Best Design Practice for Real Time Clock(RTC) PCB Design.

 

When it comes to PCB design for Real Time Clock(RTC), it’s best to keep it simple and adhere to all the recommended best practices. The reason is, you may not realize the implication of your mistakes until the PCB is mass produced and operating in the field.

One real life problem with a poorly designed RTC circuitry resulted in the time drifting off by minutes in a couple of weeks after being deployed in automated parking machines. The problem, however, could not be replicated when we brought the PCB back to the lab.

A detailed check on the PCB and reveal a design similar to this :Note that there is a VCC trace (in blue) crossing and passing between the the crystal traces.

Real Time Clock(RTC) PCB Design1

 

Best Design Practice for Real Time Clock(RTC) PCB Design.

 

We believe that this and the less than ideal site environment contributed to the inaccuracy of the clock.

The cost of rectifying and redeployment of the affected PCB is costly to the company and the problem could be avoided in had the PCB been designed with industry best practice in mind.

Here are some guidelines that would avoid similar problems.

 

Best Practice For Real Time Clock(RTC) PCB Design.

  1. Never have any other trace (low speed/high speed/power) routed in-between or underneath the crystal trace. This could cause unwanted interference directly coupled to the crystal clocking signal.

 

  1. Place the crystal as close as possible to the RTC and keep the crystal trace as short as possible. A short antenna length will decrease the possibility of noise coupling.

 

Real Time Clock(RTC) PCB Design2

  1. Create a Ground plane on adjacent and beneath the RTC circuitry to properly shield it from unwanted interference. This isolated plane should be at least 40 mil away from the neighboring plane.

 

  1. Do not route any high-speed signals within the vicinity of the RTC trace. Keep high-speed signals and the RTC crystal trace separated at a minimum 200 mil distance.

 

  1. Some RTC came with the capacitor built into the IC. For those without, you will need to choose the right capacitor that matches the load capacitance value of the crystal. Do note that the crystal may oscillate at a slower or faster speed if you use a capacitor with a mismatched value.

 

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