Datasheet

© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com
FUSB2805 • Rev. 1.0.3 6
FUSB2805 — USB2.0 High-Speed OTG Transceiver with ULPI Interface
OTG Module
The OTG module contains several sub-blocks that
provide the functionality required by the USB On-The-
Go Rev. 1.3 supplement. Specifically, it provides the
following circuits:
 ID detector to sense the ID pin of the micro-USB
cable. The ID pin dictates which device is initially
configured as the host and which is configured as
the peripheral.
 V
BUS
comparators to determine the V
BUS
voltage
level. This is required for Host Negotiation Protocol
(HNP) and Session Request Protocol (SRP).
 Resistors to temporarily charge and discharge
V
BUS
. This is required for SRP.
ID Detector
Detects which end of the mini-USB (or micro-USB) cable
is plugged in. The ID detector must first be enabled by
setting the ID_PULLUP register bit to 1b. If the
FUSB2805 senses a value on ID different from the
previously reported value, an RX CMD status update is
sent to the USB link controller or an interrupt is asserted.
If the micro-B end of the cable is plugged in, the
FUSB2805 reports that ID_GND is logic 1 and the USB
link controller changes to peripheral mode.
If the micro-A end of the cable is plugged in, the
FUSB2805 reports that ID_GND is logic 0 and the USB
link controller changes to host mode.
The ID pin has a weak pull-up resistor (400kΩ) to avoid
floating conditions. This resistor is connected when
ID_PULLUP register bit is 0. If the application does not
use the ID pin, tie this pin to V
CC3V3
.
V
BUS
Comparators
The FUSB2805 provides three comparators for
detecting the V
BUS
voltage level, as listed in Table 9 and
Table 12. The comparators are as follows:
V
BUS
-Valid Comparator
This comparator is used by an A-device (or host) to
determine whether or not the voltage on V
BUS
is at a
valid level for operation. The minimum threshold for
the V
BUS
valid comparator is 4.4 V. Any voltage on
V
BUS
below this threshold is considered a fault.
During power-up, the comparator output is ignored.
Session-Valid Comparator
The session-valid comparator is a TTL-level input that
determines when V
BUS
is high enough for a session
to start. Both the A-device and the B-device use this
comparator to detect when a session is being started.
The A-device also uses this comparator to determine
when a session is completed. The session valid
threshold is between 0.8 V to 2.0 V.
Session-End Comparator
The session-end comparator determines when V
BUS
is below the B-device session-end threshold of 0.2 V
to 0.8 V. The B-device uses this threshold to
determine when a session has ended.
SRP Charge and Discharge Resistors
The FUSB2805 provides on-chip resistors for short-term
charging and discharging of V
BUS
. These are used by
the B-device to request a session, prompting the A-
device to restore V
BUS
power. First, the B-device
verifies that V
BUS
is fully discharged from the previous
session by setting the DISCHARGE_VBUS register bit
to 1b and waiting for SESS_END to be 1b. Then the B-
device charges V
BUS
by setting the CHARGE_VBUS
register bit to 1b. The A-device detects that V
BUS
is
charged above the session-valid threshold and starts a
session by turning on V
BUS
power.
Analog Reference Module
The analog reference module provides stable internal
voltage and current references for biasing internal
analog circuitry. This module requires an accurate
external reference resistor. A value of 12 k 1%
resistor is required between the R
REF
pin and GND.