What to check in a schematic before you order boards
The schematic mistakes that most often cost a board revision, and how to catch them against the datasheets before you pay for fabrication.
RJC Systems3 min read
- schematic-review
- hardware
- design
Fixing a schematic mistake costs a few minutes. Finding the same mistake after fabrication costs a rework, or a new revision with weeks of lead time and another fab and assembly bill. A careful review before you order is the cheapest insurance a hardware team can buy.
Here is what we check, in the order we check it. The method is simple: open every part's datasheet next to the schematic and confirm each pin, value, and net against it, instead of trusting memory or a reference design.
Power
Power problems are the most common reason a new board will not start.
- Regulator ratings. Check that the input voltage range, output current, and dropout voltage cover the worst case, not just the typical case.
- Linear regulator heat. An LDO dissipates (Vin − Vout) × I. Dropping 5 V to 3.3 V at 300 mA is about 0.5 W. Multiply by the package's thermal resistance from the datasheet to estimate the temperature rise. Small packages often can't handle it.
- Input and output capacitors. Use the values and types the regulator datasheet calls for. Some regulators need a minimum ESR, or a minimum capacitance, to stay stable.
- Feedback dividers. Recalculate the output voltage from the resistor values on the schematic.
- Enables and sequencing. Every enable pin should be driven or tied, never left floating, and the rails should come up in the order the datasheets require.
Decoupling
- Every supply pin has its own capacitor of the value the datasheet recommends.
- Analog supply and reference pins get the filtering the datasheet shows. They are easy to treat as ordinary supply pins and get wrong.
- Each rail has bulk capacitance where it enters the board.
Pinouts and footprints
- Confirm the package variant. Many parts come in several packages with different pinouts.
- For connectors, confirm which side pin 1 is on and whether the mating cable flips the order.
- For anything with a thermal pad, confirm the pad is connected to the net the datasheet specifies, usually ground.
Communication buses
| Bus | What we check |
|---|---|
| I2C | Pull-ups on SDA and SCL, sized for the bus speed and capacitance; no address conflicts between devices; matching voltage levels |
| SPI | Each chip select pulled to its inactive state so devices stay deselected during boot; matching SPI modes |
| UART | TX connects to RX in each direction, with the net names unambiguous about direction |
Two of these come up again and again. One is an I2C bus with no pull-ups, or with a pull-up on every breakout module so the combined resistance is far too low. The other is a UART where TX was wired to TX because the net names didn't say which side they described.
Also check voltage levels anywhere two supply domains meet. A 3.3 V part driving an input powered from 1.8 V needs a level shifter or a part rated for it.
Reset, boot, and programming
- The reset pin has the pull-up and capacitor the datasheet calls for.
- Boot-mode and strapping pins default to the state you want at power-up, with a way to change them for recovery.
- A programming and debug header (SWD, JTAG, or the vendor's equivalent) is broken out, along with ground and the target voltage.
Unused pins and protection
- Unused inputs are tied off as the datasheet recommends. Floating CMOS inputs can draw extra current and behave unpredictably.
- External connectors have ESD protection, and the power input has reverse-polarity protection if a user could plug it in backwards.
Testability
Bring-up is much faster when you can measure the board. Before you order, add:
- Test points on every rail and a few ground points sized for a probe clip.
- A UART for debug output, even if the product doesn't use one.
- Jumpers or zero-ohm resistors that let you isolate each regulator during bring-up.
Doing the review
Go part by part, with the datasheet open, and mark each net as checked. Write down every question, even the ones that turn out to be fine. That list is what lets someone else confirm the review was complete.
When we review a schematic, that list is what you receive: every finding, documented against the datasheet it came from, for a flat fee agreed before we start.