It's the new year at last. 2021, eh?
While many of us are putting our feet up for just a little bit — reveling in "the void" before work starts up again, some of us might have earned that break a bit more than others.
One example of someone who probably should take a well earned holiday, is the ever assiduous Greg Davill, who somehow managed a miraculous stint of a project per day, pouring out practically a PCB a day, in place of the somewhat more traditional "chocolate-based" advent calendars.
Although many of us would likely find ourselves struggling to keep up with perhaps a basic, well-designed breakout board a day, the Git commits Davill has been making are anything but basic. The list below reads as a formidable game plan for a design team with half a year to burn. For one man, in one month, this is a momentous effort!
With a whole month's worth — 31 full project folders, there's a lot to dig through here, but we're going to start with the one that caught my eye, and likely would yours also — Davill's take on the ESP32-S2 breakout board, with a few extras thrown in that should make fit for any low power ESP32-S2-based application.
Striking silkscreen artwork aside, there is an initial thought of "woah, that's huge," when compared to the designs Davill is normally known for turning out.
Besides accommodating a generous array of SMD and PTH pads for piggybacking additional prototyping circuitry onto the ESP32-S2 core — there's a secondary reason for the size of this PCB that we'll touch on in just a second — first up, let's take a look at the smaller details that make up this striking circuit board.
Based around one of the newer Espressif parts, the ESP32-S2 is the single core variant of the ESP32 that we all know and love. Aimed at doubling down at what the ESP silicon is known best for doing — a great Wi-Fi MCU — this single core sibling of the Espressif silicon family also bids farewell to the Bluetooth functionality previously found on the ESP32 die. putting the focus purely on connectivity based around straight-up Wi-Fi, or alternatively, perhaps the ESP-NOW orESP-MESH frameworks.
With not a single pin left unmapped on the ESP32-S2 IC, this may as well be one of the reference designs for this hot new silicon, even if it takes a little teasing apart to pick up on all the small details packed into the schematic page.
Things like, the built-in USB transceiver of the ESP32-S2, with the net names USB_N/USB_P giving away the alternate functionality of GPIO19 and GPIO20!
We can see Pcbnew confirms this pretty freaking awesome feature enhancement — with no USB-Serial converter to be seen in-between the edge-mounted USB-C connector, and the ESP32-S2 itself.
All there is to be seen sitting on the USB differential lines is the well placed USBLC6-2, providing some ESD protection to this external interface, along with the typical inline series resistors.
It is Interesting to note that despite the built in RTC of the ESP32-S2, the lack of any 32kHz crystal on the associated ESP32 pins indicates that Davill has chosen to instead employ an external device for this task.
Snooping around the schematic quickly reveals the the RV-8803-C7 RTC device from Micro Crystal.
This all-in-one, tiny SON packaged device occupies a footprint of just 3.2mm x 1.5mm and, even more impressively, incorporates its own 32kHz crystal oscillator — similar to other such I2C RTC devices as the Maxim DS3231M that we have been seeing lately.
We've touched on the benefits of such co-packaged crystal RTC devices before, and when looking at the timekeeping current of these chips, the 240nA of the RV-8803 is certainly quite appealing in comparison to some of the competition.
With a SMD coin-cell battery to back up the RTC power domain, this PCB should be able to keep tabs on time for quite a while between the occasional bit of juice when it's plugged in to USB!
Sometimes you just want some well proven parts when it comes to picking components for your power stage — so it's little surprise to see Texas Instruments stealing the show here.
With portability in mind, this board is fitted out to run either from the USB supplied 5V DC, or, alternatively from a Li-ion battery, with a TI BQ24072 managing the safe charging of the cells chemistry.
Aimed at making things a bit simpler, the BQ24072 is a fully integrated, stand-alone device, able to keep tabs on the safe management of the Li-Ion cell, without any host intervention required. Sometimes it's nice to have these things "just work" — this is one of those times.
With a TLV62569 handling the 3.3V rail at up to 2A of current, there's no risk of brown out on anything this board should be doing...
But with the ESP32-s2 more than happy to run at 3.3V, we wonder, what is the purpose of the TPS63900 buck-boost converter, configured to supply a decent 5V rail?
With a recent slew of project that look to make use of a recent fire-sale of some very attractively priced 400 x 240 2.7" memory LCD panels, this breakout board has a secret identity — as a base board for all things memory LCD related!
These panels are a half-way house between a conventional LCD panel, and an ePaper display, both in terms of power consumption, and functionality.
While offering a fantastic display update rate, with a speedy SPI interface, they do require a little more int the way of supply voltage, with glitching occurring anywhere below the ~3.6V mark - meaning the 3.3V rail, although capable, is not going to cut it here.
Now we see the reason for the 5V DC/DC boost converter — that will not only happily feed this display, but give a whole bunch of overhead, should you have any need for 5V on the prototyping area of this PCB.
One other oddity of these displays is that the require an approximate 1Hz duty cycle square wave to be applied to one of the display input pins — as part of the mechanism to keep the display image driven (I have a feeling these displays are internally, clock-less, requiring this external time base to toggle things along in order to keep the pixels positioned as required).
This is where we see a little ingenuity from Davill, with the 100R resistor between the nets RTC_OUT and LCD_COM indicating that he has configured the RTC device to provide this signal.
This has the advantage of allowing for some really low power display options — the RTC can kick the ESP32 awake, update the display frame buffer, and then hold the image in place while the ESP32 goes back to sleep for another nap.
That's a clever two-birds-one-stone sorta hack, that we love to see!
If you've ever done much display work, you will know that 400 x 240 pixels, even in monochrome, is a sizable amount of memory to manage, and most MCU would benefit from some external storage onto which to sling extra bitmap blobs or GIF graphics.
With a hefty 32Mbit of SPI flash, there's likely no limit the amount of graphics data you could decide to add to your next ESP32-s2 project, and if (somehow?!) 32Mbit is still not enough, well, thankfully the DFN-8 package is simple enough to swap out.