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+---
+title: "Etlas: e-paper dashboard"
+date: 2024-09-05
+layout: post
+project: true
+thumbnail: thumb_sm.jpg
+---
+
+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)