Nomad
I would like to ask forum members who have been using the EZ-B v4 for awhile if they have noticed any hardware problems that seem to happen randomly. By this I mean com ports that have errors or hang, inconsistent analog to digital conversions, spurious resets, memory errors, etc. Why do I ask?
I purchased a v4 recently and intend to use it as a quick way to evaluate prototype mechanisms. As part of my due diligence I opened up the v4 and inspected the circuit boards. While the pcb with the wifi module is okay, the pcb with the ARM mcu has many issues of concern. While some of these may be just my opinions they are based on common practices learned in designing over 20 circuit boards during my 30+ years as an electrical engineer. Then there are the issues where the manufacturer's pcb layout recommendations regarding capacitor use and placement have been ignored. The data sheet for the mcu (STM32F205RET6) says:
"Caution: Each power supply pair (VDD/VSS, VDDA/VSSA ...) must be decoupled with filtering ceramic capacitors as shown above. These capacitors must be placed as close as possible to, or below, the appropriate pins on the underside of the PCB, to ensure good device operation. It is not recommended to remove filtering capacitors to reduce PCB size or cost. This might cause incorrect device operation." ( -DocID15818 Rev 12, bottom of page 68, from www.st.com )
The multiple Vdd pins supply different modules within the mcu. They could be supplied with 1 pin but the others are provided for the express purpose of being able to place a filter cap close to each of these internal modules. The mcu version used on the v4 has 5 power supply pins but only 1 of them has the required cap.
Moving on to the buck converter regulator (RT8209), the data sheet recommends 2 - 10uf ceramic caps on the VIN pin. The v4 has 2 - 68uf electrolytic caps. Electrolytic caps have much higher ESR than ceramic caps and this limits their effectiveness. The higher the value the worse it is so using 68uf caps instead of 10uf aggravates the situation.
The data sheet recommends 2 - 22uf caps on the output. The v4 uses 2 - 220uf caps which are more expensive but won't work as well.
One of the small ceramic filter caps (C3) is much too far away and with too narrow a trace to do its job properly.
In conclusion, the capacitor errors should affect the reliability of the mcu board but it may be only when it is operating at extremes of speed, temperature, current draw and/or emi. Either that or the mcu's designers don't know what they are talking about.
I've been using two v4 boards almost every day with multiple servos, sensors ect since last October, and I haven't noticed any random hardware problems with mine. There have been times where they would disconnect, but that was down to either low WiFi signals, low battery, or some bad scripting on my part.
Hello @Nomad, Welcome to the forum!
Lol, while I don't have 30+ years of experience I have learned from some great mentors over the years. It's funny I have never kept count of how many circuits and PCBs I have designed but it's likely over 50 by now.
Honestly, it's true that the ez-bv4 is one of my more unique designs. We had many design challenges, some that you may not have encountered all in one place. Remember that the electronics industry is always changing. I was surprised myself at the low ESR Electrolytic capacitor technologies available now. As a fellow design engineer you likely know that reduced component count (and board space) is extremely important due to assembly time and costs so sometimes that means dialing back the application circuit to something that is more practical than theoretical. There has been a lot considered in our design and rest assured, we thoroughly tested the ez-bv4 before release.
I would suggest that you place your ez-b back together and give it a go! You might be surprised at how stable it really is
Hi @jeremie, thanks for the welcome although I have posted before.
It seems you are saying that you left off the 4 capacitors on the mcu power lines because you wanted to save money. The caps cost 0.6 cents each. You're willing to risk reliability for that?
You have put me in the position where I have to choose whether to listen to you or the part manufacturer's design engineers. Sorry, you lose.
However, for me the point is moot as I have come to realise that the EZ-B is too limiting for my project.
Be alert for some good deals on ebay!
Where is Richard R when you need him?