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<h2 class="center" id="title">512KB E-READER</h2>
<h5 class="center">24 OCTOBER 2023</h5>
<br>
<div class="twocol justify"><p>First project with e-paper.</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-button interface
(prev/next/sleep).</p>
<p>Memory: 512KB SRAM + 4MB flash. Internal flash unsuitable for storing books due
to P/E cycle limit. Used HTTP Range requests to stream them on-demand.
Progress saved to RTC memory to survive deep sleep without flash wear.</p>
<p>PDFs are rasterized and stored as sequences of bitmaps on a server. 1 byte = 8
pixels, 1 page = 48KB (display resolution), headerless. Optimized for Range
requests without server-side logic:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>int r0 = ((page_n - 1) * PAGE_SIZE);
int rn = page_n * PAGE_SIZE - 1;
int n = snprintf(NULL, 0, "bytes=%d-%d", r0, rn) + 1;
char *buf = malloc(sizeof(char) * n);
snprintf(buf, n, "bytes=%d-%d", r0, rn);
esp_http_client_set_header(http_client, "Range", buf);
esp_http_client_perform(http_client);
</code></pre></div></div>
<p>Three pages (prev/current/next) held in a buffer—maximum possible. Upon
request, embedded software cycles the buffer, updates the screen, prefetches
the next page.</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>c_page_num++;
pg.page_num = c_page_num + 2;
pg.page_buf = pages[(c_page_num + 1) % PAGE_LEN];
xSemaphoreGive(mutex);
xQueueSend(http_evt_queue, &pg, portMAX_DELAY);
epd_draw_async(pages[c_page_num % PAGE_LEN], PAGE_SIZE);
epd_draw_await();
</code></pre></div></div>
<p>System isn’t as responsive as I’d hoped. Scheduling GPIO, SPI, and HTTP tasks
on a single thread causes input lag. Pinned GPIO/SPI tasks to one core and the
HTTP task to the other.</p>
<p>Better, but screen updates block user input.</p>
<p>Moved the SPI buffers to DMA and made the transfers async. Few more cycles
saved.</p>
<p>Can’t think of anything else.</p>
<p>Verdict: Functional 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>
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<p class="post-author right">by W. D. Sadeep Madurange</p>
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