68HC705JB2 datasheet(79/128 Pages) NXP

79 / 128 page

August 28, 1998

GENERAL RELEASE SPECIFICATION

UNIVERSAL SERIAL BUS MODULE

REV 1.1

10-11

10.3

CLOCK REQUIREMENTS

The low speed data rate is nominally 1.5 Mbs. The OSCXCLK signal driven by the

oscillator circuits is the clock source for the USB module and requires that a 6

MHz oscillator circuit be connected to the OSC1 and OSC2 pins. The permitted

frequency tolerance for low speed functions is approximately

±1.5% (15000 ppm).

This tolerance includes inaccuracies from all sources: initial frequency accuracy,

crystal capacitive loading, supply voltage on the oscillator, temperature, and

aging. The jitter in the low speed data rate must be less than 10 ns. This tolerance

allows the use of resonators in low cost, low speed devices.

10.4

HARDWARE DESCRIPTION

The USB module as previously shown in Figure 10-1 contains four functional

blocks: a 3.3 volt Regulator, a LS USB transceiver, the USB control logic, and the

USB registers. The following will detail the function of the regulator, transceiver

and control logic. See Section 10.5 for the register discussion.

10.4.1 Voltage Regulator

The USB data lines are required by the USB Speciﬁcation to have a maximum

output voltage between 2.8V and 3.6V. The data lines are also required to have an

external 1.5K

Ω pullup resistor connected between a data line and a voltage

source between 3.0V and 3.6V. Since the power provided by the USB cable is

speciﬁed to be between 4.4V and 5.0V, an on-chip regulator is used to drop the

voltage to the appropriate level for sourcing the USB transceiver and external

pullup resistor. An output pin driven by the regulator voltage is provided to source

the 1.5K

Ω external resistor. Figure 10-11 shows the worst case electrical

connection for the voltage regulator.

Figure 10-11. Regulator Electrical Connections

Transceiver

3.3V

R1 = 1.5K

Ω ±5%

R2 = 15K

Ω ±5%

Regulator

USB Data Lines

Host

Hub

USB Cable

4.4V

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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