Schematics design review checklist
Create and study BOM, powerlist and netlist
- Use as few different components as possible.
- A single BOM line is costing roughly 100-200 CHF (ordering,
putting roll on machine etc.) and you will save time for
yourself (checking BOM, ordering).
It will also reduce the risk of mounting the wrong components. - Print the BOM, check the resistor values used and see if you can
remove some values by putting them in parallel or series (or
actually taking existing values that are as good).
The same for capacitors (cf. SVEC: 9 C-types, 16 R-types). - Check if high precision or high power components that are used (e.g. 0.1% or 0.5W) are really needed.
- Avoid through-hole components as they should be manually mounted.
- See if potentiometers in the final design can be replaced by a fixed value resistor (reduces production and calibration costs).
- A single BOM line is costing roughly 100-200 CHF (ordering,
putting roll on machine etc.) and you will save time for
yourself (checking BOM, ordering).
- Print out the power and ground list for all ICs and check if they’re correctly connected.
- Print out the netlist, sort alphabetically and check for any
inconsistencies (in naming, bus signals all used).
This takes some time, but really can show non-obvious mistakes. - Check if all capacitors are correctly de-rated for their operating voltage. See Capacitor voltage rating analysis script
- For any newly created symbols, check the pin numbering.
- CERN specific: check if only symbols created by the design office are used.
- Verify that the BOM is describing fully the components used.
- Manufacturer, Manufacturer full ordering number (including package and speed): e.g., LINEAR TECHNOLOGY LT1931ES5#TRMPBF.
- For generic components: all parameters are defined in the symbol: e.g., GENERIC R0603_220R_1%_0.1W_100PPM.
- Example BOM
- Verify that the BOM is completely generated out of the schematics. If not, create the correct symbol.
- Verify that the BOM does not contain any components that are at end-of-life (EOL), obsolete or hard to get.
- Non-mounted components should be marked in the schematics as such. And it should be clear from the BOM that they should not be mounted.
Study schematics
- Buses: check if all bits are used.
- Differential signals: check if both _P and _N are used (and have really this polarity).
- Check each connection of ICs. Notably hard-wired settings of ICs (division/amplification factors, operating modes etc.). Add a note.
- Check polarity of capacitors, notably on those connected to negative power supplies.
- Check correct polarity of diodes and LEDs.
- Check if enough decoupling for each IC.
- Check global decoupling of power supplies (large cap at point of generation or entry).
- Check protection circuits on the signals. Verify in detail where current flows through.
- Check if signal levels are compatible between outputs and inputs (LVTTL etc.).
- Check if direction of signals correct (i.e. hierarchy connectors, FMC uni-directional)
- Check for any crosses on wires (showing no connection) and check connectivity of crossing wires (a dot should show connected).
- Check if there is a note about hard-wired settings of ICs (division/amplification factors, operating modes etc.).
- Calculate output voltage of DC/DC converters using formulas from datasheet and check if corresponds with note on schematic.
- Check if components are aligned to make the schematics look clear.
- Check consistency of naming and numbering of schematic pages.
- Check for testability: can all signals be tested on a production test system (e.g., without needing probes, calibration etc.).
- Check if version or revision of a PCB can be accessed via gateware.
- Check if the grid is identical throughout the design (50 mil recommended)
- Check if no element is placed outside the grid (Altium: check for Off Grid elements shall be enabled)
Schematics conventions
- Use same page size (e.g., A3) for all schematics pages.
- Check that schematics read from left to right. Make the signals flow that direction: i.e., inputs on the left, outputs on the right of the page.
- Show clearly from which power supply each component is powered.
- Power symbol for positive supply should point up. Those for negative supplies and GND should point down.
- Name power P3V3, P5V, M5V, GND, AGND etc.
- Clock signals: add frequency in name (eg. CLK_100, CLK20_VCXO).
- Negative signal names: suffix _N (e.g. PB_RESET_N), do not use over-line that disappears in text files.
- Number components simply, like R1, R3, C1, L1 etc. Do not use names like Rbias1 and Cin2 as one may see used in datasheets.
- Unused signals available on connectors (e.g. P12V, VADJ): do not connect a wire to them, leave open.
- Mark resistor values as follows: 47 (for 47 Ohm), 4R7 (for 4.7 Ohm), 4K7 (for 4.7 kOhm = 4700 Ohm)
FMC mezzanine cards
- An FMC mezzanine should have a specific EEPROM (24C02 or a 24C32, not a
24AA64T-I/MC, or a DS1624 temp sensor with EEPROM). Reason. - EEPROM: GA1 should connect to A0, GA0 should connect to A1 (Observation 5.22 ANSI/VITA 57.1 spec - CERN only).
- EEPROM: should be powered by P3V3AUX, not P3V3.
- TDI (D30) and TDO (D31) are connected together if not used on the mezzanine.
- Have a note on the schematic about allowed Vadj level (e.g. most carriers on ohwr can provide only 2V5).
- Foresee decoupling capacitors near the FMC connector (allowing power pins to work as signal return too).
- Connect all mounting holes to ground (this is not in the specification, but is the best practice).
- Consider using a thermometer/unique ID IC (DS18B20U+) as is used on most OH FMC designs (this is not in the specification, but is the best practice).
Documentation
- Check that each schematics page has on the same place the Open Hardware Licence text of its corresponding variant (-S, -W, -P, likely -W):
"Copyright mycompany 2020. This source describes Open Hardware and is licensed under the CERN-OHL-W v2 ..."
- This text can be found in the corresponding 'Guide' at CERN OHL version 2.
- Check for this same OHL text on the mechanical drawings (front-panel and 'arrangement').
- Make sure that all texts, indications and annotations are written in English.
- Check and remove any DRC error flags.
- Document all inputs and outputs on the schematics pages
- voltage and current on external power connectors
- voltage level and input impedance on coax inputs
- voltage level and output impedance or drive level on outputs
- make clear which connector type is used.
Erik van der Bij - 20 May 2022