= 35.4 Ohms (Rounding up or down to the nearest standard resistor value)
= 33 Ohms
Method 2 of 2: (More accurate)
Set an I/O pin as an output, driving a logic high into a 1k resistor to ground. Many I/O pins have selectable
drive strength, hence configure the pin the same as required for the high-speed signals in the application.
Record the measured MCU VDD and the VOH across the 1k resistor
.
– Driver Impedance = ((VDD-VOH) * 1K) / VOH
– PCB signal series termination resistor(s) value = (Driver Impedance - Trace Impedance)
Unterminated signal frequencies >30 MHz range can experience over/under shooting in the 50% range.
Unterminated high-speed signals can be a significant contributor to the radiated EMI/EMC signature and
crosstalk.
2. Minimize the use of vias throughout the design on high-speed signals. Vias add capacitance and distort
impedance to the high-speed signal traces.
3. In general, review all signal cross talk design rules to avoid cross talk problems. Use the 3-W rule to provide
enough trace separation to avoid cross talk problems. Guard traces may also be utilized to minimize cross talk
problems on high-speed signals.
17.4 PCB Trace Considerations
1. PCB traces must be routed using 45° corners when changing directions. 90° corners must never be used.
2. All component leads to any power plane or ground plane must be as short as possible. The best solutions
are plane connection vias inside the surface mount pads. When using vias outside the surface mount pads,
pad-to-via connections must be less than 5-10 mils in length. Trace connections must be as wide as possible
to lower inductance. This will include any power ferrite beads feeding power planes, fuses feeding power
planes, and so on.
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Comprehensive PCB Layout Guidelines and Recommenda...
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