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----
-title: Experimental e-reader
-date: 2023-10-24
-thumbnail: thumb_sm.png
-layout: post
----
-
-This project features an experimental e-reader powered by an ESP-WROOM-32
-development board and a 7.5-inch <a href="https://www.waveshare.com/"
-class="external" target="_blank" rel="noopener noreferrer">Waveshare</a>
-e-paper display built with the intention of learning about e-paper displays.
-
-<video style="max-width:100%;" controls="" poster="poster.png">
-  <source src="ereader.mp4" type="video/mp4">
-</video>
-
-## Introduction
-
-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.
-
-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.
-
-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.
-
-## The file format
-
-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).
-
-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.
-
-The pdftoebm.py script enclosed in the tarball at the end of the page converts
-PDF documents to EBM files.
-
-## How does it work?
-
-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. 
-
-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.
-
-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.
-
-## Epilogue
-
-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.
-
-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.
-
-Files: [source.tar.gz](source.tar.gz)