A Z-Wave Developer’s Journey | Half 6

Half 6: Z-Wave {Hardware} Finest Practices, focuses on sensible {hardware} design strategies for engineers growing low-cost, dependable Z-Wave IoT units. It covers PCB format suggestions, price optimization, RF issues, manufacturing finest practices, and design pointers drawn from real-world engineering expertise.

Introduction

Value delicate wi-fi good dwelling units have totally different necessities in comparison with designing cell telephones or laptops. The printed circuit boards (PCB) are normally only some sq. inches and sometimes solely two layers. To suit within {an electrical} outlet field your entire product have to be small and value issues require built-in microcontrollers to produce most of marketings necessities.

Fundamentals of IoT System {Hardware}

Can AI design and format a PCB? I lately tried one of many new AI PCB design instruments with their free 14-day trial. I uploaded the ZRADmini schematic into the “free” model of the instruments which ran out of ACUs (Agentic Compute Items) earlier than it might even digest the library elements. Possibly if I had paid for a subscription, I would be capable to get one thing helpful, however on my restricted price range I didn’t spend extra time or cash making an attempt the device out. Low-cost IoT units have easy, one-page schematics and small PCBs. A lot of the effort is connecting the wi-fi MCU and designing the antenna. Each of which require experience that comes from years of designing IoT merchandise.

A key function of getting the engineer do the PCB design, is that the engineer can simply change the MCU pinout to unravel PCB format issues. Since each Silicon Labs and Tridents chips can route almost any peripheral to nearly any GPIO, I typically swap GPIOs whereas routing the PCB. Make a fast change to the schematic, reread the netlist within the format device and voilà, downside solved! Simply bear in mind to additionally change the pinout within the firmware.

I normally manually design, format and hand-assemble prototypes. If I have to construct greater than 5, I contract meeting out as a result of meeting rapidly turns into an enormous time sink. I take advantage of the open supply KiCAD digital design software program. I’ve contributed just a few library elements to the repo together with the Silicon Labs ZG23 and Tridents CZ20. Many PCB distributors can immediately import KiCAD recordsdata, however I desire to ship them good ol’ Gerber recordsdata to make sure I get precisely what I need. With the KiCAD recordsdata, the seller might modify parameters such because the solder masks clearance which leads to no solder masks between the wonderful pitch pins of the MCU leading to nothing however solder shorts between the tiny pins. I don’t even use the autorouter because it tends to make a large number of issues with manner too many vias. The routing is normally easy sufficient to finish in just a few hours and is an pleasurable sport of close-the-rats-nets.

Maintaining Prices Low

The principle price driver for shopper electronics is quantity. The upper the annual quantity, the decrease the price for almost each line merchandise; PCB manufacturing, the MCU, passives, ICs, meeting, check and packaging. If the product quantity isn’t at the very least within the 10K models/12 months vary, then it have to be distinctive sufficient to command a excessive value. There’s a important knee-of-the-curve on the 10k threshold.

Don’t spend an excessive amount of time checking, rechecking, and reviewing the schematic and format. The turnaround time for a brand new prototype PCB could be as little as in a single day, and the price of a brand new board is only a few {dollars}. Thus, it is sensible to ship the board out rapidly and count on to spin it a few times to repair a few errors. I by accident put the mistaken dimension passive part on the board and solely found the error when it barely suits within the footprint on the board. However that’s simply fastened and a brand new board is prepared subsequent week. I’ve additionally reversed the pinout of a connector or another particular part that provides the underside view within the datasheet as a substitute of the highest view. Once more, simply fastened and I’ve new boards in a couple of week. Watch out with GPIO choice because the 800 sequence has limitations on energy area and wakeup capabilities of sure pins. See my GPIO weblog for extra particulars.

Guidelines For Low-cost IoT Merchandise

Naturally these “guidelines” aren’t absolute necessities however as a substitute are suggestions. Engineering is the artwork of balancing the answer to satisfy the necessities in a cost-efficient method. Usually it’s worthwhile to push again on a requirement if the price is just too excessive so don’t merely settle for the necessities as non-negotiable. The primary rule right here is an instance of those engineering tradeoffs: a four-layer PCB is costlier, however you get higher impedance (thus higher RF) and certain a smaller PCB. The stability between 2 vs. 4 layers is exclusive for each product which suggests it have to be engineered.

