Micrel, Inc.
MIC2555
VTRM
VBUS Comparators
VBUS Comparators monitor the voltage level of VBUS. As
described in the USB On-The-Go Supplement, VBUS not
only supplies power but also is used to signal various
operational conditions as part of the SRP protocol.
Depending upon the voltage on VBUS, three states of
operation can be defined:
Controlled by
Register Bits
1.5k
1.5k
DAT_VP
SE0_VM
OE_INT/
•
•
•
VBUS Valid
Session Valid
Session End.
RCV
Single
Ended
VBUS Valid Comparator
Decoder
This comparator is used by an A-device to determine
whether the voltage on VBUS is at a valid level for
operation. The minimum threshold for the VBUS valid
comparator is 4.4V. Any voltage on VBUS below the
threshold of the VBUS valid comparator is considered a
fault. During power up, it is expected that this
comparator’s output will be ignored.
1.5k
1.5k
Controlled by
Register Bits
Session Valid Comparator
Resistors and Circuitry associated with D+, D– Pins
The session valid comparator determines when VBUS is
high enough for a session to start. Both the A-device and
B-device use this comparator to detect when a session
is being started. The A-device also uses this comparator
to indicate when a session is over.
ID Detector
The ID function, defined within the USB On-The-Go
supplement, represents a new addition to the USB
standard. It is used to detect the presence or removal of
a peripheral device as well as to differentiate between
USB and non-USB peripherals. ID is unique to the mini-
USB connectors and receptacles.
The session valid window for an A-device is 0.8 – 2.0V
while the session valid window for a B-device is 0.8 –
4.0V. Because these ranges overlap the A-device
window is typically chosen to service both requirements
and a single comparator can be used. This is the case
with MIC2555.
MIC2555’s ID Detector is operational in both the Active
and Suspended power modes, and differentiates
between three conditions:
Session End Comparator
ID Pin Condition
Device Status
VID
The USB OTG Supplement specifies that a B-device
cannot initiate SRP unless VBUS is below the B-device
Session End threshold of 0.8V. Monitoring VBUS with a
comparator will give an exact and positive determination
of when VBUS has dropped below 0.8V, but the USB
OTG supplement allows that the 0.8V limit can also be
inferred, by discharging VBUS through a low value resistor
for a predetermined period. The MIC2555 provides both
a session-end comparator and a discharging resistor. To
accommodate either technique, the designer can use
them individually or, in concert as he so chooses.
Floating
No device present
VID > 0.85VBAT
USB device
present
Grounded
VID < 0.15VBAT
Grounded through
a Resistor
Non-USB device
present
0.15VBAT < VID 0.85VBAT
<
Here, ‘Grounded through a Resistor’ means a resistor of
considerable value, typically 100kꢀ. The ID
comparators are set to ignore the modest resistances
contributed by the cables and connector contacts.
a
That a non-USB device is present (ID = resistive) is
inferred from the interrupt register by the indication of an
interrupt (ID has changed state) and that neither ID =
GND or ID = Float are true. Viewing the Interrupt source
register will give the real time status of the ID
comparator outputs. Viewing this register is necessary to
determine the true state of affairs as insertion of the USB
plug can produce multiple rail-to-rail transitions. These
Pull-up/down Resistors on D+, D-
MIC2555 supplies the pull-up and pull-down resistors for
termination
and
signaling
required
by
USB
specifications. These resistors are integrated within the
chip and switched into the circuit, as needed, via
individual control bits in the control registers.
M9999-060805
(408) 955-1690
June 2005
13
MICREL [ MICREL SEMICONDUCTOR ]
