Device operable as a host
An accessory device ( 12 ), such as a DVB-T receiver is operable as a USB host to a mobile telephone ( 11 ) to which it is connected via a USB bus ( 13 ). When acting as a host, the accessory device ( 12 ) provides a 4V supply voltage on V BUS to the mobile telephone, which includes a regulator ( 18 ) regulating the voltage to 3.3 Volts. When a host device (PC) is also connected to the USB bus ( 13 ), the voltage on V BUS rises, and this is detected by a comparator ( 21 ). In response, the accessory device ( 12 ) sends a USB reset command sends its USB lines tri-state, and ceases to supply V BUS , thereby relinquishing host status. Loss of the PC ( 10 ) is detected by detecting a low voltage on V BUS , following which the accessory device reassumes host status.
1 . A device operable as a host device comprising: a port connected to a bus and, means for detecting the presence of another host connected to the bus and for relinquishing host status in response thereto.
2 . A device as claimed in claim 1 , including a power supply for providing a supply voltage on a voltage supply line of the bus.
3 . A device as claimed in claim 2 , in which the supplied voltage is less than a minimum allowed voltage for the voltage supply line of the bus.
4 . A device as claimed in claim 1 , in which the detecting means includes means for detecting a change in voltage on a voltage supply line of the bus, thereby detecting the presence of the other host.
5 . A device as claimed in claim 4 , in which the change is an increase.
6 . A device as claimed in claim 4 , in which the detecting means includes a comparator.
7 . A device as claimed in claim 1 , in which the device is arranged for causing at least some lines of the port to be forced tri-state on detecting the presence of another host.
8 . A device as claimed in claim 1 , including means to send a reset command via the bus in response to detecting the presence of another host.
9 . A device as claimed in claim 1 , including means for detecting the loss of the other host, and for reassuming host status in response thereto.
10 . A device as claimed in claim 9 , in which the loss detecting means includes means for detecting a reduction in voltage on a voltage supply line of the bus, thereby detecting loss of the other host.
11 . A device as claimed in claim 1 , in which the device includes first and second ports.
12 . A device as claimed in claim 1 , in which the device is a battery pack.
13 . A device as claimed in claim 1 , in which the device includes a digital video broadcast receiver.
14 . A device as claimed in claim 1 , wherein the device is operable as a USB host and having at least one USB port connectable to a USB bus, the presence detecting means being means for detecting the presence of another USB host.
15 . A device as claimed in claim 1 , operable as a USB host device, the device including first and second USB ports connected directly to each other and to a USB host module.
16 . A device as claimed in claim 1 , wherein the device is a mobile telecommunications device comprising first and second USB ports each connected to a USB bus, and one of a) a USB non-host module and b) a USB dual-mode module connected to the USB bus.
17 . A system including a device as claimed in claim 1 , and a host device connected to the port.
18 . A system as claimed in claim 17 , including a non-host device connected to a second port of the device.
19 . A method of operating a device operable as a host, the method comprising: detecting a change in voltage on a voltage supply line forming part of a bus; and relinquishing host status in response thereto.