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Questions on Teensy 3.1 components
As part of a project I'm working on, I may want to build my own integrated Teensy 3.1 (using a MINI54 from Paul) and I have questions about what components are used on the Teensy 3.1. Based on the schematic available, I'm looking for make/model/value for the following components:
- Diodes labeled 1A on the VBAT circuit and on the VIN circuit
- The three ferrite beads
- The 16MHz crystal attached to PTA18/PTA19
- The switch going to VSS on the MINI54 (not critical, but I like the feel of the switch used on the Teensy 3.1 )
- The LED on pin 13 (not critical either)
Anything else I need to know re: hardware?
PS - Thanks to Paul for sharing the schematic!
I can't help with all the components but I can help with a couple of these.
Diodes labeled 1A on the VBAT circuit and on the VIN circuit
The 16MHz crystal attached to PTA18/PTA19
Hey potato - curious about something, perhaps you know more than I do. Do you foresee any problems using http://www.digikey.com/product-detai...2-1-ND/2122191 instead of the PMEG diode? I'm using the BAS40's right now, they're working pretty well, but I do have an odd, hard to pin down problem with some of my boards (there's two teensy's per PCB, and for no apparent reason, one of those sections will, very rarely, refuse to boot until I power cycle it), and I'm curious if maybe this could be the cause of it.
That and I'm using http://www.digikey.com/product-detai...8CT-ND/1802879 as crystals (they seem to have tighter frequency tolerances, but more capacitance and lower ESR), which might also be the cause of the problem...
Oh, and EK - if you don't already know - pin 33 on the MK20's can't be tied to ground at boot. It 'causes all sorts of problems' to quote Paul. Looks like it should/might be fixed in 1.21 whenever that comes out, but if you can you should avoid using it as anything but an output.
Last edited by MuShoo; 10-26-2014 at 09:52 AM.
The crystal is particularly sensitive. It should have a ground plane underneath, with only a single connection to GND near the MK20 chip. For a 2-layer PCB, this means not running any traces underneath the crystal, and not using the GND lines or plane underneath the route the ground. On a 4-layer PCB (like Teensy 3.1), dedicating layer 2 as a ground plane without any signal traces is usually good enough.
Originally Posted by MuShoo
All crystals are sensitive, but these miniature ones with load capacitance under 15 pf are particularly so.
In fact, Arduino Uno R2 had traces routed underneath its crystal, which caused the board to crash with certain programs the rapidly switched those signals. Don't make that PCB layout mistake!
Very interesting! I wonder if some of my past problems may have been influenced by the crystal...
I always had a solid GND plane under it and kept any signals as far as I could away from the crystal or te very short traces to the contact but I never segmented the GND plane to have a small dedicated GND section just for the crystal.
I presume you'd recommend connecting the GND plane for the crystal at the GND pin right next to the crystal, ie pin 30 or 31 if memory serves?
Thank you everyone! Any info on the ferrite beads?
Potatotron says the diodes labeled 1A are these: http://www.digikey.com/product-detai...4-1-ND/2531811
Can anyone confirm? These are Schottky diodes with three pads: two diode inputs and a common cathode. In the schematic, each diode has one input and one output. Unless, the dual diodes shown in the schematic are together this one component? Thanks!
Last edited by MikeP; 05-29-2015 at 02:13 AM.
Yes that's the correct part, a NXP PMEG4010CPA. It is one physical component that has 2 diodes inside it, with a common cathode. The chip has 2 inputs and 1 output; the inputs are main power and battery power, and the output goes to the VBAT pin of the MCU.
I have an additional question about components: Are all capacitors shown in the datasheet electrolitic? or are some of them ceramic? does this matter? Thank you in advance
I don't think Paul's ever published an official BOM so a definitive answer would have to come from him but I don't think any of the caps are electrolytic...I think all of those are in a can form factor and not the 0402 etc. sizes that I can see on my Teensy.
Originally Posted by idc
My understanding is that all of the caps used on the Teensy as shipped are ceramic, including the 2.2uF Vout and VUSB decoupling capacitors. Ceramic capacitors are bipolar, i.e. they don't care what end is positive and which end is negative.
Do pay close attention to claimed vs. actual values on ceramic caps. That can be very tricky depending on the grade, used voltage, frequency, and so on. I generally do not use any ceramics above 1uF for that reason. Paul knows what he's doing and presumably tested a lot.
The user has the option of adding a polarized 100uF Tantalum cap on the backside (connection to VUSB, per schematic), but my understanding is that cap is supposed to be used for USB host mode only.
