Maxim Integrated MAX3420E - Manual (Page 3)

operation.
3. How many endpoints does the MAX3420E support?
The MAX3420E contains four endpoints:
EP0, bidirectional CONTROL endpoint, 64 byte FIFO.
EP1, OUT BULK or INT endpoint, 2 x 64 byte double-buffered FIFOS
EP2, IN BULK or INT endpoint, 2 x 64 byte double-buffered FIFOS
EP3, IN BULK or INT endpoint, 64 byte FIFO
With these endpoints, it is possible to build USB peripherals that support popular USB class
drivers, such as a Human Interface Device (HID), Mass Storage, Picture Transfer Protocol
(PTP), and Printer.
4. Why does the MAX3420E not support ISOCHRONOUS transfers?
ISOCHRONOUS transfers require fast interfaces and large buffers, neither of which is
consistent with the MAX3420E design goals (a low-cost part with an SPI interface that can run
at any speed). Most applications that seem to require ISOCHRONOUS bandwidth can actually
be handled with BULK or INTERRUPT transfers. This is because most of the ISOCHRONOUS
bandwidth available to a USB device in a system is also typically available for BULK/INT
transfers.
5. Can I use the MAX3420E in a self-powered peripheral?
Absolutely. In fact, the MAX3420E has features specifically intended to support self-powered
applications. For example, in a self-powered application the peripheral needs to know when
the device is plugged into a powered USB port. The MAX3420E's VBCOMP (V
BUS
comparator) pin is connected to V
BUS
, and routed to an internal comparator that provides an
interrupt request at plug-in (VBUSIRQ) and another interrupt request at disconnect
(NOVBUSIRQ). As another example for the MAX3420E, a bit called VBGATE (V
BUS
gate) can
be set to automatically disconnect the D+ pullup resistor whenever V
BUS
is detected to be off.
This is a required USB specification.
6. Can I use the MAX3420E in a bus-powered peripheral?
Yes. In a bus-powered application, a 3.3V voltage regulator is connected to the USB
connector's V
BUS
pin. Whenever the peripheral is plugged into USB, the chip and the SPI
master driving it are powered. So there is no need to connect the MAX3420's VBCOMP pin to
V
BUS
. In this case, the VBCOMP input can be used as an extra general-purpose input. Care
must be taken to ensure that input signals to this pin meet the threshold requirements noted in
the
MAX3420 Electrical Characteristics table.
7. What external circuitry do I need to connect the MAX3420E to USB?
The MAX3420E requires a V
CC
supply of 3.3V. Bus-powered peripherals need a 3.3V
regulator to convert the power available on the V
BUS
pin (4.4V to 5.25V) to 3.3V. In addition,
the MAX3420E requires an external crystal (parallel resonant, 12MHz ±0.25%) with load
capacitors from each pin to ground, and two series resistors (33Ω, 1%) between the D+/D-
outputs, and the USB "B" connector.
8. Can you recommend a 3.3V regulator?
The MAX6349TL is ideal. It supplies 150mA at 3.3V, and contains a power-on-reset (POR)
circuit which can be connected directly to the MAX3420E RES# pin. A good external POR
circuit is important to have in a hot-plugged design such as a USB peripheral.
Page 3 of 9