDD). The UART pins are I/O pins, are they not? All the other I/O pins - on higher pin-count versions - are flow-control and active-low too, so why would they put them on those and not also the actual RX/TX? So I don't think I need to worry about adding pull-ups on the bridge side.


--- Quote from: Peabody on November 08, 2018, 09:33:20 pm ---The TI datasheet for the LVC chip says A&B go tristate if either Vcc supply pin is at "ground".  As opposed to floating, I assume.  If USB is unconnected, or your project supply is off, will the relevant Vcc be grounded?

--- End quote ---

This has been nagging the back of my mind the last couple of days. :-\

When they say "at ground", I believe they mean that either of the VCCx pins is at the same potential as the GND pin. The datasheet for the NXP version of the 74LVC1T45 even specifically states this: "When either VCC(A) or VCC(B) is at GND level, the device goes into suspend mode" - suspend mode being with A & B tristated.

The whole device will share a common ground, so, I suppose the question is, when USB is disconnected, will VBUS be at the same potential as ground? I don't know. I am going to be using a TVS diode package on the USB (ST USBLC6-2), so maybe the VBUS would be gently pulled to ground via the leakage current of the diodes? :-//

Peabody:

--- Quote from: HwAoRrDk on November 10, 2018, 04:37:24 pm ---
--- Quote from: Peabody on November 08, 2018, 09:33:20 pm ---The TI datasheet for the LVC chip says A&B go tristate if either Vcc supply pin is at "ground".  As opposed to floating, I assume.  If USB is unconnected, or your project supply is off, will the relevant Vcc be grounded?

--- End quote ---

This has been nagging the back of my mind the last couple of days. :-\

When they say "at ground", I believe they mean that either of the VCCx pins is at the same potential as the GND pin. The datasheet for the NXP version of the 74LVC1T45 even specifically states this: "When either VCC(A) or VCC(B) is at GND level, the device goes into suspend mode" - suspend mode being with A & B tristated.

The whole device will share a common ground, so, I suppose the question is, when USB is disconnected, will VBUS be at the same potential as ground? I don't know. I am going to be using a TVS diode package on the USB (ST USBLC6-2), so maybe the VBUS would be gently pulled to ground via the leakage current of the diodes? :-//

--- End quote ---

For what it's worth, I used the MC74VHCT50A in a similar situation using the CP2102 adpater.  That part also claims tristating when Vcc is grounded.  I specifically tested for that, and found that there was no tristating when Vcc is floating.  In that example, the solution was to drive the 50A's Vcc from the 3.3V regulated output of the CP2102, which itself goes to ground when the CP2102 is unpowered.  It's not low-impedance ground, but close enough.  So, you're going to have to physically test this in the configuration you want to use.  Also, it's not clear to me from the datasheet whether you still get tristating if one Vcc is grounded but the other is floating.  Taken literally, that should be the case, but I think you would want to test that.

So the LVC behavior is going to depend on what happens to the Vccx sources when they are powered down.  But if those sources don't go to ground when off, you may be able to power from other sources that do behave as you need.  For example, you might be able to power from one of the Holtek outputs, such as DTR, instead of VBUS. You just need something that is normally high when On, but will sink a little current when power is Off.




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