Some users have reported performance improvements as a function of adding additional capacitors for 'peaky' loads. I would ensure that the capacitance on both sides of the voltage regulator is similar or even higher on the VIN/VUSB side (if you use the built-in regulator). This is to prevent potentially back-feeding the regulator when you disconnect the Teensy from power.
They're all ceramic, X7R and X5R type.
Thank you Paul for clarifying. I hage an additional question: I have seen in a different post the use of diodes to be able to have either Vusb or a battery to VREGIN. Is it suitable to use the diodes already shown in the schematic, one with a VUSB input and the other one with Battery input, so that VUSB takes over when connected? Is this safe?
Thank you in advance.
This connection would be similar to the one used for the VBAT pin in the MK20 to select from external VBAT or internal 3.3V.
Originally Posted by idc
First, I don't have a perfectly clear idea of exactly how you're proposing to connect things, based only on this description. A diagram or quick-and-dirty schematic sketch would really help. Small details matter, so diagrams that clearly show the wiring are much better than just text.
If you do go to the trouble to draw something, I and other very well may comment with opinions. I often do comment on such things, but I also need to mention that's pretty far outside the standard support that comes with a Teensy. We really are about helping you do DIY electronics, but when you're making your own PCB with your own schematic design, the best anyone can do without directly taking on the design is offer only opinions.
First of all thank you for answering my post. I want to be clear that I am in no way trying to replicate the teensy itself, on the contrary, I bought 4 teensy's and I have been prototyping with them, but now that I want to go to a custom product, includying the teensy itself in to my design will be a size issue, so that is why I am going for using the MK20 and the MINI54 on a custom design.
Attached you can see what I meant about the diodes. The idea is to use the 1A diodes to allow the input of the internal regulator of the MK20 to receive VBAT 3.7, but to switch to VUSB functioning when it is available. It would be something like the schematic you have on the web page, but moving the part you suggest to cut for battery powered teensy to the wire that connects the anodes of the 1A diodes, and actually cutting it. The first image I attach shows how I mean this.
A second concern I had when using a 3.7V Battery, is that the MK20 datasheet says that the internal regulator only guarantees 3.3V output for VREGIN > 3.6, which means the 3.3V output wont be stable any more when my battery is just a bit lower (3.6V or 3.5V). Considering this I have been thinking to include an external regulator as shown in my second image, so that I can guarantee 3.3V when the input ranges from 3.3V to 3.7V. Do you have any comments on this? Is it better to use the external regulator than the internal one due to this specification I mention? I also include a screenshot of the Datasheet saying that below 3.6V the output would not be mantained and it enters in "pass through mode".
Thank you in advance for your attention.
The diode approach seems fine.
Yes, an external regulator with lower dropout spec will allow running longer with the 3.3V power in full regulation.
The USB is specified to work down to 3.0 volts, and in testing it usually performs well down to 2.7 and often to about 2.5. While that's out of spec, it still usually works fine, so maybe that's an acceptable trade-off for the last several percent of the battery's charge? The extra LDO regulator with lower dropout spec would be the more proper way, of course.
You might also want to consider a way to monitor the battery voltage, or estimate the remaining charge, so you can shut down to avoid deep discharge of the battery.
Wow, your Teensy must be getting pretty popular, Paul. I went to Digikey to look for the dual diodes, and the suggested "what other people are looking at" at the bottom of the page was the Freescale MK20 processor and the 16Mhz crystal. Big leagues now!
The reason for said search was a stupid mistake on my first Teensy 3.1. Using a break-out board https://www.tindie.com/products/pico...ac_prod_search , it has the diode from VUSB to VIN. Not thinking, I powered it from USB and the BOB's 3.3v without cutting the jumper on the Teensy and the smoke leaked out of both of the doide pairs and the MINI54. I can't find if that is the normal failure mode, but it looks reasonable after looking at the schematic. Tuition to the school of life. It's not the cost that bothers me, its the abrupt halt in playing with the thing.
I'm coming from the Microchip camp and so far, Arduino, and now Teensy are pretty cool. Thank you Paul, and all the others that are offering support for this neat little marvel.
kindly tell the digikey part number for ferrite
Many professors and hobbyists learn about PIC Chips and just get stuck in the proverbial don't know what they don't know!
Originally Posted by boroko
As posted in reply #7:
That's a very easy spec to look up on DigiKey. Pick one in the package size you prefer (I use 0603 whenever I can personally).
Originally Posted by npashine
Any more info on these?:
- The button?
- The LED?
- The optional 32.768kHz crystal? What is it for?
- The fuse?
Also, what are the two squares in the schematic next to the optional 100uF cap? Is it a solder jumper? Why?