AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 7

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

Power Supply Considerations
There are two power supply voltages:
In some cases, the different supplies may be connected internally on the chip by ESD protection diodes
but these connections are not intended for power distribution. Each pair of supply pins must be considered
individually and all power supply pins must be connected appropriately on the PCB.
The core must not be powered down separately from the I/O.
The 5V supply must not be switched off while the core is powered from an external 2.5V supply.
When using the internal voltage regulator, the 2.5V supply pins (VDD1, VDD2 and VDDPLL) should not
be connected together on the pcb. They should be connected only to appropriate decoupling capacitors.
The internal voltage regulator is not designed to supply external circuitry.
When using an external voltage regulator, the 2.5V supply pins must be connected on the PCB with
decoupling capacitors close to each supply pin pair — take steps to keep the VDDPLL supply clean from
noise.
The PCB must be carefully laid out to ensure optimal operation of the internal voltage regulator as well as
of the MCU itself. In general, it is advisable to route the MCU power and oscillator first. A recommended
layout can be found in the Voltage Regulator section of the relevant Device User Guide.
Power Supply Layout Guidelines
Every supply pair must be decoupled by a ceramic capacitor, preferably surface mount, connected as
close as possible to the corresponding pins. Keep the decoupling capacitors on the same side of the PCB
as the MCU with no vias in the connections from the decoupling capacitor to the MCU pins.
Use low impedance connections to VSS1, VSS2, and VSSR. If not using a full board ground plane,
connect the ground pins in a star arrangement with the central point of the ground star at the VSSR pin.
Where there is a full board ground plane, connect all VSS supplies directly to the plane, except for
VSSPLL and the oscillator circuit ground.
Connect VSSPLL directly to VSSR (maintaining the star layout).
The example layout in the Device User guides shows a copper fill under the MCU, connecting the
grounds, with exclusion slots in the fill. The intent is to provide low impedance connectivity with a star
configuration.
In general, the oscillator ground return should be considered as a separate ground, routed directly back
to VSSPLL and not connected elsewhere to the digital ground. The TEST pin is a static logic input that
can also be connected to the oscillator ground to facilitate oscillator layout and minimize the impedance
of the oscillator ground.
Freescale Semiconductor
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2.5V supply for logic, the CPU core, PLL, and oscillator,
5V supply (VDD5) for the I/O buffers, voltage regulator, and ATD.
Designing Hardware for the HCS12 D-Family, Rev. 0
Power Supply Considerations
7