MAX3420E Просмотр технического описания (PDF) - Maxim Integrated

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производитель

MAX3420E

USB Peripheral Controller with SPI Interface

Maxim Integrated 

MAX3420E

USB Peripheral Controller with SPI Interface

1.0µF

CERAMIC

0.1µF

12MHz

CXI

CXO

+3.3V

USB

"B" CONNECTOR

VBUS

D+

D-

GND

33Ω

33Ω

1.0µF

CERAMIC

VCC VL

RES

D+

MAX3420E

D-

VBCOMP

XI XO

GPX

INT

MOSI

MISO

SCLK

SS

N.C.

µP

N.C.

GND

GPIN GPOUT

4

4

GPIO

Figure 17. MAX3420E in a Self-Powered Application

The VBUS detect input, VBCOMP, is an important

MAX3420E feature. Because the µP is powered whether

the USB device is plugged in or not, it needs some way to

detect a plug-in event. A comparator inside the MAX3420E

checks for a valid VBUS connection on VBCOMP and

provides a connect status bit to the µP. Once connected,

the µP can delay the logical connection to the USB bus

to perform any required initialization, and then connect by

setting the CONNECT bit to 1 in the MAX3420E register

USBCTL (R15). This connects the internal 1.5kΩ resistor

from D+ to VCC, to signal the host that a device has been

plugged in.

If a host turns off VBUS while the device is connected,

the USB rev. 2.0 specification requires that the device

must not power its 1.5kΩ pullup resistor connected to

D+. The MAX3420E has two features to help service

this event. First, the NOVBUSIRQ bit indicates the loss

of VBUS. Second, the µP can set a bit called VBGATE

(VBUS gate) to instruct the MAX3420E to disconnect the

pullup resistor anytime VBUS goes away, regardless of

the CONNECT bit setting.

Crystal Selection

The MAX3420E requires a crystal with the following

specifications:

Frequency: 12MHz ±0.25%

CLOAD: 18pF

CO: 7pF max

Drive level: 200µW

Series resonance resistance: 60Ω max

Note: Series resonance resistance is the resistance

observed when the resonator is in the series resonant

condition. This is a parameter often stated by quartz

crystal vendors and is called R1. When a resonator is used

in the parallel resonant mode with an external load capac-

itance, as is the case with the MAX3420E oscillator circuit,

the effective resistance is sometimes stated. This effective

resistance at the loaded frequency of oscillation is:

R1 x ( 1 + (CO / CLOAD))2

For typical CO and CLOAD values, the effective resis-

tance can be greater than R1 by a factor of 2.

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