Merge pull request adafruit#3 from ladyada-eagleclaw/fix-wire-end-portability

Fix NVM example on ESP8266

Author: ladyada

README.md

@@ -79,14 +79,19 @@ 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:
+Release the I2C peripheral before saving, then restore it afterward. ESP8266 is
+the exception because its software I2C core does not provide `Wire.end()`:
 
 ```cpp
+#ifndef ESP8266
 Wire.end();
+#endif
 bool saved = gp8403.saveToNVM(SDA, SCL);
 Wire.begin();
 ```
 
+On ESP8266, `saveToNVM()` takes control of SDA and SCL directly.
+
 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

@@ -10,7 +10,7 @@
  *
  * WARNING: NVM programming uses a special open-drain waveform, not a normal
  * I2C transaction. Wire must be stopped before saveToNVM() and restarted
- * afterward.
+ * afterward. ESP8266 is the exception because it does not provide Wire.end().
  *
  * 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. ESP8266's software I2C implementation is the exception
+  // because it does not provide Wire.end(). If your project uses custom pins
+  // or bus settings, restore that same configuration below.
+#ifndef ESP8266
   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() {
+#ifndef ESP8266
   Wire.end();
+#endif
   bool saved = gp8403.saveToNVM(SDA, SCL);
   Wire.begin();
   return saved;