1. Decrease PCB layers
   1. 2 works for many Z-Wave designs
   2. 4 prices ~40% extra for naked PCB however has higher impedance/RF
2. Decrease dimension
   1. PCB prices are computed by the sq. millimeter
   2. Bear in mind the GND aircraft of the PCB is half of the antenna
3. Rectangular define
   1. Easy form maximizes array utilization
      
   2. Ideally v-scored for minimal waste
4. 0.16-0.20mm (6-8mil) wire spacing
   1. comply with the design guidelines of the low-cost PCBs producer
5. Decrease particular holes
   1. Slots, uncommon sizes, edge options all price $
6. All SMT elements on one facet – cheaper, simpler and better yield
7. 0402/0603 passive elements with sufficient house for straightforward Choose&Place
   1. 0201s are the scale of a grain of sand – tough to handle
   2. Bigger sizes price extra with no important profit
8. Decrease handbook meeting & soldering
9. PCB price is pushed by turnaround time
   1. Ready for an additional week is cheaper

PCB Design Suggestions

1. Use the Mini Simplicity debug footprint
   1. Keep away from customized footprints!
      1. Requires fragile & costly jig for debug/manufacturing
   2. Use the usual Tag-Join footprint & cable
   3. The spring publish cable alignment pins enabling dependable programming and checkhas
   4. Ideally make the footprint seen beneath a battery door to permit (re)programming after the gadget has been inserted into the enclosure.
2. No wires beneath the RF traces
   1. Ideally all RF traces are all on part facet
   2. Decrease vias that are impedance discontinuities
   3. Encompass with floor vias
3. Use the seller really useful crystals as they’ve been vetted by the seller
4. Steady Floor fills
   1. Ideally have a steady floor fill on the again for higher RF
5. Add thru-hole vias to the VCOM TX sign
   1. See when printfs are popping out on a scope in comparison with present consumption
   2. Will be executed by making a by way of bigger (1mm)
6. Join additional GPIOs to thru-hole check factors
   1. Accessible from each side of the PCB
   2. Simply prototype {hardware} modifications or future options
7. Add a {hardware} model GPIO Resistor Divider Pair or capacitor
   1. excessive ohms so little present
   2. Model 1=pulled up, Model 2=pulled down
   3. Model 3 onward makes use of the ADC to measure voltage or a cap and a timer
8. Overview the bottom aircraft flood fills
   1. Guarantee present paths don’t require altering layers
   2. No antennas
   3. No floating areas
   4. Array vias between prime and backside

      

9. Keep away from wires beneath the Z-Wave chip
   1. The EP prevents this anyway

10. Silkscreen sign names on connectors and check factors
11. All floor connections will need to have thermal reliefs
    1. WSTK has a number of pins flood stuffed to a thru-hole – almost not possible to solder!
12. Analog has particular guidelines! Observe them!
13. Have a guidelines, comply with it, add new gadgets to keep away from earlier errors
    1. DRC guidelines should go
    2. All nets routed

Sources

Listed below are just a few hyperlinks to suppliers I take advantage of or have used up to now. There are a lot of PCB producers and digital meeting homes, thus it’s possible there may be one native to you. One rule I take advantage of to search out native distributors is that if they’ve tanks or planes or navy {hardware} images on their website online, they don’t seem to be going to be price aggressive for IoT.

• Digital Parts
• PCB producers
• Stencil suppliers
  • OSHStencils – linked with OSHPark
  • The opposite PCB distributors can produce stencils from the Gerbers
• Meeting
  • ScreamingCircuits – Full service from PCB to assembled prototypes
    • You’ll ship them the MCU and any unusual elements
  • Seeed Studio – I purchase their maker boards for GPS and shows however additionally they have a division for prototyping

See my public ZRAD github repo for detailed procedures for going from schematics to PCB format to assembling prototypes.

Subsequent Steps

Half 7 of the Z-Wave Developer’s Journey discusses tips on how to debug and optimize battery life. Matching the correct kind of battery to the wants of your product and attaining the specified battery life requires basic engineering expertise. I’ll go over just a few suggestions from my years of optimizing coin cell battery units to final for years. As we proceed alongside the Z-Wave Developer’s Journey, I welcome your feedback and questions.  Please be at liberty to achieve out to me immediately by way of electronic mail.

Concerning the Creator

Eric Ryherd has been on the forefront of Z-Wave innovation since 2003, starting as a advisor and later serving as a Subject Utility Engineer at Silicon Labs. Over the course of his profession, he has contributed to the design and growth of a variety of Z-Wave merchandise, together with sensors, distant controls, motorized window shades, and in-wall dimmers, lots of that are available on the market in the present day.

Though he “retired” in 2022, Eric stays deeply engaged in embedded techniques and Z-Wave growth by way of his weblog, DrZWave.weblog, and ongoing IoT consulting initiatives. He’s additionally a well-known face at Z-Wave Alliance Unplug Fests, the place he ceaselessly serves because the lead coordinator, supporting interoperability and developer collaboration.