Universal Serial Bus (USB)
MCF5272 ColdFire Integrated Microprocessor User’s Manual, Rev. 3
12-36
Freescale Semiconductor
12.5
Line Interface
The recommended line interface is shown in
Figure 12-24. The transistor used to control the 1.5–k
Ω
pull-up resistor is optional.
Figure 12-24. Recommended USB Line Interface
12.5.1
Attachment Detection
The USB specification requires that a pull-up resistor must be placed on the D+ signal for the upstream
hub to detect attachment of a full-speed device. The user may optionally want to control the attach/detach
detection by software instead of only at power on and off. With software control of the pull-up resistor, the
user has unlimited time to initialize the USB module. The software controlled pull-up resistor can be
implemented with only a few discrete components. The recommended circuit for implementing software
12.5.2
PCB Layout Recommendations
The device has input protection on all pins and may source or sink a limited amount of current without
damage.
The most important considerations for PCB layout deal with noise: noise on the power supply, noise
generated by the digital circuitry on the device, and noise resulting from coupling digital signals into the
analog signals. The best PCB layout methods to prevent noise–induced problems are as follows:
Keep digital signals as far away from analog signals (D+ and D-) as possible.
Use short, low inductance traces for the analog circuitry to reduce inductive, capacitive, and radio
frequency noise sensitivities.
Use short, low inductance traces for digital circuitry to reduce inductive, capacitive, and radio
frequency radiated noise.
GP I/O pin
USB_D+
USB_D-
USB
Series B
Receptacle
Vbus
D+
D–
GND
3.3 V
1k
Ω
100
Ω
33
Ω
33
Ω
1.5k
Ω±5%
MCF5272