USB Device Disconnect-On-Deman

The uPSD32xx family, from ST, consists of Flash programmable system devices with a 8032 Microcontroller Core. Of these, the uPSD3234A-40 and uPSD3254A-40 are notable for having a complete implementation of the USB hardware directly on the chip. This USB hardware complies with the Universal Serial Bus Specification Revision 1.1, and has a low-speed (1.5Mbit/s) device capability. Up to 8 bytes can be transmitted or received in one packet. The typical sustainable bus throughput is up to 6 KBytes per second on USB1.1, up to 11 KBytes per second with certain types of USB2.0 host.There are times, such as to force a reset, when a physical USB device would be physically disconnected,and then reconnected. This application note presents a possible way of emulating, on demand by software,a disconnect of an embedded USB device, within the uPSD32xx, without needing to add any active components.
USB DEVICE CONNECT AND DISCONNECT SIGNALLING
When no function is attached to the downstream port of the host or hub, its pull-down resistors cause both D+ and D- to be pulled below the single-ended low threshold of the host or hub port. This creates an SE0 state on the downstream port.
A connect condition is detected when the hub detects that one of the data lines is pulled, for more than 2.5μs, above its VIH threshold (2.0V, according to Universal Serial Bus Specification Revision 1.1). Figure 1 depicts this for a low-speed USB device, and Figure 2 for a full-speed one.