Fix NVM example on ESP8266

Co-authored-by: Limor Fried <ladyada@users.noreply.github.com>

Author: ladyada

README.md

@@ -79,14 +79,20 @@ and voltages above the selected range are rejected without an I2C write.
 Saving is never automatic. Do not call it repeatedly or from a loop; the
 datasheet does not specify the nonvolatile-memory endurance.
 
-The I2C peripheral must be released before saving and restored afterward:
+When the Arduino core supports `Wire.end()`, release the I2C peripheral before
+saving. Restore `Wire` afterward on every platform:
 
 ```cpp
+#ifdef WIRE_HAS_END
 Wire.end();
+#endif
 bool saved = gp8403.saveToNVM(SDA, SCL);
 Wire.begin();
 ```
 
+Some software I2C cores, including ESP8266, do not provide `Wire.end()`.
+`saveToNVM()` takes control of SDA and SCL directly on those platforms.
+
 If your application uses custom pins, speed, or other bus settings, restore the
 same configuration instead of using the parameterless `Wire.begin()` shown
 above.

examples/nvm_persistence/nvm_persistence.ino

@@ -9,8 +9,8 @@
  * the NVM write endurance, so save only when the output really must change.
  *
  * WARNING: NVM programming uses a special open-drain waveform, not a normal
- * I2C transaction. Wire must be stopped before saveToNVM() and restarted
- * afterward.
+ * I2C transaction. When the Arduino core supports Wire.end(), Wire must be
+ * stopped before saveToNVM() and restarted afterward.
  *
  * WARNING: NVM programming always uses the GP8403's hardcoded 0x58 address.
  * It cannot save a device configured at another I2C address.
@@ -60,9 +60,12 @@ void setup() {
   waitForCommand('S');
 
   // saveToNVM() temporarily controls SDA and SCL directly as open-drain GPIO.
-  // Stop Wire first so the I2C peripheral releases those pins. If your project
-  // uses custom pins or bus settings, restore that same configuration below.
+  // Stop Wire first when the Arduino core supports it. Some software I2C cores,
+  // including ESP8266, do not provide Wire.end(). If your project uses custom
+  // pins or bus settings, restore that same configuration below.
+#ifdef WIRE_HAS_END
   Wire.end();
+#endif
   bool saved = gp8403.saveToNVM(SDA, SCL);
   Wire.begin();
 

hw_tests/03_nvm_persistence/03_nvm_persistence.ino

@@ -127,7 +127,9 @@ char waitForCommand() {
 }
 
 bool saveNVM() {
+#ifdef WIRE_HAS_END
   Wire.end();
+#endif
   bool saved = gp8403.saveToNVM(SDA, SCL);
   Wire.begin();
   return saved;