12-36
MCF5272 User’s Manual
Line Interface
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 control of the
pull-up resistor is shown in Figure 12-24.
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.
•
Bypass capacitors should be connected between the Vdd and GND pairs with
minimal trace length. These capacitors help supply the instantaneous currents of the
digital circuitry, in addition to decoupling the noise that may be generated by other
sections of the device or other circuitry on the power supply.
•
Use short, wide, low inductance traces to connect all of the GND pins together and,
with a single trace, connect all of the GND pins to the power supply ground. This
helps to reduce voltage spikes in the ground circuit caused by high-speed digital
current spikes. Suppressing these voltage spikes on the integrated circuit is the
reason for multiple GND leads. A PCB with a ground plane connecting all of the
digital and analog GND pins together is the optimal ground configuration,
producing the lowest resistance and inductance in the ground circuit.
•
Use short, wide, low inductance traces to connect all of the Vdd power supply pins
together and, with a single trace, connect all of the Vdd pins to the 3.3V power
supply. Connecting all of the digital and analog Vdd pins to the power plane would
be the optimal power distribution method for a multi-layer PCB with a power plane.
•
The 48-MHz oscillator must be located as close as possible to the chip package. This
is required to minimize parasitic capacitance between crystal traces and ground.
Summary of Contents for DigitalDNA ColdFire MCF5272
Page 1: ...MCF5272UM D Rev 0 02 2001 MCF5272 ColdFire Integrated Microprocessor User s Manual ...
Page 38: ...xxxviii MCF5272 User s Manual TABLES Table Number Title Page Number ...
Page 58: ...1 10 MCF5272 User s Manual MCF5272 Specific Features ...
Page 90: ...2 42 MCF5272 User s Manual Exception Processing Overview ...
Page 96: ...3 6 MCF5272 User s Manual MAC Instruction Execution Timings ...
Page 158: ...5 46 MCF5272 User s Manual Motorola Recommended BDM Pinout ...
Page 184: ...7 12 MCF5272 User s Manual Interrupt Controller Registers ...
Page 338: ...13 44 MCF5272 User s Manual Application Examples ...
Page 414: ...18 6 MCF5272 User s Manual PWM Programming Model ...
Page 452: ...19 38 MCF5272 User s Manual Power Supply Pins ...
Page 482: ...20 30 MCF5272 User s Manual Reset Operation ...
Page 492: ...21 10 MCF5272 User s Manual Non IEEE 1149 1 Operation ...