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To the casual reader: this is not a problem you are going to encounter by using your usual USB device such as a USB flash drive (thumb stick). This is an issue when using certain USB hubs which are self-powered or other self-powered devices. What this means is the device you plug into Montage's "To Device" port will itself have a power supply connected. This mostly deals with some of the unsupported configurations like using an external USB-connected monitor and using hubs which themselves you may elect to use powered hubs. Not a common use-case, so do not take this as being alarmist for most users.
To Yamaha: although it would be great for downstream devices to "play nice" and only treat VBUS from Yamaha's USB port as a source - and never itself source power on VBUS - this is a lofty expectation. Yamaha cannot control what a user does - or what other devices do - and so any USB port designed in the world must assume the worst - or fall victim to misbehaving devices. In this case, Yamaha as a victim does not blow up - it just places Montage in a mode whereby the user cannot power-off Montage without disconnecting the USB device first.
The issue is this: VBUS allows current to flow from a downstream connected target device and "backfeed" VBUS. Therefore, if a self-powered downstream device is itself supplying power to VBUS - then when you turn the power off Montage - the VBUS voltage rail on Montage will still be powered. This "residual voltage" will prevent Montage from powering down because apparently the internal power tree is such that 5V (VBUS) is also a source for the regulators that supplies power to everything else.
The fix is fairly simple - to isolate VBUS from the rest of the power domains. It's an artwork spin - so that's a bad thing (not a firmware fix). Maybe there's a conditioning component already there that can be "swapped out" so it's a BOM-only fix. Probably not, probably requires a netlist change.
This is something to consider for follow-on products. Also something to place in on the radar of possibilities for support calls if you get a "I have a USB device plugged in, now I can't power down Montage" type call.
Also, would be good to add to the test matrix. Both residual voltage on USB when powered off (coming from Montage) as well as isolation-type-testing when power is supplied on VBUS from an external source.
Posted : 06/05/2017 11:02 pm
Interesting but they really don't recommend connecting anything additional to the Montage. Just for the record. From the Reference Manual...
[USB To Device] Terminal
Used to connect this instrument to a USB flash memory device. This lets you save data created on this instrument to an external USB flash memory device and load data from a USB flash memory device to the instrument. Save/Load operations can be performed: [UTILITY][Contents] [Store/Save] or [Load].
NOTE
• Only USB flash memory devices can be recognized by this instrument. No other USB devices (such as a hard disk drive, CD- ROM drive and USB hub) can be used.
• The instrument supports the USB 1.1 to 3.0 standard. However, note that the transfer speed differs depending on the data type and the condition of this instrument.
Posted : 06/05/2017 11:16 pm
Topic starter
Right - exactly. My disclaimer I gave earlier was that this is an issue using Montage counter to recommendations. Currently from a system design perspective there are plenty of safe-guards in Montage to protect it from use outside of intended use. I'm suggesting to add this to the list in the design since a) it's cheap b) it's easy c) it helps reduce service calls and possible warranty events d) it's good practice.
The unwanted response to an unsupported configuration can be solved with a tiny component costing pennies.
If the complexity or cost were higher, I wouldn't recommend it and would instead stick to the guns of not, in any way, accommodating/handling unsupported use.
Posted : 06/05/2017 11:26 pm
