Learn USB Protocol
USB is proliferating due to the fact that RS232 is disappearing and that the end user’s experience is very simple.
Everything is strictly specified. The USB cable and connector have been defined by the USB consortium all the
way down to the USB symbol dimensions. This provides confidence that any device will connect to another
Hot pluggable/Automatic configuration – Enumeration (which we will get into detail later) enables a device to
be plugged in and its operating parameters communicated to the host. Also, the USB spec takes into account
inrush currents for devices that are going to draw their power from the bus.
When we refer to lossless data transfers we are referring to the fact that the protocol provides a mechanism to
retransmit data if required in all but one transfer type. This enables the data transfers to be reliable and we will
go over that later in the course. There are three transfer speeds for USB 2.0. The transfer speed number listed
here is the bandwidth on the bus. This is not directly equal to data throughput. The throughput is less than these
numbers for any USB device due to USB overhead. Later in the presentation we will show some numbers for
The USB 2.0 protocol is fully backwards compliant. This is why the C8051F32x and 34x devices are full speed
devices and also USB 2.0 compliant.
The 500 uA listed here applies to devices that are consuming power from the USB host. If a device is fully selfpowered, the Suspend mode current is not applicable because it is not drawing power from the USB host. Note
that the USB 3.0 specification allows for 2.5mA suspend current. Another aspect of the newer specification is
that the micro connectors are now the preferred connector solution. The USB implementers forum deprecated
the use of the mini-A and mini-AB (May 23, 2007).
The benefit of this aspect of the System Design Guide means that user’s don’t need to buy external hardware and
developers know that USB is an interface their customers will definitely have.
Existing USB root hubs and external hubs in the field (and still manufactured by most companies) that are USB
2.0 compliant based on the pre-ECN spec will still enforce the 500 uA limitation. Only newly certified hubs
would need to comply with the post-ECN spec. In summary, it is best to make your product obey the stricter rule
of 500 uA for the widest compatibility with existing PCs and hubs.