User's Manual

FLC-BTM501 Datasheet
Flaircomm Microelectronics Confidential
-18-
7.1.5 of the USB specification v1.2. The internal pull-up pulls USB_DP high to at least 2.8V when
loaded with a 15kΩ ±5% pull-down resistor (in the hub/host) when VDD =3.1V. This presents a
Thevenin resistance to the host of at least 900Ω. Alternatively, an external 1.5kΩ pull-up resistor can
be placed between a PIO line and DP on the USB cable.
4.7.2.1 Self-Powered Mode
In self-powered mode, the module 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, 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
module via a resistor network (Rvb1 and Rvb2), so the module can detect when VBUS is powered
up. The module will not pull USB_DP high when VBUS is off.
Self-powered USB designs (powered from a battery or LDO) must ensure that a PIO line is allocated
for USB pull-up purposes. A 1.5KΩ 5% pull-up resistor between USB_DP and the selected PIO line
should be fitted to the design. Failure to fit this resistor may result in the design failing to be USB
compliant in self-powered mode. The internal pull-up in the module is only suitable for bus-powered
USB devices, e.g., dongles.
Figure 6: USB Connections for Self-Powered Mode
Note:
USB_ON is shared with the module PIO terminals.
Identifier
Value
Function
R
s
27Ω Nominal
Impedance matching to USB cable
Rvb1
22kΩ 5%
VBUS ON sense divider
Rvb2
47kΩ 5%
VBUS ON sense divider
Table 6: USB Interface Component Values