10 Tips for Better Schematic Checking – Processor Boards
- Tricky power
Be aware of the power rail, which powers a particular pin. A wrong connection can cause current leaking, not defined signal level or can prevent board from booting. Check how the pin is connected inside the chip – is there a pull up/down, is there a protection diode, ….?
Always have an option to disable on board watchdog reset.
- Input pins
Be sure, all input pins have defined level (if unconnected, check if internal pull up/down is used). Be sure, the pin level is what you expect.
Use series termination resistor on CLOCK signals, place it close to the clock output.
- Buffer type
Be aware of different buffer types. For example, differential pair clocks can have different voltage levels or impedances.
- Reset timing
Be aware of the longest required reset timing in your board. Not meeting this requirement may prevent a chip from working properly.
- Power rail
Be sure you always check required voltage level. Some power pins require unusual voltage levels.
- Maximum current
Always check if your power supply can deliver required current. Don’t forget to make sum of currents flowing to all peripherals connected to one power rail.
Always check if your power pins are actually connected to power. Sometimes happen, you may forget to change the power net name when you copy it from reference schematic.
- Double check voltage dividers
If a voltage divider is used to set output voltage of a regulator, be sure you double check resistor values. You don’t want to damage your board with higher voltage.
- Tricky “power” pins
Some pins don’t appear as power pins, but may have unusual requirements e.g. “VBUS – Connect directly to 5V”
- Read Errata
Errata document describes known bugs of a chip – and some of the errata are really important.
- Use Checklist
Some chip manufacturers provides Schematic / Layout / Design checklists. When you finish your schematic, don’t forget to read them.
Following video shows some examples: