Schematic review - Page 1
Forum section: electronics / projects

author: Edgarus
author_rank: Newbie
timestamp: October 04, 2015, 07:20:02 am
content: HiI have a small schematic of a device which I work on.Can someone more experienced tell me if there are any issues at this schematic?Briefly, device roles:- monitor board temperature- monitor air temperature- communicate via i2c with other boards (used P82B715TD extender)- some functions will be based on DS3231 RTC, and according to specified time of day, commands will be send to other boards.Any help and criticism appreciated..Best regards.PG

author: Phoenix
author_rank: Frequent Contributor
timestamp: October 04, 2015, 07:56:41 am
content: I havn't looked at the circuitry in detail, but a bit of feedback on your layout might be good. The more effort you put into a good drawing the more likely people are to help with electrical/circuit checking.The top answer to this looks like a great place to start on a better layouthttp://electronics.stackexchange.com/questions/28251/rules-and-guidelines-for-drawing-good-schematicsSpecifically, you have drawn most things in seperate blocks on their own, which makes the schematic much harder to follow. You've also tended to draw parts as they would look as physical components, this also makes the schematic harder to read.The vast majority of the circuitry you have drawn can be connected using wires, not nets. Consider a left to right flow.Left - Power regulationMiddle - MicrocontrollerRight - Sensors + interfaces.Or even power at the top, micro on the left, sensors/interface on the right. You'll have to see what works.

author: Edgarus
author_rank: Newbie
timestamp: October 04, 2015, 09:28:22 am
content: Thank you Phoenix. I know that it does not look like it should be, I will take your advice.But besides this lack of proper element placement, do you see any issues with improper connections, bad element usage, etc?Best Regards

author: Dave
author_rank: Super Contributor
timestamp: October 04, 2015, 09:44:34 am
content: Those 20ohm resistors in the feedback loops of those buffers seem unnecessary.Why did you put 22ohm resistors on the USB lines?PDI clock is aclock. A signal that tends to oscillate a lot. Why did you put a 1ms RC constant on that?

author: Edgarus
author_rank: Newbie
timestamp: October 04, 2015, 09:52:33 am
content: Quote from: Dave on October 04, 2015, 09:44:34 am20ohm were to increase the accuracy of the buffer (on opamp).22ohm were as a small protection of usb port on xmega.PDI_CLK is also a reset, so resistor and the capacitor are reset circuit (values taken from atmel).

author: Dave
author_rank: Super Contributor
timestamp: October 04, 2015, 10:24:18 am
content: Quote from: Edgarus on October 04, 2015, 09:52:33 amPlease elaborate.QuoteUSB requires a differential impedance of 90 ohms. This is matched internally in the xmega. Simply adding random resistors in series is ridiculous.QuoteSo how exactly is that PDI programmer supposed to clock the controller during programming?You could also take away that BJT and connect the LED directly to the microcontroller. It's more than capable of driving a small LED.Another thing, you generally don't want to connect the USB cable shield directly to your circuit ground. That would defeat the purpose of the shield.

author: Edgarus
author_rank: Newbie
timestamp: October 04, 2015, 10:59:56 am
content: Quote from: Dave on October 04, 2015, 10:24:18 amThe output voltage of the buffer will not be equal to the output voltage of the MCP97000 thermistor, so I've added the resistor with the same value as output resistance of the device connected to non inverting input, to get as much precise value on the output.QuoteThese to 22Ohms were also to remove potential interference, and they will not interfere in usb itself, but I was thinking if I add the USBLC6 device, should I put these resistors.QuoteYes, I've got your point.QuoteI was thinking about it, but I'm not sure if driving anything directly from uC port is a good idea..QuoteGood point. I will fix it.Thank you very much Dave.Best regards

author: Yansi
author_rank: Super Contributor
timestamp: October 04, 2015, 12:36:20 pm
content: QuoteBullshit. As was said, remove them. USB is a (quite) high speed differential comms bus, where impedance should be matched, not modified by someone that thought that ...There would be more "interference" (reflection) on the bus if the impedance is not matched. These resistor definitely will decrease performance, not increase, as you might think.

author: Edgarus
author_rank: Newbie
timestamp: October 05, 2015, 07:45:32 am
content: Quote from: Yansi on October 04, 2015, 12:36:20 pmThank you Yansi.

author: daqq
author_rank: Super Contributor
timestamp: October 05, 2015, 08:16:58 am
content: Beware the reverse current of the Schottky diode - depending on the temperature, it can be comparable or higher than the current consuption of the DS3231.I don't think you need an amplifier for the MCP9700.The LM1117 might not like the series inductance directly before the bulk capacitor.What is the purpose of C13 - the PDI_CLK is a pretty high frequency signal, this could kill it.Why can't you drive the LED directly? IIRC the ATxMega series has a pretty powerful push/pull IO.Add ESD protection for the SD card interface.

author: tggzzz
author_rank: Super Contributor
timestamp: October 05, 2015, 08:47:27 am
content: A schematic shold showintention, the layout showsexecution.Signal flow: left to right, top down.Where a circuit has a standard representation (e.g. an op amp inverting amplifier), use it.Connect signals together, so people can quickly seeallthe places that are connected."Miscellaneous" component, e.g. decoupling capacitors can be drawn anywhere and are typically in a PSU section. Exception: those near high speed analogue subcircuits.

author: Neganur
author_rank: Unknown
timestamp: October 05, 2015, 10:01:55 am
content: Quote from: Yansi on October 04, 2015, 12:36:20 pmWhat do you mean, those resistors are there to terminate the bus/load matching.https://www.fairchildsemi.com/application-notes/AN/AN-5052.pdf

author: Dave
author_rank: Super Contributor
timestamp: October 05, 2015, 02:07:54 pm
content: Quote from: Neganur on October 05, 2015, 10:01:55 amNope, the resistors in Edgarus' circuit are there to ruin the matching of the termination.Quote from: Dave on October 04, 2015, 10:24:18 amI didn't pull this information out of my ass.Atmel Application note AVR1017: XMEGA - USB Hardware Design RecommendationsQuote from: Section 3.2.1Quote from: Section 4

author: Neganur
author_rank: Unknown
timestamp: October 05, 2015, 02:11:43 pm
content: Despite the language, I appreciate the source. Now your response makes much more sense

author: dan3460
author_rank: Frequent Contributor
timestamp: October 05, 2015, 05:06:53 pm
content: If may I say something, we all appreciate the contributors that take their time to look, analyze and answer posts to these forums. Many of us are new to electronics and we are learning, many times we fail to understand that the information we obtained from one source is incorrect or it was to be applied in a different way.Please be patient, it is not our intention to bother you.Thanks to all,

author: Dave
author_rank: Super Contributor
timestamp: October 05, 2015, 05:54:58 pm
content: There is a reason why I didn't post the information from the upper post before. I am currently designing my own circuit with an ATxmega and USB, so I thoroughly researched these things sometime during the last week. I knew the information was correct, I just couldn't remember where exactly I found it, so I didn't include the source.OP's attitude towards this whole thread ticked me off. He asked for help and then rejected the answers that he was given. Who does that?Neganur, I apologize if I came off a bit rude, that wasn't my intent.