colintd · Mar 24, 2026
diff --git a/example/PPStest/PPStest.ino b/example/PPStest/PPStest.ino
@@ -0,0 +1,41 @@
+// Minimal test app making calls to PPS module in tight loop, printing state to Serial
+// Used to verify stability of the I2C interface to the module.
+#include <M5Unified.h>
+#include "M5ModulePPS.h"
+
+M5ModulePPS pps;
+
+#define OUTPUT_VOLTAGE 5.0f
+#define OUTPUT_CURRENT 5.0f
+
+void setup() 
+{
+    M5.begin();
+    Serial.begin(1000000);
+
+    Serial.printf("Attempting to connect to PPS\n");
+
+    while (!pps.begin(&Wire, M5.In_I2C.getSDA(), M5.In_I2C.getSCL(), MODULE_POWER_ADDR, 400000U)) 
+    {
+        Serial.println("Connect error");
+        delay(100);
+    }
+
+    Serial.printf("PPS connected\n");
+
+    pps.setOutputVoltage(OUTPUT_VOLTAGE);
+    pps.setOutputCurrent(OUTPUT_CURRENT);
+    pps.setPowerEnable(true);
+}
+
+int cycle=0;
+void loop() 
+{
+
+    float voltage_readback = pps.getReadbackVoltage();
+    float current_readback = pps.getReadbackCurrent();
+    float vin              = pps.getVIN();
+    const char * modestr   = ((pps.getMode() <= 1) ? "CV" : "CC");
+
+    Serial.printf("%d: Vin=%0.3fV Vout=%0.3fV Iout=%0.3fA Mode=%s\n",cycle++,vin,voltage_readback,current_readback,modestr);
+}
diff --git a/src/M5ModulePPS.cpp b/src/M5ModulePPS.cpp
@@ -19,17 +19,40 @@ bool M5ModulePPS::begin(TwoWire *wire, uint8_t sda, uint8_t scl, uint8_t addr,
     }
 }
 
+// If a second I2c transaction is made to the module, less than 100us after the end of the previous call we get NACK errors
+// The underlying cause looks like an issue in the latest ESP-IF I2C library, but until that is fixed we need a workaround
+// The below wrappers for the I2C functions ensure the minimum gap is enforced, with minimal performance impact even with
+// tight looping to the PPS libraries.  It allows applications to run indefinitely vs a few seconds (with the demo app on a
+// Core2).  Once the I2C library is fixed, simply remove the MIN_GAP definition to disable the workaround.
+#define MIN_GAP 100
+
+#ifdef MIN_GAP
+  #define I2C_ENTRY() {uint32_t gap=micros()-_micros;if(gap<MIN_GAP)delayMicroseconds(MIN_GAP-gap);}
+  #define I2C_EXIT() {_micros=micros();}
+#else
+  #define I2C_ENTRY()
+  #define I2C_EXIT()
+#endif
+
 bool M5ModulePPS::writeBytes(uint8_t addr, uint8_t reg, uint8_t *buffer,
                              uint8_t length) {
+    bool rc=false;
+    I2C_ENTRY();
+
     _wire->beginTransmission(addr);
     _wire->write(reg);
     _wire->write(buffer, length);
-    if (_wire->endTransmission() == 0) return true;
-    return false;
+    if (_wire->endTransmission() == 0) rc=true;
+
+    I2C_EXIT();
+    return rc;
 }
 
 bool M5ModulePPS::readBytes(uint8_t addr, uint8_t reg, uint8_t *buffer,
-                            uint8_t length) {
+                                uint8_t length) {
+    bool rc=false;
+    I2C_ENTRY();
+
     uint8_t index = 0;
     _wire->beginTransmission(addr);
     _wire->write(reg);
@@ -38,9 +61,11 @@ bool M5ModulePPS::readBytes(uint8_t addr, uint8_t reg, uint8_t *buffer,
         for (uint8_t i = 0; i < length; i++) {
             buffer[index++] = _wire->read();
         }
-        return true;
+        rc=true;
     }
-    return false;
+
+    I2C_EXIT();
+    return rc;
 }
 
 void M5ModulePPS::float_to_bytes(float s, uint8_t *d) {

diff --git a/src/M5ModulePPS.h b/src/M5ModulePPS.h
@@ -31,6 +31,7 @@ class M5ModulePPS {
     uint8_t _sda;
     uint8_t _scl;
     uint32_t _speed;
+    uint32_t _micros;
     bool writeBytes(uint8_t addr, uint8_t reg, uint8_t *buffer, uint8_t length);
     bool readBytes(uint8_t addr, uint8_t reg, uint8_t *buffer, uint8_t length);
     void float_to_bytes(float s, uint8_t *d);