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diff --git a/_log/etlas.md b/_log/etlas.md new file mode 100644 index 0000000..c274401 --- /dev/null +++ b/_log/etlas.md @@ -0,0 +1,115 @@ +--- +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. + + + +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) |