Design Considerations
9.2 Electrical Design Considerations
CAUTION
This device contains protective circuitry to guard against
damage due to high static voltage or electrical fields. However,
take normal precautions to avoid application of any voltages
higher than maximum-rated voltages to this high-impedance
circuit. Reliability of operation is enhanced if unused inputs are
tied to an appropriate voltage level.
Use the following list of considerations to assure correct operation of the device:
• Provide a low-impedance path from the board power supply to each VDD pin on the
device and from the board ground to each VSS (GND) pin.
• The minimum bypass requirement is to place 0.01–0.1µF capacitors positioned as
near as possible to the package supply pins. The recommended bypass configuration
is to place one bypass capacitor on each of the VDD/VSS pairs, including VDDA/VSSA
Ceramic and tantalum capacitors tend to provide better tolerances.
.
• Ensure that capacitor leads and associated printed circuit traces that connect to the
chip VDD and VSS (GND) pins are as short as possible.
• Bypass the VDD and VSS with approximately 100 µF, plus the number of 0.1 µF
ceramic capacitors.
• PCB trace lengths should be minimal for high-frequency signals.
• Consider all device loads as well as parasitic capacitance due to PCB traces when
calculating capacitance. This is especially critical in systems with higher capacitive
loads that could create higher transient currents in the VDD and VSS circuits.
• Take special care to minimize noise levels on the VREF, VDDA, and VSSA pins.
• Using separate power planes for VDD and VDDA and separate ground planes for VSS
and VSSA are recommended. Connect the separate analog and digital power and
ground planes as near as possible to power supply outputs. If an analog circuit and
digital circuit are powered by the same power supply, you should connect a small
inductor or ferrite bead in serial with VDDA and VSSA traces.
• Physically separate analog components from noisy digital components by ground
planes. Do not place an analog trace in parallel with digital traces. Place an analog
ground trace around an analog signal trace to isolate it from digital traces.
• Because the flash memory is programmed through the JTAG/EOnCE port, SPI, SCI,
or I2C, the designer should provide an interface to this port if in-circuit flash
programming is desired.
• If desired, connect an external RC circuit to the RESET pin. The resistor value
should be in the range of 4.7 kΩ–10 kΩ; the capacitor value should be in the range of
0.22 µF–4.7 µF.
MC56F844xx Advance Information Data Sheet, Rev. 2, 06/2012.
60
Freescale Semiconductor, Inc.
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