M68HC08 datasheet(176/332 Pages) FREESCALE

176 / 332 page

Universal Serial Bus Module (USB)

Technical Data

MC68HC908JB16 — Rev. 1.1

176

Universal Serial Bus Module (USB)

Freescale Semiconductor

11.5.5 Low-Speed Device

Low-speed devices are configured by the position of a pull-up resistor on

the USB D– pin of the MCU. Low-speed devices are terminated as

shown in Figure 11-9 with the pull-up on the D– line.

Figure 11-9. External Low-Speed Device Configuration

For low-speed transmissions, the transmitter’s EOP width must be

between 1.25

µs and 1.50µs. These ranges include timing variations due

to differential buffer delay and rise/fall time mismatches and to noise and

other random effects. A low-speed receiver must accept a 670ns SE0

followed by a J transition as a valid EOP. An SE0 shorter than

330ns or an SE0 not followed by a J transition are rejected as an EOP.

Any SE0 that is 8

µs or longer is automatically a reset.

11.6 Clock Requirements

The low-speed data rate is nominally 1.5 Mbps. The OSCXCLK

÷2

(6MHz) signal driven by the oscillator circuits is the clock source for the

USB module and requires that a 12MHz oscillator circuit be connected

to the OSC1 and OSC2 pins. The permitted frequency tolerance for low-

speed functions is approximately

±1.5% (15,000 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 10ns.

1.5 kΩ

D+

D–

VREG (3.3V)

USB LOW-SPEED CABLE

MCU

M68HC08 Datasheet(PDF) 176 Page - Freescale Semiconductor, Inc