Experimental e-reader

EXPERIMENTAL E-READER

24 OCTOBER 2023

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This project features an experimental e-reader powered by an ESP-WROOM-32 +development board and a 7.5-inch Waveshare +e-paper display built with the intention of learning about e-paper displays.

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Introduction

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The prototype e-reader comprises an ESP32 microcontroller, an e-paper display +HAT, and three buttons: yellow, blue, and white for turning the page backwards, +forwards, and putting the device to sleep, respectively. The prototype does not +store books on the microcontroller. It streams books from a server over HTTP. +The e-reader employs RTC memory to record the reading progress between +sessions.

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The most formidable challenge when trying to build an e-reader with an ESP32 is +its limited memory and storage. My ESP-WROOM-32 has a total of 512KB of SRAM +and 4MB of flash memory, which the freeRTOS, ESP-IDF, and the e-reader +application must share. To put things into perspective, a Kindle Paperwhite has +at least 256MB of memory and 8GB of storage. That is 500x more memory than what +I’d have to work with.

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Despite its size, as microcontrollers go, ESP32 is a powerful system-on-a-chip +with a 160MHz dual-core processor and integrated WiFi. So, I thought it’d be +amusing to embrace the constraints and build my e-reader using a $5 MCU and the +power of C programming.

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The file format

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The file format dictates the complexity of the embedded software. So, I’ll +begin there. The e-reader works by downloading and rendering a rasterized +monochrome image of a page (a .ebm file).

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The EBM file contains a series of bitmaps, one for each page of the book. The +dimensions of each bitmap are equal to the size of the display. Each byte of +the bitmap encodes information for rendering eight pixels. For my display, +which has a resolution of 480x800, the bitmaps are laid out along 48KB +boundaries. This simple file format lends well to HTTP streaming, which is its +main advantage, as we will soon see.

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The pdftoebm.py script enclosed in the tarball at the end of the page converts +PDF documents to EBM files.

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How does it work?

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As the e-reader has no storage, it can’t store books locally. Instead, it +downloads pages of the EBM file over HTTP from the location pointed to by the +EBM_ARCH_URL setting in the Kconfig.projbuild file on demand. To read a +different book, we have to replace the old file with the new one or change the +EBM_ARCH_URL value. The latter requires us to recompile the embedded +software.

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Upon powering up, the e-reader checks the reading progress stored in the RTC +memory. It then downloads three pages (current, previous, and next) to a +circular buffer in DMA-capable memory. When the user turns a page by pressing a +button, one of the microprocessor’s two cores transfers it from the buffer to +the display over a Serial Peripheral Interface (SPI). The other downloads a new +page in the background. I used the ESP-IDF task API to schedule the two tasks +on different cores of the multicore processor to make the reader more +responsive.

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I designed the EBM format with HTTP streaming in mind. Since the pages are laid +out in the EBM file along predictable boundaries, the e-reader can request +pages by specifying the offset and the chunk size in the HTTP Range header. Any +web server will process this request without custom logic.

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Epilogue

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My fascination with e-paper began back in 2017, when I was tasked with +installing a few displays in a car park. Having no idea how they worked, I +remember watching the languid screens refresh like a Muggle witnessing magic. +This project was born out of that enduring curiosity and love of e-paper +technology.

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Why did I go to the trouble of building a rudimentary e-reader when I could +easily buy a more capable commercial e-reader? First of all, it’s to prove to +myself that I can. More importantly, there’s a quiet satisfaction to reading on +hardware you built yourself. You are no longer the powerless observer watching +the magic happen from the sidelines. You become the wizard who makes the +invisible particles swirl into form by whispering C to them. There’s only one +way to experience that.

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Files: source.tar.gz

by W. D. Sadeep Madurange