MC68HC908JB8 Datasheet, PDF (132/286 Pages) Motorola, Inc – MICROCONTROLLERS

Universal Serial Bus Module (USB)
9.5.5 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 9-9 with the pull-up on the D– line.
MCU
V_REG (3.3V)
1.5 kΩ
D+
D–
USB LOW-SPEED CABLE
Figure 9-9. External Low-Speed Device Configuration
For low-speed transmissions, the transmitter’s EOP width must be
between 1.25µs 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 8µs or longer is automatically a reset.
9.6 Clock Requirements
The low-speed data rate is nominally 1.5 Mbps. 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% (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.
Technical Data
132
MC68HC908JB8•MC68HC08JB8•MC68HC08JT8 — Rev. 2.3
Universal Serial Bus Module (USB)
Freescale Semiconductor