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Hello,
I have a MCBSTR9 dev board and have implemented a USB virtual serial port using the example from ST's website. It works fine as long as the jlink JTAG device is plugged in before I plug the board's usb port into my PC. If the jlink device is not plugged in, the device does not enumerate properly. Windows reports a vender and device id of 0x0000, and no driver will load. I have jumper J4 on the board set to ground. Is there a jumper or setting somewhere I am missing? Has anyone experienced this before? Thanks,
-Steve
This snippet from www.sss-mag.com/usb.html might explain your problem.
I note a typical current of 150mA for this board from the Keil website. http://www.keil.com/mcbstr9/specs.asp
It looks like you need the two cables on your machine!
There are several hardware requirements for devices that are placed on the USB bus. Five volts is the nominal supply voltage on the bus. A device that requires 100mA or less can be powered from the host or any hub, provided that the total available power hasn't already been exhausted by other devices. A device on the bus can draw up to 500mA from it. However, not all USB hosts (especially a battery powered PC) or bus-powered hubs will allow a device to draw more than 100mA from the bus. For this reason, a USB device that draws more than 100mA should, in most cases, be self-powered
Thank you Patrick.
Thanks, makes sense.
"A device on the bus can draw up to 500mA from it."
I thought that 500mA was only available after negotiation?
ie, any "dumb" device can just plug into a USB socket and draw up to 100mA(?) but, if over 100mA(?) is required, the device has to specifically request it using the USB protocol.