Wenting Is Back to Wow Us Again, This Time with Preader — a Raspberry Pi RP2040-Powered E-Reader!

E-readers, bi-stable displays, electronic shelf labels — and more — are all the rage over on Twitter these days.

With low-power designs becoming a more and more common a target for product design teams and hobbyists alike, LCD TFT display panels are looking a little long in the tooth.

While immensely popular, and even with many, many how-tos and myriad references abound, the heyday and glamour us times of brightly backlit, high-contrast IPS panels may ultimately be coming to and end.

Low-power LCD alternatives

By now, you will all have heard of bi-stable displays. Although this is a catchment terms, that also includes such panel technologies as E Ink, the group also includes what are referred to as Cholesteric Liquid Crystal Display panels, which are another type of bi-stable display, more often seen used in applications such as electronic shelf labels.

They have some advantages over traditional E Ink displays — in that they often have a higher contrast, especially with colored ink applications (yup, they can also do multicolour panels, just as e-ink do!) — however, they do suffer from poor update rates at lower temperatures!

With older models of ESL already showing up on the usual secondhand auction and retail sites, those who want to get their hands on a few bits of hardware to hack upon, or some — at times — heavily discounted display modules to play with, know just where to look.

ChLCDs from secondhand ZBDs

When Wenting found one or two of the ZBD epop 900RB tags to purchase — FCCID for the 900 here — they quickly jumped upon the chance to get to grips with this panel technology!

With hardware in hand, Wenting set about upon the task of designing a custom circuit around the tag's somewhat secretive display panel.

This requires first and foremost, figuring out the functionality — and finer details — of the device layout that was originally derived by the designers at ZBD.

The eagle eyed will spot some signs that this is already going to be a fun foray into circuit design — see those rails up at top of edge-mounted display ZIF connector footprint? Yup, some 40-odd volts here aren't a typo, or poor measurement — these displays need some spiky voltages to generate the required electrostatic potential needed to polarize the pixels within the display panel glass.

Some will say well why no just reprogram the application memory on the EPOP900's original CC1110 32-bit MCU?

Well, that's certainly possible — the effects of random I/O bit banging can be seen above! — it adds it's own set of frustrations in reverse engineering the existing binaries and intimately getting to know each and every register of yet another chip.

Also, doing so that would leave out chance to play with the shiny, new RP2040, and some more appealing peripheral functions of the of the chip, including... USB Host Controller mode!

That is a bit of a leap of faith however, especially when the Pico development board skips over the inclusion of a compatible A-type, micro-USB connector - what is one to do when looking to explore the host mode features of the Foundations recent featherweight board?

Hack on, of course.

Wield that solder and iron and get that hardware hacked into line!

That's some tidy soldering, but also does make you wonder if the cost of a standard, A-type OTG connector — in volume — was enough to get cut down in cost optimization at production time.

With CircuitPython supporting the RP2040, we can see many users wanting to make use of such a feature for code updates, or perhaps media storage using MSD or even using controller peripherals like a keyboard or mouse — with HID class!

Powering on from prototyping....

With a grok on USB OTG, and USB Host / Device modes of the RP2040, some firmware to flip bits in the display modules, and a low level look at the signalling layer of the LCD, Wenting is confident enough to forge ahead with the first revision of his RP2040-powered ChLCD e-reader — the Preader.

Designed as a pair of boards — feature-rich controller PCB, along with a font facing board, that acts as a frame for the fragile LDC glass, and also features capacitive finger slider elements for some user feedback!

A fully functional RP2040 EPD interface board

Even without the front facia, this board can function fully independently, letting the RP2040 read from microSD, and write to the display panel data buffers, as is aptly demonstrated in the photo-op below!

Digging into the design...

We'll take a quick look at the hardware / firmware design for the Preader, as it's clean, well thought out and accessible — as far as most designs go!

There are a few points to muse upon, namely focused around the design and control of the various power supply and delivery components.

A boosted bias voltage for a bi-stable display...

We noted earlier those spiky, significant supply lines that are fed to the displays electrostatic potential bias supply pins. These rails don't need to be up at the 40V initially measured, and are happily driven in the region of 15-20V, for decent contrast during operation.

If we dig into the aptly named "lcd" schematic sheet, we find the display's requisite ZIF connector, and all of the various support circuitry needed to these picky panels into life. The main supply rail for these various bias points is generated from a LT1615, switched DC/DC boost converter IC.

It's a proven part that brings a KISS approach to the generation of this boosted rail — VEE.

The MIC5233 more manageably modulates the main VEE rail

Within the same sheet, we see four, separate implementations of the MIC5233 high-voltage input, low Iq LDO regulators, with inputs being fed from VEE.

These regulators are available in fixed output or adjustable versions, and we can see the adjustable part used in this design.

What isn't visible in the above screenshot is source of the reference voltage being fed to the junction of R511 and R512 (as shown above)!

If we follow the schematic, we arrive at a Maxim IC DS4424 I2C controlled, seven-bit current sink/source digital-to-analog converter (DAC).

By supplying or sipping small amounts of current to or from the base voltage generated by the resistive divider of the MIC5233 feedback loop, the output can be finely modulated as required by the application. Neat!

Power management made as easy as... a PMIC?

There's a lot to keep an eye on in a portable device, such as this one.

Starting with USB, there's a 5V input. There's also the system bus, at 3.3V that needs to be catered for.

There's also a battery that needs to be charged, and up/down regulated to system VCC as it discharges.

There was also talk of USB-OTG, which necessitates that the gadget in question must be able to supply power to the USB device attached to its OTG port. This would mean taking that same battery input, and boosting it beyond 3.3V, all the way to 5V.

This sounds like a potential muddle of DC/DC switching regulators, load switches, feedback loops and a lot of monitoring — made up of sub-modules that each handle their specific task. That's not only complex, but costly.

If only there was a cheaper, simpler way to farm out the management of all these supplies and sources!

A PMIC, or power management integrated circuit, is one that is application specific to just that — designed to offer a one-stop-shop solution to your systems supplies, while seamlessly offering safekeeping over the lithium-chemistry rechargeable cell, from which it is fed.

The Analog Devices ADP5360, seen in this design, is able to handle not only the safe charging of the LiPo cell, but also keeps an eye on the estimated "juice" remaining, thanks to a method known as Columb counting — monitoring the flow of current in, and out of the cell, in order to gauge the remaining capacity.

More than just a fancy battery charger, the device will also provide two output rails, at programmable voltages — by feeding the cell supply to its two, built-in DC/DC switching regulators — one buck, one buck/boost. The buck regulator feeds SYS at 3.3V, while the buck/boost feeds USB-OTG, at 5V.

With status and control registers accessible over I2C — naturally mapped to the RP2040 I2C peripheral — this is a complete solution to getting the required rails wrangled, when working with a single-cell lithium-ion-powered application.

Get reading!

While Wenting is whetting our appetite with wonderful updates over on Twitter (@zephray_wenting), but the impulsive would likely get a kick from getting a grok over at the Preader project sources, over on GitHub here.