Exactly what I would do. If you aren’t happy with it there will likely be things that neither of us have picked up, so getting a physical prototype in-hand is helpful, all other factors aside (like schedule / budget). You can incorporate more changes in that first revision.
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No need for routing differential. If it was multi megabit, then maybe. Wavelength at 100MHz is 2m. Say 1/50th is outside waveguide theory, you need 40mm trace before it matters. At 9600 it is 4km if my math is right
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Thanks Klaus, I’ve been thinking about this for a few days. If I calculate the wavelength of 100MHz of an electromagnetic wave through free space then I get 3m. However, if my understanding of how the wave propagates within a PCB is correct, we are not moving through free space, rather the dielectric medium. Therefore, would I be right in saying the speed is going to be different, and in turn, changes the wavelength?
Also, is the 1/50th based on the harmonic frequencies of the wave being negligible at that point?
RF is not my my best field, but:
Speed of light is different in materials. FR4 is actually half, so 150*10^6m/s
So for 100MHz, the wavelength is 1.5m
The dipole emits different power wrt the length. So lower than the wavelength the power is reduced by the square of the reduction in length
So imagine you have a cable connected to your box that is 1.5m long. It will emit lots of radiation
A PCB trace on the PCB with 3cm (1/50th) has 2500 times lower radiation
When you get close to the wavelength (L is larger then 1/5), then it begins to radiate proportional to the length (not squared)
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My PCB arrived on Thursday. I had ordered the board (almost) fully assembled, so after adding the FT232 IC’s, the programming header and the Wiz850io module the fun began.
I started by plugging in the master usb port. The 5V and 3V lets turned on. At least there wasn’t a catastrophic problem. The LEDs on the boot mode and debug mode switches worked as expected. Connecting the ST-Link, I was able to communicate with the MCU. LED’s were then all checked along with the reset switch.
Clock frequency was then tested with my frequency counter and was measured at 15.9999347MHz giving a 0.00000408% deviation. My guess is that the deviation comes from the stray capacitance being slightly different from the guessed value. I would need to verify against the other boards to see if it would be worth tweaking the load capacitor values on future boards.
Next I programmed the FT232 IC’s which didn’t provide any unwanted excitement. These were then tested at 115200bps and exhibited no data integrity issues. Enabling the WizNet 850io also didn’t provide any issues (SPI bus @ 16Mbit). Last to test was the last minute addition of an M24C64 EEPROM. Again this worked as expected.
So all in all, the PCB behaves as designed. If I was revising it I would choose a different EEPROM - something like a 24AA02E64 which would have had the addition benefit of providing a unique MAC address for the 850io.
Thank you to @smerrett79 @kvk @JuliaTruchsess for keeping me right. Now onto the fun of writing the software!
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Great job on getting everything working so fast. Now you have a platform that is working 
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Do you have a picture?
I’m away from home for a few days but I’ll get a couple of pics when I’m back.
A thing of beauty - long may those LEDs blink! How do you find the plated annuli and clearance of the mounting holes, in relation to your mounting screw cap OD or standoff OD and taller parts on the board? I have been upping my mounting hole clearances more, recently, as either a washer goes further than I would like towards a nearby copper pour (albeit usually GND) or I haven’t left good access for a driver bit holder to get in.
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Thanks Simon. As you can see below with a M3 security torx head there is a small overlap of plating. With M3 pozi it’s probably slightly small. Holes were M3.2 to allow a little bit of wiggle room. There would be room for a nylon washer if required. In the torx photo you’ll see the hole is isolated to maintain isolation of that ground in the unlikely event it goes in a metal enclosure. Also, there is plenty of room for my fingers to get a fastener in.
Where I’ve made a, lets call it a security feature, and not left enough room for a fastener I’ve used this type of hex screw successfully before since they have a slightly smaller OD and it’s easy to get a hex key in.
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