diff --git a/.gitignore b/.gitignore
index 50d898c..ef58455 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /nbproject/private/
+/build
 /avrrfm.elf
 /avrrfm.hex
 /avrrfm.lst
diff --git a/avrrfm.c b/avrrfm.c
index 658bfb8..45a0aed 100644
--- a/avrrfm.c
+++ b/avrrfm.c
@@ -34,6 +34,9 @@
 #include "dejavu.h"
 #include "unifont.h"
 
+#define TRANSMIT_FAST   4  // 4 ~ 32 seconds
+#define TRANSMIT_SLOW   38 // 38 ~ 5 minutes
+
 #define LABEL_OFFSET    10
 #define BLACK           0x0000
 #define RED             0xf800
@@ -47,6 +50,10 @@
     #define RECEIVER    1
 #endif
 
+static volatile uint8_t watchdogInts = 0;
+static uint8_t measureInts = TRANSMIT_FAST;
+static uint8_t timeoutCount = 0;
+
 /* Temp. label coordinates */
 static x_t xl = 0;
 static y_t yl = 0;
@@ -58,7 +65,7 @@
  * Called when the watchdog barks to wake up the transmitter.
  */
 ISR(WDT_vect) {
-    barkRadio();
+    watchdogInts++;
 }
 
 /**
@@ -69,6 +76,13 @@
 }
 
 /**
+ * Called when an INT0 interrupt occurs.
+ */
+ISR(INT0_vect) {
+    intRadio();
+}
+
+/**
  * Sets up the pins.
  */
 static void initPins(void) {
@@ -241,7 +255,7 @@
  * @param flags
  */
 static void handlePayload(PayloadFlags flags) {
-    uint8_t rssi = readRssi();
+    uint8_t rssi = getRssi();
     Temperature temp = readTemp();
 
     // communicate RSSI back to transmitter
@@ -254,15 +268,21 @@
 
 /**
  * Waits for a response from the receiver with timeout.
+ *
+ * @return true if timeout occurred, false otherwise
  */
-static void waitResponse(void) {
+static bool waitResponse(void) {
     uint8_t response[1];
     int8_t len = receivePayload(response, sizeof (response), true);
     if (len > 0) {
         // receiver RSSI
         int8_t rssi = divRoundNearest(response[0], 2);
         setOutputPower(rssi);
+
+        return false;
     }
+
+    return true;
 }
 
 /**
@@ -339,7 +359,9 @@
         // _delay_ms(1000);
 
         if (!RECEIVER) {
-            if (wouldTransmit()) {
+            if (watchdogInts % measureInts == 0) {
+                watchdogInts = 0;
+
                 enableSPI();
                 wakeTSens();
                 wakeRadio();
@@ -347,7 +369,16 @@
                     transmitData();
                 } else {
                     transmitTemp(NODE1);
-                    waitResponse();
+                    bool timeout = waitResponse();
+                    if (timeout) {
+                        if (++timeoutCount > MAX_TIMEOUTS) {
+                            measureInts = TRANSMIT_SLOW;
+                            timeoutCount = 0;
+                        }
+                    } else {
+                        timeoutCount = 0;
+                        measureInts = TRANSMIT_FAST;
+                    }
                 }
                 sleepTSens();
                 sleepRadio();
diff --git a/rfm69.c b/rfm69.c
index f751713..66738ca 100644
--- a/rfm69.c
+++ b/rfm69.c
@@ -10,12 +10,8 @@
 
 static volatile uint8_t irqFlags1 = 0;
 static volatile uint8_t irqFlags2 = 0;
-
-static uint8_t watchdogInts = 0;
-static uint8_t measureInts = TRANSMIT_FAST;
-static uint8_t timeoutInts = 0;
-static uint8_t timeoutCount = 0;
-static bool timeoutEnabled = false;
+static volatile uint8_t timeoutInts = 0;
+static volatile bool timeoutEnabled = false;
 
 /**
  * Selects the radio to talk to via SPI.
@@ -89,19 +85,6 @@
  */
 static void timeout(void) {
     irqFlags1 |= (1 << 2);
-    if (++timeoutCount > MAX_TIMEOUTS) {
-        measureInts = TRANSMIT_SLOW;
-        timeoutCount = 0;
-    }
-}
-
-/**
- * Reads interrupt flags when a radio interrupt occurs.
- */
-ISR(INT0_vect) {
-    irqFlags1 = regRead(IRQ_FLAGS1);
-    irqFlags2 = regRead(IRQ_FLAGS2);
-    // printString("irq\r\n");
 }
 
 void initRadio(uint64_t freq, uint8_t node) {
@@ -213,33 +196,12 @@
     printString("Radio init done\r\n");
 }
 
