Etlas: e-paper dashboard

ETLAS: E-PAPER DASHBOARD

05 SEPTEMBER 2024

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Etlas is a news, stock market, and weather tracker powered by an ESP32 NodeMCU -D1, featuring a 7.5-inch Waveshare e-paper display and a -DHT22 sensor module.

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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.

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The following diagram outlines the Etlas’s overall system architecture.

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architecture

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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.

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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:

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static inline int dht_await_pin_state(int state, int timeout)
-{
-    int t;
-    static const uint16_t delta = 1;
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-    for (t = 0; t < timeout; t += delta) {
-        ets_delay_us(delta);
-        if (gpio_get_level(DHT_PIN) == state)
-          return t;
-    }
-    return 0;
-}
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-static inline int dht_get_raw_data(unsigned char buf[DHT_DATA_LEN])
-{
-    int rc;
-    unsigned char i, pwl, pwh;
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-    gpio_set_level(DHT_PIN, 0);
-    ets_delay_us(1100);
-    gpio_set_level(DHT_PIN, 1);
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-    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;
-    }
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-    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;
-}
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I ported this implementation from ESP8266 -to ESP32—all credit for the algorithm belongs to them.

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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 -examples for Arduino and STM32 -platforms.

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Etlas has been running reliably for over a year since August 2024.

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Files: source.tar.gz

by W. D. Sadeep Madurange