ETLAS: E-PAPER DASHBOARD
05 SEPTEMBER 2024
Repurposed e-reader prototype into something for regular use. News, stocks, weather dashboard. ESP32 NodeMCU D1 + 7.5” Waveshare e-paper + DHT22 sensor.
 |
 |
 |
|
Stocks: Two weeks EOD prices from Polygon.io (max SRAM holds). Flask app on VPS relays feed, lets me swap symbols via config. FastCGI + httpd + htpasswd.
gui_plot_stocks() triggers WDT. vTaskDelay() prevents that. Stepped graph was easier to code for the bit-packed framebuffer (not enough memory for much else), but it’s messy–too messy.
Note to self: Refactor gui_plot_stocks(). Bresenham’s line algorithm perhaps?
News: Channel NewsAsia RSS. MCU parses XML directly. Character glyphs stored as bitmaps in sprites directory.
Weather: DHT22 via PWM. 26µs/50µs/70µs pulses too fast for standard ESP32 APIs. Compare relative pulse widths:
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[BUFLEN])
{
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 < BUFLEN; 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;
}
Ported from ESP8266.
After flashing new versions, display would stop responding. Didn’t observe this with e-reader. Issue occurs spontaneously. Couldn’t find root cause. Making a change like adding a comment or tweaking delay in refresh() (then changing back) fixed it. Noisy electrical connections?
Once running, it runs: August 2024 - December 2025.
Commit: a92c86a