-void setNodeAddress(uint8_t address) {
-    regWrite(NODE_ADDR, address);
-}
-
-void setOutputPower(uint8_t rssi) {
-    uint8_t pa = 0x40; // -18 dBm with PA1
-    // adjust power from -2 to +13 dBm
-    pa += min(max(rssi - 69, PA_MIN), PA_MAX);
-    regWrite(PA_LEVEL, pa);
-}
-
-uint8_t getOutputPower(void) {
-    return regRead(PA_LEVEL);
-}
-
-void barkRadio(void) {
-    watchdogInts++;
-}
-
-bool wouldTransmit(void) {
-    if (watchdogInts % measureInts == 0) {
-        watchdogInts = 0;
-
-        return true;
-    }
-
-    return false;
+/**
+ * Reads interrupt flags when a radio interrupt occurs.
+ */
+void intRadio(void) {
+    irqFlags1 = regRead(IRQ_FLAGS1);
+    irqFlags2 = regRead(IRQ_FLAGS2);
 }
 
 void timeRadio(void) {
@@ -259,6 +221,25 @@
     _delay_ms(5);
 }
 
+void setNodeAddress(uint8_t address) {
+    regWrite(NODE_ADDR, address);
+}
+
+uint8_t getRssi(void) {
+    return regRead(RSSI_VALUE);
+}
+
+void setOutputPower(uint8_t rssi) {
+    uint8_t pa = 0x40; // -18 dBm with PA1
+    // adjust power from -2 to +13 dBm
+    pa += min(max(rssi - 69, PA_MIN), PA_MAX);
+    regWrite(PA_LEVEL, pa);
+}
+
+uint8_t getOutputPower(void) {
+    return regRead(PA_LEVEL);
+}
+
 void startReceive(void) {
     // get "PayloadReady" on DIO0
     regWrite(DIO_MAP1, 0x40);
@@ -266,10 +247,6 @@
     setMode(MODE_RX);
 }
 
-uint8_t readRssi(void) {
-    return regRead(RSSI_VALUE);
-}
-
 PayloadFlags payloadReady(void) {
     PayloadFlags flags = {.ready = false, .crc = false};
     if (irqFlags2 & (1 << 2)) {
@@ -311,8 +288,6 @@
     clearIrqFlags();
     if (ready) {
         timeoutEnable(false);
-        timeoutCount = 0;
-        measureInts = TRANSMIT_FAST;
     }
     setMode(MODE_STDBY);
 
diff --git a/rfm69.h b/rfm69.h
index bb470e2..1f87026 100644
--- a/rfm69.h
+++ b/rfm69.h
@@ -81,9 +81,7 @@
 #define F_STEP          6103515625ULL
 #define CAST_ADDRESS    0x84
 
-#define TRANSMIT_FAST   4  // 4 ~ 32 seconds
-#define TRANSMIT_SLOW   38 // 38 ~ 5 minutes
-#define TIMEOUT_INTS    3  // about 100 milliseconds
+#define TIMEOUT_INTS    3  // about 100 milliseconds @ 30 Hz
 #define MAX_TIMEOUTS    9  // slow down tx attempts after so many timeouts
 
 /**
@@ -93,6 +91,29 @@
 void initRadio(uint64_t freq, uint8_t node);
 
 /**
+ * Should be called when a radio interrupt occurred, i.e. 'PayloadReady'.
+ */
+void intRadio(void);
+
+/**
+ * Gives a timer pulse to the radio. Used to time-out blocking functions,
+ * i.e. transmitter waiting for a response from the receiver.
+ * TIMEOUT_INTS must be adjusted according to the frequency with that 
+ * this function is called.
+ */
+void timeRadio(void);
+
+/**
+ * Shuts down the radio.
+ */
+void sleepRadio(void);
+
+/**
+ * Wakes up the radio.
+ */
+void wakeRadio(void);
+
+/**
  * Sets the node address.
  * 
  * @param address
@@ -100,6 +121,13 @@
 void setNodeAddress(uint8_t address);
 
 /**
+ * Returns the current RSSI value.
+ * 
+ * @return rssi value
+ */
+uint8_t getRssi(void);
+
+/**
  * Sets the output power based on the given receiver RSSI.
  * 
  * @param rssi
@@ -114,45 +142,11 @@
 uint8_t getOutputPower(void);
 
 /**
- * Gives a watchdog "wakeup" pulse to the radio.
- */
-void barkRadio(void);
-
-/**
- * Returns true if the transmitter would actually transmit.
- * 
- * @return transmit or not
- */
-bool wouldTransmit(void);
-
-/**
- * Gives a timer pulse to the radio.
- */
-void timeRadio(void);
-
-/**
- * Shuts down the radio.
- */
-void sleepRadio(void);
-
-/**
- * Wakes up the radio.
- */
-void wakeRadio(void);
-
-/**
  * Sets the radio to receive mode and maps "PayloadReady" to DIO0.
  */
 void startReceive(void);
 
 /**
- * Returns the current RSSI value.
- * 
- * @return rssi value
- */
-uint8_t readRssi(void);
-
-/**
  * Returns true if a "PayloadReady" interrupt arrived and clears the
  * interrupt state.
  * 
diff --git a/sdcard.c b/sdcard.c
index f791d85..aecdf5f 100644
--- a/sdcard.c
+++ b/sdcard.c
@@ -200,12 +200,12 @@
 
     // CMD0 - go to idle state
     response[0] = sendIdle();
-    if (response[0] > 0x01) {
+    if (response[0] != 0x01) {
         // retry solves failure possibly caused by another component
         // on the same SPI bus
         response[0] = sendIdle();
     }
-    if (response[0] > 0x01) {
+    if (response[0] != 0x01) {
         printString("SD card error 0\r\n");
         return false;
     }
diff --git a/types.h b/types.h
index 6378677..f15e271 100644
--- a/types.h
+++ b/types.h
@@ -55,6 +55,11 @@
 } Temperature;
 
 /**
+ * Pointer to a function that takes no argument and returns void.
+ */
+typedef void (*Function)(void);
+
+/**
  * Pointer to a function that takes an array of bytes
  * and returns a boolean.
  */