AN2826 Freescale Semiconductor / Motorola, AN2826 Datasheet - Page 11

• Manufacturer Part Number: AN2826
• Description: DDR-SDRAM Layout Considerations for MCF547x/8x Processors
• Manufacturer: Freescale Semiconductor / Motorola

MCF547x/8x Validation Board DDR Layout
decoupling capacitors that provide filtering on the three supply rails +3.3V, +2.5V and +1.5V DC in an
attempt to by-pass as much digital noise as possible. An important consideration for the system designer
is the selection of the value and dielectric material of these by-pass capacitors. Remember, it is not just the
fundamental core MPU frequency (up to 266 MHz) that we need to filter. The third, fifth and seventh
harmonics of this fundamental frequency (798 MHz, 1330 MHz, and 1862 MHz for a core frequency of
266 MHz) will also contain significant amounts of energy that can cause problems when attempting to get
a finished system through EMC/CE testing. This is why there are not only 0.1uF and 1nF bypass capacitors
in the design, but also 100pF capacitors on the MPU supplies close to the MPU. For bypass capacitors less
than 0.1uF in value, a dielectric of either COG or NPO is preferred to X7R, as the former materials are far
more self-resonant at these frequencies, and therefore liable to absorb far more digital switching noise.
Figure 7. MCF547x/8x Validation Board Top Layer
Figure 7
details the top layer of signalling routing on the validation board between the MPU and the DDR
SDRAM DIMM. There are several things of interest to the system designer. Firstly, look at some of the
DDR-SDRAM Layout Considerations for MCF547x/8x Processors, Rev. 1
Freescale Semiconductor
11