From 4c9bfbf32b651f54aa64c1a88b972819fddbab75 Mon Sep 17 00:00:00 2001 From: Sadeep Madurange Date: Tue, 16 Dec 2025 23:26:14 +0800 Subject: Ereader and some other language fixes. --- _site/projects/e-reader/index.html | 2 +- _site/projects/etlas/index.html | 160 ++++++++++++++------------ _site/projects/matrix-digital-rain/index.html | 2 +- 3 files changed, 91 insertions(+), 73 deletions(-) (limited to '_site/projects') diff --git a/_site/projects/e-reader/index.html b/_site/projects/e-reader/index.html index 2e04274..ea997c4 100644 --- a/_site/projects/e-reader/index.html +++ b/_site/projects/e-reader/index.html @@ -127,7 +127,7 @@ and RTC memory.

features like an offline library of books and touch screens, there’s something to be said about reading on hardware you built yourself. You are no longer the powerless Muggle watching others perform magic. You are the wizard who makes -the invisible particles swirl into form by whispering C to it.

+the invisible particles swirl into form by whispering C to them.

Files: source.tar.gz

diff --git a/_site/projects/etlas/index.html b/_site/projects/etlas/index.html index 6b2906b..ae30cb6 100644 --- a/_site/projects/etlas/index.html +++ b/_site/projects/etlas/index.html @@ -44,7 +44,9 @@

ETLAS: E-PAPER DASHBOARD

05 SEPTEMBER 2024

-

Etlas is a weather, news, and stock price tracking system.

+

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.

@@ -53,80 +55,96 @@
-

Acknowledgements

+

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 e-paper display driver in the epd.c file of the project is derived from Waveshare examples for Arduino -and STM32 platforms.

- -

The exceptionally elegant algorithm in the file dht.c, which reads DHT22 sensor -data by comparing consecutive pulses, I ported directly from this implementation for ESP8266 -modules to my ESP32. All credit for the algorithm belongs to them.

- -

Overview

- -

Etlas comprises an embedded system featuring an ESP32 NodeMCU D1 -microcontroller, a 7.5″ Waveshare e-paper display, a -DHT22 weather sensor, and a server backend consisting of a FastCGI-wrapped -Flask app. The following diagram outlines this system architecture.

+

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

architecture

-

The embedded application, written in C with the help of the ESP-IDF v5.2.1, -connects to a (2.4GHz) WiFi network at startup and obtains time from an NTP -server. It then composites and updates an internal pixel buffer using data -acquired from various data sources and renders to the screen by writing to its -Serial Peripheral Interface.

- -

Price curves

- -

On the e-paper display, the top-left panel displays end-of-day price curves -over two weeks from the Polygon.io API. The -microcontroller’s 512KB SRAM limits the price curves to two weeks. Instead of -using raster images for complex graphics—like e-paper projects often do—Etlas -computes the price curves from CSV data on the fly using the ESP32’s 160MHz -microprocessor. To avoid the overhead of floating-point arithmetic, all -computations are performed on integers. The gui_plot_stocks() function in the -gui.c file contains the rendering logic.

- -

Proxying the price feeds through the Flask app permits me to configure the -tickers I’m interested in via its application settings. The Flask app cycles -through the tickers in its configuration file as it receives requests from the -embedded software. The Flask app is secured with basic authentication -implemented using htpasswd and OpenBSD’s httpd web server. This FastCGI-wrapped -architecture dramatically reduces the complexity of the server code and -increases system reliability.

- -

News feed

- -

The more prominent panel on the right displays news from Channel News Asia. The embedded program downloads -and parses the RSS feed before rendering it to the display. The character -glyphs used for rendering news are stored in header files in the sprites -directory as bitmaps. I created the glyphs manually by saving them as images in -GIMP and rasterizing them with ImageMagick.

- -

I connected the embedded system directly to the RSS feed to write less server -code (the focus of this project was the embedded system). In hindsight, -however, it limits the feeds from which Etlas can receive data. In a future -version, I will relay the RSS feed through the backend (like the stock prices) -to make it more flexible.

- -

Weather data

- -

The bottom panels (middle and right) display the temperature and relative -humidity from a DHT22 sensor. The DHT22 driver, arguably the most interesting -part of the software, reads real-time sensor data by comparing relative pulse -widths. The pulses themselves are too quick for the ESP32 to reliably measure -directly.

- -

Much of the heavy lifting of acquiring, interpreting, and rendering data from -different data sources is performed on the microcontroller using less than 512 -KB of memory. The embedded software that makes that possible is written in C -using the ESP-IDF v5.2.1. My e-paper display driver is a port of Waveshare examples for Arduino -and STM32 platforms.

- -

I’ve been using Etlas daily (for a couple of hours on weekdays and all day on -weekends) since August 2024. As of October 2025, it’s been running reliably for -over a year.

+

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 this 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 +examples for Arduino and STM32 +platforms.

+ +

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

Files: source.tar.gz

diff --git a/_site/projects/matrix-digital-rain/index.html b/_site/projects/matrix-digital-rain/index.html index 2893c92..b2107b4 100644 --- a/_site/projects/matrix-digital-rain/index.html +++ b/_site/projects/matrix-digital-rain/index.html @@ -45,7 +45,7 @@
12 JANUARY 2024

“All I see is blonde, brunette, red head.” The iconic digital rain from The -Matrix in C, with zero dependencies - not even ncurses.

+Matrix in C, with zero dependencies—not even ncurses.