Added option to use GPIO21 instead of GPIO4

README.md

@@ -21,15 +21,21 @@ Where:
 
 Other options:
 * -d dma_channel - Specifies the DMA channel to be used (0 by default), type 255 to disable DMA transfer, CPU will be used instead
-* -b bandwidth - Specifies the bandwidth in kHz, 200 by default
+* -b bandwidth - Specifies the bandwidth in kHz, 100 by default
 * -r - Loops the playback
 
 After transmission has begun, simply tune an FM receiver to chosen frequency, You should hear the playback.
 ### Raspberry Pi 4
-On Raspberry Pi 4 power saving options probably interfers somehow with this software making transmitting not possible on standard FM broadcasting frequencies. However it is possible to transmit on lower frequencies i.eg. 66MHz. It is recommended to change either ARM core scaling governor settings to "performance" or to change ARM minium and maximum core frequencies to one constant value (see: https://www.raspberrypi.org/forums/viewtopic.php?t=152692 ).
+On Raspberry Pi 4 other built-in hardware probably interfers somehow with this software making transmitting not possible on all standard FM broadcasting frequencies. In this case it is recommended to:
+1. Compile executable with option to use GPIO21 instead of GPIO4 (PIN 40 on GPIO header):
+```
+make GPIO21=1
+```
+2. Change either ARM core frequency scaling governor settings to "performance" or to change ARM minium and maximum core frequencies to one constant value (see: https://www.raspberrypi.org/forums/viewtopic.php?t=152692 ).
 ```
 echo "performance"| sudo tee /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
 ```
+3. Using lower FM broadcasting frequencies (below 93 MHz) when transmitting.
 ### Supported audio formats
 You can transmitt uncompressed WAV (.wav) files directly or read audio data from stdin, eg.:
 ```

makefile

@@ -1,6 +1,10 @@
 EXECUTABLE = fm_transmitter
 VERSION = 0.9.4
 FLAGS = -Wall -O3 -std=c++11
+TRANSMITTER = -fno-strict-aliasing -I/opt/vc/include
+ifeq ($(GPIO21), 1)
+	TRANSMITTER += -DGPIO21
+endif
 
 all: main.o mailbox.o sample.o wave_reader.o transmitter.o
 	g++ -L/opt/vc/lib -lm -lpthread -lbcm_host -o $(EXECUTABLE) main.o mailbox.o sample.o wave_reader.o transmitter.o
@@ -15,7 +19,7 @@ wave_reader.o: wave_reader.cpp wave_reader.hpp
 	g++ $(FLAGS) -c wave_reader.cpp
 
 transmitter.o: transmitter.cpp transmitter.hpp
-	g++ $(FLAGS) -fno-strict-aliasing -I/opt/vc/include -c transmitter.cpp
+	g++ $(FLAGS) $(TRANSMITTER) -c transmitter.cpp
 
 main.o: main.cpp
 	g++ $(FLAGS) -DVERSION=\"$(VERSION)\" -DEXECUTABLE=\"$(EXECUTABLE)\" -c main.cpp

transmitter.cpp

@@ -46,6 +46,7 @@
 #define DMA0_BASE_OFFSET 0x00007000
 #define DMA15_BASE_OFFSET 0x00e05000
 #define CLK0_BASE_OFFSET 0x00101070
+#define CLK1_BASE_OFFSET 0x00101078
 #define PWMCLK_BASE_OFFSET 0x001010a0
 #define GPIO_BASE_OFFSET 0x00200000
 #define PWM_BASE_OFFSET 0x0020c000
@@ -236,12 +237,22 @@ class ClockDevice : public Device
 class ClockOutput : public ClockDevice
 {
     public:
+#ifndef GPIO21
         ClockOutput(unsigned divisor) : ClockDevice(CLK0_BASE_OFFSET, divisor) {
             output = reinterpret_cast<uint32_t *>(peripherals->GetVirtualAddress(GPIO_BASE_OFFSET));
             *output = (*output & 0xffff8fff) | (0x04 << 12);
+#else
+        ClockOutput(unsigned divisor) : ClockDevice(CLK1_BASE_OFFSET, divisor) {
+            output = reinterpret_cast<uint32_t *>(peripherals->GetVirtualAddress(GPIO_BASE_OFFSET + 0x08));
+            *output = (*output & 0xffffffc7) | (0x02 << 3);
+#endif
         }
         virtual ~ClockOutput() {
+#ifndef GPIO21
             *output = (*output & 0xffff8fff) | (0x01 << 12);
+#else
+            *output = (*output & 0xffffffc7) | (0x02 << 3);
+#endif
         }
         inline void SetDivisor(unsigned divisor) {
             clock->div = (0x5a << 24) | (0xffffff & divisor);