[RF69] Fixed known binary receive length (#408)

src/modules/RF69/RF69.cpp

@@ -347,9 +347,12 @@ int16_t RF69::readData(uint8_t* data, size_t len) {
   RADIOLIB_ASSERT(state);
 
   // get packet length
-  size_t length = len;
+  size_t length = getPacketLength();
-  if(len == RADIOLIB_RF69_MAX_PACKET_LENGTH) {
+  size_t dumpLen = 0;
-    length = getPacketLength();
+  if((len != 0) && (len < length)) {
+    // user requested less data than we got, only return what was requested
+    dumpLen = length - len;
+    length = len;
   }
 
   // check address filtering
@@ -361,6 +364,11 @@ int16_t RF69::readData(uint8_t* data, size_t len) {
   // read packet data
   _mod->SPIreadRegisterBurst(RADIOLIB_RF69_REG_FIFO, length, data);
 
+  // dump the bytes that weren't requested
+  if(dumpLen != 0) {
+    clearFIFO(dumpLen);
+  }
+
   // clear internal flag so getPacketLength can return the new packet length
   _packetLengthQueried = false;
 
@@ -935,4 +943,11 @@ void RF69::clearIRQFlags() {
   _mod->SPIwriteRegister(RADIOLIB_RF69_REG_IRQ_FLAGS_2, 0b11111111);
 }
 
+void RF69::clearFIFO(size_t count) {
+  while(count) {
+    _mod->SPIreadRegister(RADIOLIB_RF69_REG_FIFO);
+    count--;
+  }
+}
+
 #endif

src/modules/RF69/RF69.h

@@ -616,7 +616,8 @@ class RF69: public PhysicalLayer {
 
       \param data Pointer to array to save the received binary data.
 
-      \param len Number of bytes that will be received. Must be known in advance for binary transmissions.
+      \param len Number of bytes that will be read. When set to 0, the packet length will be retreived automatically.
+      When more bytes than received are requested, only the number of bytes requested will be returned.
 
       \returns \ref status_codes
     */
@@ -950,6 +951,7 @@ class RF69: public PhysicalLayer {
 #endif
     int16_t setMode(uint8_t mode);
     void clearIRQFlags();
+    void clearFIFO(size_t count);
 };
 
 #endif