Initial commit

.gitignore

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+.pio
+.vscode/.browse.c_cpp.db*
+.vscode/c_cpp_properties.json
+.vscode/launch.json
+.vscode/ipch

.vscode/extensions.json

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+{
+    // See http://go.microsoft.com/fwlink/?LinkId=827846
+    // for the documentation about the extensions.json format
+    "recommendations": [
+        "platformio.platformio-ide"
+    ],
+    "unwantedRecommendations": [
+        "ms-vscode.cpptools-extension-pack"
+    ]
+}

include/README

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+
+This directory is intended for project header files.
+
+A header file is a file containing C declarations and macro definitions
+to be shared between several project source files. You request the use of a
+header file in your project source file (C, C++, etc) located in `src` folder
+by including it, with the C preprocessing directive `#include'.
+
+```src/main.c
+
+#include "header.h"
+
+int main (void)
+{
+ ...
+}
+```
+
+Including a header file produces the same results as copying the header file
+into each source file that needs it. Such copying would be time-consuming
+and error-prone. With a header file, the related declarations appear
+in only one place. If they need to be changed, they can be changed in one
+place, and programs that include the header file will automatically use the
+new version when next recompiled. The header file eliminates the labor of
+finding and changing all the copies as well as the risk that a failure to
+find one copy will result in inconsistencies within a program.
+
+In C, the convention is to give header files names that end with `.h'.
+
+Read more about using header files in official GCC documentation:
+
+* Include Syntax
+* Include Operation
+* Once-Only Headers
+* Computed Includes
+
+https://gcc.gnu.org/onlinedocs/cpp/Header-Files.html

lib/README

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+
+This directory is intended for project specific (private) libraries.
+PlatformIO will compile them to static libraries and link into the executable file.
+
+The source code of each library should be placed in a separate directory
+("lib/your_library_name/[Code]").
+
+For example, see the structure of the following example libraries `Foo` and `Bar`:
+
+|--lib
+|  |
+|  |--Bar
+|  |  |--docs
+|  |  |--examples
+|  |  |--src
+|  |     |- Bar.c
+|  |     |- Bar.h
+|  |  |- library.json (optional. for custom build options, etc) https://docs.platformio.org/page/librarymanager/config.html
+|  |
+|  |--Foo
+|  |  |- Foo.c
+|  |  |- Foo.h
+|  |
+|  |- README --> THIS FILE
+|
+|- platformio.ini
+|--src
+   |- main.c
+
+Example contents of `src/main.c` using Foo and Bar:
+```
+#include <Foo.h>
+#include <Bar.h>
+
+int main (void)
+{
+  ...
+}
+
+```
+
+The PlatformIO Library Dependency Finder will find automatically dependent
+libraries by scanning project source files.
+
+More information about PlatformIO Library Dependency Finder
+- https://docs.platformio.org/page/librarymanager/ldf.html

platformio.ini

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+; PlatformIO Project Configuration File
+;
+;   Build options: build flags, source filter
+;   Upload options: custom upload port, speed and extra flags
+;   Library options: dependencies, extra library storages
+;   Advanced options: extra scripting
+;
+; Please visit documentation for the other options and examples
+; https://docs.platformio.org/page/projectconf.html
+
+[env:rp2040zero]
+platform = https://github.com/maxgerhardt/platform-raspberrypi.git
+board = waveshare_rp2040_zero
+framework = arduino
+board_build.core = earlephilhower
+upload_protocol = picotool
+monitor_speed = 115200
+lib_deps =
+    adafruit/Adafruit NeoPixel @ ^1.12.0

