I need help with the PCB board development process

A

ATORTeensyMan

Active member
Introduction:
I am new to electrical design & PCB design, and I am just looking for guidance through the process, because I am still learning. I would like to integrate the chipset & bootloader of one of the Teensy's into this PCB. I am kind of uncertain on the differences between the Teensy 4.0 & 4.1 chipsets if there is any. The project was breadboarded with a Teensy 3.5 & the speed was okay, but I would like the computational speed increase of the newer chipsets.

Power:
On my breadboard I used a power regulator to step a 9v battery down to 5v to power the Teensy, however I'd like to integrate that into my PCB if possible. Do I need to use 5v or can I build the chipset around a 3.3v supply voltage to keep it simple since my pressure sensor is that voltage already? The pressure sensor I picked uses a 3.3v I2C signal and both SDA & SCLK have a 10k ohm resistor on it. Currently to check the battery level of the 9v battery I have a voltage divider of 220k & 100k ohms and just created a lookup table on excel to tell when the discharge rate of the battery is down to ~2.33v from ~2.81v. However, I was seeing around ~2.3v results when reading the voltage from the battery unless I used
Code: 
pinMode(battery_pin, INPUT_PULLUP);
, so how would I implement this pull-up resistor in my pcb if I actually need it or maybe i did something wrong?

MCU IC Chip:
I read that I need to use a chip and bootloader specifically from PJRC, so where can I get schematics/footprints/3dmodels for these things. For uploading code would usb-b micro or ISP be the best route to go? I wouldn't need to power the board through the usb-b micro at all, I'd only need it for loading code from Arduino IDE.

Component Constraints:
3.3v - (DIP) mounted pressure sensor +/- 30 inH2O http://www.allsensors.com/cad/DS-0355_Rev_B.PDF
3x pins for LEDs https://www.amazon.com/dp/B08G4TVTHG/ref=twister_B08LKC43C2?_encoding=UTF8&th=1
1x pin for reading the battery voltage w/ a voltage divider & pull-up resistor (maybe?)
1x 3.3v I2C bus w/ 10k ohm resistors

Sorry if this is a lot! Thanks!
 
1) For power supply if all of your parts are 3.3V then yes, it makes sense to go directly to 3.3V. However please keep in mind if you want the option to power your device from USB for programing then that will be supplying 5V.
Don't use a linear regulator to go from 9 to 3.3, you'd waste 2/3 of the power, use a switching regulator.
Unless you are experienced with switching regulator design or it's vital to save every penny I'd recommend an off the shelf DC-DC regulator module, doing it yourself can be very layout sensitive. A module may cost a little more but makes life a lot easier.

2) The 4.0 and 4.1 are basically the same parts on a different PCB.

3) A voltage divider to reduce the battery voltage to a suitable level for the ADC input is fine. You should set the input to no pull up or pull down. Put a capacitor (say 0.1uF) on the ADC pin. When the ADC takes a sample its input current spikes up, if you have a 100k impedance on that signal that current spike is going to make the voltage dip a little. The capacitor will prevent this.

4) Copy the 4.1 schematic and layout as much as is practical. For high speed processors like this lots of capacitors on the power rails, their placement and the power / ground routing are critical to reliable operation.

5) The LEDs you show will be very dim if you drive them from 3.3V. They look to be fairly standard ~2.5V LEDs with a 680 ohm resistor in series. This means that at 3.3V you'll be getting around 1 mA current. You should still see it but it won't be bright. You could use a FET to drive them, that would let you use the 9V from the battery directly. Make sure you pick a FET with a suitable threshold voltage since the gate will only be driven by 3.3V not the 5V that a lot of "suitable for arduino" type parts assume.
 
How do I find more information on what each pin on the chip does exactly? What's the difference between GPIO_EMC_#, GPIO_B#_#, GPIO_SD_B#_#, and GPIO_AD_B#_#. I can see that "AD" probably means both analog and digital capabilities. That being said, would any of these that aren't an "AD" be connected as the digital i/o (D1-D#) like on a real Teensy? Then I will treat the "AD"'s as the analog i/o A1-A#?

How much does it change the electrical if I want my usb to only be there to compile code to the bootloader and I don't ever want to power the teensy that way? In my situation I'd want to attach a power rail to the pcb and just power the system using a external battery.

Where do I find the symbol, footprint & 3dmodel for the bootloader IC?

I'm going to post pictures of my pcb schematic as I go, thanks for the help!
 
ATORTeensyMan said:
How do I find more information on what each pin on the chip does exactly? What's the difference between GPIO_EMC_#, GPIO_B#_#, GPIO_SD_B#_#, and GPIO_AD_B#_#. I can see that "AD" probably means both analog and digital capabilities. That being said, would any of these that aren't an "AD" be connected as the digital i/o (D1-D#) like on a real Teensy? Then I will treat the "AD"'s as the analog i/o A1-A#?
Click to expand...

You'll want to read the reference manual. Then you'll want to use the same pins as on a Teensy, unless you want to edit the pin definitions.

ATORTeensyMan said:
How much does it change the electrical if I want my usb to only be there to compile code to the bootloader and I don't ever want to power the teensy that way? In my situation I'd want to attach a power rail to the pcb and just power the system using a external battery.
Click to expand...

Not much. You can avoid connecting the bus voltage on the USB connector. You'll still need ground and the data pins. I think you still need to supply power to the USB circuitry on the 1062.

ATORTeensyMan said:
Where do I find the symbol, footprint & 3dmodel for the bootloader IC?
Click to expand...
This is the chip that's being used by PJRC, but you need to purchase from PJRC so that it has the bootloader software loaded.
https://www.mouser.com/ProductDetail/NXP-Semiconductors/MKL02Z32VFG4R?qs=RnZx02BLotJ2qOD8TpW3pw==

You might consider starting with something simpler, like designing around the Teensy MicroMod board first. Then you don't need to deal with the complex bits of a custom 1062 board (high speed decoupling, power staging, and the clock).
 
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