Using the AT89RFD-08 Board
AT89RFD-08 Hardware User Guide
2-19
4382E–MP3–05/06
2.10
Layout Guide
Signal integrity on Power Supply Decoupling, USB and Audio lines are the main con-
straints of the PCB.
2.10.1
Power Supply
Considerable care must be taken to provide quiet supplies such as the analog section:
n AUDVREF: Audio Voltage Reference pin for decoupling.
n AUDVCM: Audio Common Mode reference for decoupling.
n AUDVDD, AUDVSS: Audio Digital Supply Voltage and Ground.
n AUDVBAT: Audio Amplifier Supply.
n UVDD, UVSS: USB Supply Voltage.
n PVDD, PVSS: PLL Supply voltage and Ground.
and digital section:
n VDD and VSS: Digital Supply Voltage an Ground.
Power Supply rails must be routed in ‘
star
’ in order to reduce as mush as possible the
noise in the lines. The common point of the power supply lines must me as close as pos-
sible of the regulator output.
Each of these power supplies shall have decoupling capacitors.
Power Supply Implementation:
n All power sources wires can carry up to 500 mA
n All Power supply types must have at least one test point access on the Bottom side of
the board for test (it can be limited to a single padstack)
n Input and Output filter capacitors must be placed as close as possible from the
Voltage Regulator
n Care must be taken on the routage of the power supply (star routage, local ground
copper area...)
n The filter tantalum capacitors must have a low ESR.
n The Voltage Regulator must be placed on the component side and above a copper
pad linked VDD.Decoupling
The role of the decoupling is to maintain stable the power-supply. In other word, if the
frequency domain is considered, the decoupling maintains low the impedance across
the power-supply pins of the device in order to short circuit the harmonics of the current
which are contains in the disturbances.
The decoupling capacitors must me as close as possible of the microprocessor.
Priority is given to placement enabling the shortest wiring.