User Manual

Bluegiga Proprietary Version 1.0
18
10 USB INTERFACE
WRAP THOR 2022-1 USB devices contain a full-speed (12Mbits/s) USB interface, capable of
driving a USB cable directly. No external USB transceiver is required. The device operates as a
USB peripheral, responding to requests from a master host controller such as a PC. Both the OHCI
and the UHCI standards are supported. The set of USB endpoints implemented behave as specified
in the USB section of the Bluetooth specification v1.1 and v1.2 part H2.
As USB is a master-slave orientated system, WRAP THOR 2022-1 only supports USB slave
operation.
10.1 USB D
ATA CONNECTIONS
The USB data lines emerge as pins USB_D+ and USB_D- on the package. These terminals are
connected to the internal USB I/O buffers of WRAP THOR 2022-1 and therefore have a low output
impedance. To match the connection to the characteristic impedance of the USB cable, series
resistors must be connected to both
USB_D+ and USB_D-.
10.2 USB P
ULL-UP RESISTOR
WRAP THOR 2022-1 features an internal USB pull-up resistor. This pulls the USB_D+ pin weakly
high when WRAP THOR 2022-1 is ready to enumerate. It signals to the PC that it is a full-speed
(12Mbit/s) USB device.
The USB internal pull-up is implemented as a current source, and is compliant with 7.1.5 of the
USB specification v1.1. The internal pull-up pulls USB D+ high to at least 2.8V when loaded with a
15Kohm-5% pull-down resistor (in the hub/host) when VDD_PADS=3.1V). This presents a
thevenin resistance to the host of at least 900 ohms. Alternatively, an external 1.5Kohm pull-up
resistor can be placed between a PIO line and D+ on the USB cable. The firmware must be alerted
to which mode is used by setting Persistent Store Key PSKEY_USB_PIO_PULLUP (0x2d0)
appropriately. The default setting uses the internal pull_up resistor.
10.3 P
OWER SUPPLY
The minimum output high voltage for USB data lines is 2.8V. To safely meet the USB
specification, the voltage on terminals must be an absolute minimum of 3.1V. CSR recommends
3.3V for optimal USB signal quality.
10.4 S
ELF-POWERED MODE
In self-powered mode, the circuit is powered from its own power supply and not from the VBUS
(5V) line of the USB cable. It draws only a small leakage current (below 0.5mA) from VBUS on
the USB cable. This is the easier mode for which to design for, as the design is not limited by the
power that can be drawn from the USB hub or root port. However, it requires that VBUS be
connected to WRAP THOR 2022-1 via a resistor network (Rvb1 and Rvb2), so WRAP THOR
2022-1 can detect when VBUS is powered up. WRAP THOR 2022-1 will not pull USB_D+ high
when VBUS is off.