2–22
The Univ ersal Se rial Bus co nne cts USB d evic es with a USB h ost, in this c ase, the Little Bo ard/P5x.
The USB physical interconnect is a tiered star topology, or tree, consisting of hubs and USB
devices. Each USB segment is a point-to-point connection between hubs or between hubs and USB
devices. The entire tree can support up to 127 USB devices. The USB interface stand ard is inten ded
for keyboards, mice, modems, digitizer pads, and other low- to medium-speed peripherals.
Each USB interface is implemented as a two-wire differential pair for data, a power wire, a ground
wire, and a shield wire. The USB port signals appear on J24, the Utility2 connector, as sho wn in
Table 2– 18.
Table 2– 18. USB Port Pinout on Utility2 Connector
J24 Pin Signal Name Function
15 USBPW R1 USB1 +5 Volt Power
17 USBP-1 USB1 Data-
19 USBP+1 USB1 Data+
21 USBGND1 Ground
23 SHIELD1 Cable Shield for USB1
16 USBPW R2 USB2 +5 Volt Power
18 USBP-2 USB2 Data-
20 USBP+2 USB2 Data+
22 USBGND2 Ground
24 SHIELD2 Cable Shield for USB2
The bus can run at 12 Mbits/se cond or 1.5 Mbits/seco nd, d epen ding o n a pu ll-up o n the p eriph eral
device. A 1.5 KOhm pull-up on the +data line sets the speed to 12 Mbits/second . A 1.5 KOhm p ull-
up on the -data line sets the speed to 1.5 Mbits/second.
The power to the peripheral device is current limited with a self-resetting fuse.