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<h2 class="center" id="title">ESP32 E-READER PROTOTYPE</h2>
<h6 class="center">24 OCTOBER 2023</h5>
<br>
<div class="twocol justify"><p>First project with e-paper displays and ESP32.</p>
<video style="max-width:100%;" controls="" poster="poster.png">
<source src="ereader.mp4" type="video/mp4" />
</video>
<p>ESP-WROOM-32, 7.5” Waveshare e-paper display, three buttons (prev/next/sleep).</p>
<p>No local storage—streams books over HTTP. RTC memory tracks reading progress
between sessions.</p>
<p>ESP32: 512KB SRAM, 4MB flash (shared with FreeRTOS, ESP-IDF). Not enough to
store books. Stream from webserver instead.</p>
<p>Custom EBM file format. Rasterized monochrome bitmaps, one per page (480x800).
One byte = 8 pixels. Pages on 48KB boundaries—HTTP Range requests work without
server logic. pdftoebm.py converts PDFs.</p>
<p>Circular buffer holds 3 pages (prev/current/next)–page table. Single-threaded
approach too slow—user input, SPI, and HTTP on one core causes lag.</p>
<p>Optimizations: pin the GPIO task (responding to user input and updating
display) to one core, pin the HTTP task to the other core.</p>
<p>Better, but system unresponsive during screen updates. Made SPI transfer async:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>void epd_draw_async(const uint8_t *buf, size_t n)
{
static spi_transaction_t t[3];
memset(&t[0], 0, sizeof(t[0]));
t[0].length = 8;
t[0].tx_data[0] = 0x13;
t[0].user = (void*) 0;
t[0].flags = SPI_TRANS_USE_TXDATA;
memset(&t[1], 0, sizeof(t[1]));
t[1].length = 8 * n;
t[1].tx_buffer = buf;
t[1].user = (void*) 1;
memset(&t[2], 0, sizeof(t[2]));
t[2].length = 8;
t[2].tx_data[0] = 0x12;
t[2].user = (void*) 0;
t[2].flags = SPI_TRANS_USE_TXDATA;
for (int i = 0; i < 3; i++)
spi_device_queue_trans(spi, &t[i], portMAX_DELAY);
}
</code></pre></div></div>
<p>Much better. Squeeze a few more cycles by moving SPI buffer to DMA.</p>
<p>Can’t think of anything else.</p>
<p>Outcome: Works but limited. Led to <a href="../etlas/">Etlas</a>.</p>
<p>Commit:
<a href="https://git.asciimx.com/esp32-e-reader/commit/?id=7f691c46093933b67aab466c0ca582ace8ab73d4">7f691c4</a></p>
</div>
<p class="post-author right">by W. D. Sadeep Madurange</p>
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