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

Repurposed [e-reader prototype](../e-reader/) into something for regular use.
News, stocks, weather dashboard.  ESP32 NodeMCU D1 + 7.5" Waveshare e-paper +
DHT22 sensor.

<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>
  <tr style="border: none;">
    <td colspan="2" style="border: none;"><img src="etlas_arch.png" alt="front" style="width: 100%"></td>
  </tr>
</table>

Stocks: Two weeks EOD prices from Polygon.io (max possible). Flask app on VPS
relays feed; manages watchlist. Backend: httpd + htpasswd + FastCGI + Flask.

gui_plot_stocks() triggers watchdog; vTaskDelay() required. Stepped graph was
easier to implement (under memory constraints), but the logic is hideous. Note
to self: Refactor with Bresenham’s?

News: Channel NewsAsia RSS. MCU parses XML directly. Character glyphs stored as
bitmaps in header files.

Weather: DHT22 single-wire protocol. 26µs/50µs/70µs pulses are too fast for
standard ESP32 APIs. Bit-banged relative pulse widths: (Ported from <a
href="https://github.com/Fonger/ESP8266-RTOS-DHT" class="external"
target="_blank" rel="noopener noreferrer">ESP8266</a>):

```
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;
}
```

Ghost in the machine: epd_init() stalls intermittently on first refresh() after
flash. Toggling delay values in refresh() resolves it. If first refresh
succeeds, it remains stable. Root cause unknown--suspected power supply issues
from powering display via MCU.

Uptime: August 2024 - January 2026

Commit:
[a92c86a](https://git.asciimx.com/etlas/commit/?id=a92c86ac1592c2137d3d1fec1668eacc2d0ca581)