Interface Questions | Maxim MAX3420E specs

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 (VBUS comparator) pin is connected to VBUS, 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 (VBUS gate) can be set to automatically disconnect the D+ pullup resistor whenever VBUS 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 VBUS 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 VBUS. 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 VCC supply of 3.3V. Bus-powered peripherals need a 3.3V regulator to convert the power available on the VBUS 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 (33, 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.

9.What does the CONNECT bit do?

The MAX3420E has a switchable internal 1500 pullup resistor between its D+ pin and VCC. The

CONNECT bit operates this switch. This switch allows a bus- powered peripheral to delay connection to USB until it finishes initialization. It also allows a self-powered peripheral to remove VCC from the pullup resistor in the absence of VBUS, as required by the USB Specification.

3.Interface Questions

1.How does a microprocessor connect to the MAX3420E?

The microprocessor connects to the MAX3420E by implementing an SPI master, using 3, 4, or 5 wires. Some microcontrollers include hardware SPI, but many do not. In this latter case, it is easy to implement an SPI master by bit-banging general-purpose IO pins.

2.You say the SPI interface is 3, 4, or 5 wires. What does this mean?

The minimum SPI interface consists of three wires: SS# (Slave Select), SCLK (Serial Clock), and MISO (configured for bidirectional MISO/MOSI data). Since this interface does not use the INT pin, the controlling microprocessor would need to poll two interrupt-request registers to determine when the MAX3420E requires service.