src/main.cpp

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+#include <Arduino.h>
+#include <Adafruit_NeoPixel.h>
+
+// CLOCK module (TX)
+const uint8_t TX_DI = 0;
+const uint8_t TX_DE = 1;
+const uint8_t TX_RE = 2;
+
+// DATA module (RX)
+const uint8_t RX_DI = 26;
+const uint8_t RX_DE = 27;
+const uint8_t RX_RE = 28;
+const uint8_t RX_RO = 29;
+
+const uint8_t LED_PIN = 16;
+Adafruit_NeoPixel led(1, LED_PIN, NEO_GRB + NEO_KHZ800);
+
+void setStatus(uint8_t r, uint8_t g, uint8_t b) {
+  led.setPixelColor(0, led.Color(r, g, b));
+  led.show();
+}
+
+// Read N bits from SSI encoder.
+// SSI convention: encoder latches on first falling edge,
+// shifts new bit on rising edge, master samples on falling edge.
+uint64_t ssi_read(uint8_t bits, uint16_t half_us) {
+  uint64_t value = 0;
+
+  // Frame start: clock idles HIGH, drop to LOW = latch trigger
+  digitalWrite(TX_DI, LOW);
+  delayMicroseconds(half_us);
+
+  for (uint8_t i = 0; i < bits; i++) {
+    digitalWrite(TX_DI, HIGH);                     // encoder shifts next bit out
+    delayMicroseconds(half_us);
+    digitalWrite(TX_DI, LOW);                      // sample on falling edge
+    value = (value << 1) | (digitalRead(RX_RO) ? 1 : 0);
+    delayMicroseconds(half_us);
+  }
+
+  // Return to idle and wait monoflop time
+  digitalWrite(TX_DI, HIGH);
+  delayMicroseconds(30);
+
+  return value;
+}
+
+void handleCommand(const String& cmd) {
+  // Format: "READ <bits> <half_us>"
+  // Example: "READ 25 5"
+  if (!cmd.startsWith("READ ")) {
+    Serial.println("ERR unknown command. Use: READ <bits> <half_us>");
+    return;
+  }
+
+  int firstSpace = cmd.indexOf(' ');
+  int secondSpace = cmd.indexOf(' ', firstSpace + 1);
+  if (secondSpace < 0) {
+    Serial.println("ERR usage: READ <bits> <half_us>");
+    return;
+  }
+
+  int bits   = cmd.substring(firstSpace + 1, secondSpace).toInt();
+  int halfUs = cmd.substring(secondSpace + 1).toInt();
+
+  if (bits < 1 || bits > 64) {
+    Serial.println("ERR bits must be 1..64");
+    return;
+  }
+  if (halfUs < 1 || halfUs > 10000) {
+    Serial.println("ERR half_us must be 1..10000");
+    return;
+  }
+
+  setStatus(0, 0, 16);                             // blue = reading
+  uint64_t raw = ssi_read(bits, halfUs);
+  setStatus(0, 16, 0);                             // green = idle
+
+  // Print as hex and decimal
+  // (Pico printf supports %llX for 64-bit)
+  Serial.printf("OK bits=%d half_us=%d hex=0x%llX dec=%llu\n",
+                bits, halfUs, raw, raw);
+}
+
+void setup() {
+  Serial.begin(115200);
+  led.begin();
+  setStatus(8, 8, 0);
+
+  pinMode(TX_DI, OUTPUT);
+  pinMode(TX_DE, OUTPUT); pinMode(TX_RE, OUTPUT);
+  pinMode(RX_DE, OUTPUT); pinMode(RX_RE, OUTPUT);
+  pinMode(RX_DI, OUTPUT);
+  pinMode(RX_RO, INPUT);
+
+  digitalWrite(TX_DE, HIGH);
+  digitalWrite(TX_RE, HIGH);
+  digitalWrite(RX_DE, LOW);
+  digitalWrite(RX_RE, LOW);
+  digitalWrite(RX_DI, LOW);
+  digitalWrite(TX_DI, HIGH);                       // SSI idle HIGH
+
+  delay(200);
+  while (!Serial && millis() < 3000) { delay(10); }
+  Serial.println("\nSSI bridge ready");
+  Serial.println("Send: READ <bits> <half_us>");
+  Serial.println("Example: READ 25 5");
+  setStatus(0, 16, 0);
+}
+
+void loop() {
+  static String buf;
+  while (Serial.available()) {
+    char c = Serial.read();
+    if (c == '\n' || c == '\r') {
+      if (buf.length() > 0) {
+        handleCommand(buf);
+        buf = "";
+      }
+    } else if (buf.length() < 64) {
+      buf += c;
+    }
+  }
+}

test/README

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+
+This directory is intended for PlatformIO Test Runner and project tests.
+
+Unit Testing is a software testing method by which individual units of
+source code, sets of one or more MCU program modules together with associated
+control data, usage procedures, and operating procedures, are tested to
+determine whether they are fit for use. Unit testing finds problems early
+in the development cycle.
+
+More information about PlatformIO Unit Testing:
+- https://docs.platformio.org/en/latest/advanced/unit-testing/index.html