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----
-title: "Etlas: e-paper dashboard"
-date: 2024-09-05
-thumbnail: thumb_sm.jpg
-layout: post
----
-
-Etlas is a news, stock market, and weather tracker powered by an ESP32 NodeMCU
-D1, featuring a 7.5-inch <a href="https://www.waveshare.com/" class="external" 
-target="_blank" rel="noopener noreferrer">Waveshare</a> e-paper display and a
-DHT22 sensor module.
-
-<table style="border: none;">
-  <tr style="border: none;">
-    <td style="border: none;"><img src="dash.jpg" alt="front" style="width: 100%"></td>
-    <td style="border: none;"><img src="pcb.jpg" alt="back" style="width: 100%"></td>
-  </tr>
-</table>
-
-The top-left panel shows two weeks of end-of-day prices—the maximum the ESP32's
-SRAM can hold—from the Polygon.io API. The price feed is relayed through a
-FastCGI-wrapped Flask app hosted on a VPS. This lets me configure stock symbols
-in its application settings. The app cycles through them as requests come in
-from the ESP32. Running the Flask app as a FastCGI process while exposing it
-via httpd with htpasswd authentication keeps the server code simple and secure.
-
-The following diagram outlines the Etlas's overall system architecture.
-
-![architecture](etlas_arch.png)
-
-The more prominent panel on the right of the display shows local and world news
-from Channel NewsAsia. The MCU downloads and parses XML data from the RSS feed
-directly before rendering it to the display. The character glyphs used are
-stored as bitmaps in the sprites directory. I skipped the proxy for news to
-avoid writing more server code, but in hindsight it limits the feeds Etlas can
-handle. I will fix this in a future version.
-
-The middle and bottom right panels display the temperature and relative
-humidity from the DHT22 sensor. The DHT22 uses pulse-width modulation to
-transmit data to the host. The 26µs, 50µs, and 70µs pulses are too fast for the
-ESP32 to measure reliably with standard APIs. Instead, the driver compares
-relative pulse widths to differentiate zeros from ones:
-
-```
-static inline int dht_await_pin_state(int state, int timeout)
-{
-    int t;
-    static const uint16_t delta = 1;
-
-    for (t = 0; t < timeout; t += delta) {
-        ets_delay_us(delta);
-        if (gpio_get_level(DHT_PIN) == state)
-          return t;
-    }
-    return 0;
-}
-
-static inline int dht_get_raw_data(unsigned char buf[DHT_DATA_LEN])
-{
-    int rc;
-    unsigned char i, pwl, pwh;
-
-    gpio_set_level(DHT_PIN, 0);
-    ets_delay_us(1100);
-    gpio_set_level(DHT_PIN, 1);
-
-    if (!dht_await_pin_state(0, 40)) {
-        rc = 1;
-        xQueueSend(dht_evt_queue, &rc, (TickType_t) 0);
-        return 0;
-    }
-    if (!dht_await_pin_state(1, 80)) {
-        rc = 2;
-        xQueueSend(dht_evt_queue, &rc, (TickType_t) 0);
-        return 0;
-    }
-    if (!dht_await_pin_state(0, 80)) {
-        rc = 3;
-        xQueueSend(dht_evt_queue, &rc, (TickType_t) 0);
-        return 0;
-    }
-
-    for (i = 0; i < DHT_DATA_LEN; i++) {
-        if (!(pwl = dht_await_pin_state(1, 50))) {
-            rc = 4;
-            xQueueSend(dht_evt_queue, &rc, (TickType_t) 0);
-            return 0;
-        }
-        if (!(pwh = dht_await_pin_state(0, 70))) {
-            rc = 5;
-            xQueueSend(dht_evt_queue, &rc, (TickType_t) 0);
-            return 0;
-        }
-        buf[i] = pwh > pwl;
-    }
-    return 1;
-}
-```
-
-I ported <a href="https://github.com/Fonger/ESP8266-RTOS-DHT" class="external"
-target="_blank" rel="noopener noreferrer">this</a> implementation from ESP8266
-to ESP32—all credit for the algorithm belongs to them.
-
-Etlas is a networked embedded system. All acquisition, processing, and
-rendering of data are performed on the ESP32's 160MHz microprocessor using less
-than 512KB of SRAM. The embedded software that makes this possible is written
-in C using ESP-IDF v5.2.1. The e-paper display driver is derived from Waveshare
-<a href="https://github.com/waveshareteam/e-Paper" class="external"
-target="_blank" rel="noopener noreferrer">examples</a> for Arduino and STM32
-platforms.
-
-Etlas has been running reliably for over a year since August 2024.
-
-Files: [source.tar.gz](source.tar.gz)