TWR-K60N512-KEIL Freescale Semiconductor, TWR-K60N512-KEIL Datasheet - Page 28

• Manufacturer Part Number: TWR-K60N512-KEIL
• Description: K60N512 Keil Tower Kit
• Manufacturer: Freescale Semiconductor
• Series: Kinetisr
• Type: MCUr

Specifications of TWR-K60N512-KEIL

• Rohs Compliant: YES
• Contents: 4 Boards, Documentation, DVD
• Peak Reflow Compatible (260 C): Yes
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• For Use With/related Products: Freescale Tower System, K60N512

PCB routing considerations
2.3.4.1 RESET_b and NMI_b
Critical input pins, like RESET_b and NMI_b should have 100nF capacitors close to the
MCU for transient protection. Each pin has a weak internal pullup, but an external 4.7kΩ
to 10kΩ pullup is recommended. As with power pin filtering, it is recommended to
minimize the ground loop for the capacitor and the VDD loop for the pullup resistor for
these pins.
The RESET_b pin also has a configurable digital filter to reject potential noise on this
input after power-up. The configuration bits are located in the SIM_SOPT6 register.
While use of this filter may negate the need for the pullup and capacitor mentioned
above, it is still recommended to use external filtering in electrically noisy environments.
2.3.4.2 General purpose I/O
General purpose inputs, such as low speed inputs, timer inputs, and signals from off-
board should have low pass filters (series resistor and capacitor to ground) to prevent data
corruption due to crosstalk or transients. The filter capacitor should be placed close to the
MCU pin, while the resistor can be placed closer to the source.
Inputs that come from connectors should have low pass filtering at the connector to
prevent noise from propagating onto the PCB. This requires a robust ground structure
around the connector. Series resistors for signals that come from off-board should be
placed as close to the connector as possible. A filter cap closer to the MCU input pin may
be required if the signal trace length is very long and can pick up noise from other
circuits.
Output pins should not have any significant capacitance placed close to the MCU. These
signals can have capacitors at the load or connector to minimize radiated emissions if
necessary.
2.3.4.3 Analog inputs
Analog inputs should have low pass filters as well. The challenge with analog inputs,
especially for high resolution analog-to-digital conversions, is that the filter design needs
to consider the source impedance and sample time rather than a simple cutoff frequency.
This topic cannot be discussed in detail here, but the general concept is that fast sample
times will require smaller capacitor values and source impedances than slow sample
times. Higher resolution inputs may require smaller capacitor values and source
impedances than lower resolution inputs.
Kinetis Quick Reference User Guide, Rev. 0, 11/2010
28
Freescale Semiconductor