MC908JB12DWE Datasheet, PDF (176/332 Pages) Freescale Semiconductor, Inc – MC68HC908JB16 Technical Data

Universal Serial Bus Module (USB)
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.
MCU
V_REG (3.3V)
1.5 kΩ
D+
D–
USB LOW-SPEED CABLE
Figure 11-9. 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.
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.
Technical Data
176
Universal Serial Bus Module (USB)
MC68HC908JB16 — Rev. 1.1
Freescale Semiconductor