AN2127 Freescale Semiconductor / Motorola, AN2127 Datasheet - Page 3

• Manufacturer Part Number: AN2127
• Description: EMC Guidelines for MPC Based Automotive Powertrain Systems
• Manufacturer: Freescale Semiconductor / Motorola

2.6 Slew Rate
2.7 Static Pin Configuration
3 Hardware Guidelines
3.1 Standard Decoupling
3.1.1 Capacitor Selection
3.1.2 Capacitor Layout
Slew rate is controlled by the SLRC[0:3] bits in the PDMCR register. Slow slew rate should be used
unless insufficient for system timing requirements. The default configuration is for slow slew rate. Slow
slew rate has a longer rise time than normal slew rate. Therefore, slow slew rate results in smaller am-
plitude harmonics.
Unused output pins should be driven low instead of driven high. In general, there is less voltage fluctu-
ation on ground than on power. Therefore, conducted emissions out of the pin will be lower when the
pin is driven low than when the pin is driven high.
All of the MPC500 family MCUs are mixed voltage parts. Each has some 5-V power supply balls for the
high-voltage I/O pins. In addition, each has another supply voltage for its core operation and low-voltage
I/O pins. These voltages are 3.3 V for the MPC55x and 2.6 V for the MPC56x. Since the low-voltage
supply is noisier than the high-voltage supply, emphasis should be placed on decoupling the low-volt-
age supply.
Three different capacitor values should be used to decouple both the high-voltage and the low-voltage
supplies. A good starting place is to use values of 10 µF, 0.1 µF, and 10 nF. Approximately 30 10-nF
caps are needed for the high frequency decoupling. Only a few 0.1 µF and 10 µF caps are required for
the low frequency decoupling. Low Equivalent Series Resistance (ESR) and low Equivalent Series In-
ductance (ESL) capacitors should be used.
AVX. Similar capacitors are also available from other manufacturers such as Murata, KEMET, and
Vishay.
There should be at least one 0.1 µF and one 10 µF capacitor for each of the low-voltage and the high-
voltage supplies. These capacitors should be placed next to the power entry point of the board and con-
nected directly to the power and ground planes with multiple vias.
Of the 30 10 nF capacitors, eight should be used to decouple the high-voltage (5-V) power plane in the
vicinity of the MCU. Place the capacitors as close as possible to the MCU surrounding the MCU uni-
formly. Each of the capacitors should connect to the 5-V power plane and the ground plane using mul-
tiple vias. The vias should be located in the solder pads for the capacitors (see
high-voltage power balls (V
EMC Guidelines for MPC500-Based Automotive Powertrain Systems
Capacitor Value
0.1
10 nF
10
Freescale Semiconductor, Inc.
µ
µ
F
For More Information On This Product,
F
DDH
) directly to the high-voltage power plane.
Table 1 AVX Capacitor Examples
Rev. 0, 15 July 2001
Go to: www.freescale.com
Table 1
provides some example components available from
03065C103KAT1W
06125C104KAT1W
AVX Part Number
TAJE106K050R
Figure
1). Connect the
MOTOROLA
3