diff --git a/hw/application_fpga/fw/testfw/main.c b/hw/application_fpga/fw/testfw/main.c
index 835d370..4302097 100644
--- a/hw/application_fpga/fw/testfw/main.c
+++ b/hw/application_fpga/fw/testfw/main.c
@@ -106,7 +106,7 @@ uint32_t wait_timer_tick(uint32_t last_timer)
 	}
 }
 
-void zero_fwram()
+void zero_fwram(void)
 {
 	for (int i = 0; i < TK1_MMIO_FW_RAM_SIZE; i++) {
 		fw_ram[i] = 0x00;
@@ -149,7 +149,7 @@ void failmsg(char *s)
 	puts("\r\n");
 }
 
-int main()
+int main(void)
 {
 	// Function pointer to blake2s()
 	volatile int (*fw_blake2s)(void *, unsigned long, const void *,
diff --git a/hw/application_fpga/fw/tk1/main.c b/hw/application_fpga/fw/tk1/main.c
index 2aa6240..60c1e71 100644
--- a/hw/application_fpga/fw/tk1/main.c
+++ b/hw/application_fpga/fw/tk1/main.c
@@ -42,9 +42,9 @@ struct context {
 	uint8_t uss[32];    // User Supplied Secret, if any
 };
 
-static void print_hw_version();
+static void print_hw_version(void);
 static void print_digest(uint8_t *md);
-static uint32_t rnd_word();
+static uint32_t rnd_word(void);
 static void compute_cdi(const uint8_t *digest, const uint8_t use_uss,
 			const uint8_t *uss);
 static void copy_name(uint8_t *buf, const size_t bufsiz, const uint32_t word);
@@ -55,9 +55,9 @@ static enum state loading_commands(const struct frame_header *hdr,
 				   const uint8_t *cmd, enum state state,
 				   struct context *ctx);
 static void run(const struct context *ctx);
-static void scramble_ram();
+static void scramble_ram(void);
 
-static void print_hw_version()
+static void print_hw_version(void)
 {
 	htif_puts("Hello, I'm firmware with");
 	htif_puts(" tk1_name0:");
@@ -81,7 +81,7 @@ static void print_digest(uint8_t *md)
 	htif_lf();
 }
 
-static uint32_t rnd_word()
+static uint32_t rnd_word(void)
 {
 	while ((*trng_status & (1 << TK1_MMIO_TRNG_STATUS_READY_BIT)) == 0) {
 	}
@@ -362,7 +362,7 @@ static void run(const struct context *ctx)
 	__builtin_unreachable();
 }
 
-static void scramble_ram()
+static void scramble_ram(void)
 {
 	uint32_t *ram = (uint32_t *)(TK1_RAM_BASE);
 	uint32_t rnd = rnd_word();
@@ -384,7 +384,7 @@ static void scramble_ram()
 	*ram_scramble = rnd_word();
 }
 
-int main()
+int main(void)
 {
 	struct context ctx = {0};
 	struct frame_header hdr = {0};
diff --git a/hw/application_fpga/fw/tk1/proto.c b/hw/application_fpga/fw/tk1/proto.c
index a78c539..b01a3f7 100644
--- a/hw/application_fpga/fw/tk1/proto.c
+++ b/hw/application_fpga/fw/tk1/proto.c
@@ -142,7 +142,7 @@ static void write(uint8_t *buf, size_t nbytes)
 	}
 }
 
-uint8_t readbyte()
+uint8_t readbyte(void)
 {
 	for (;;) {
 		if (*can_rx) {
diff --git a/hw/application_fpga/fw/tk1/proto.h b/hw/application_fpga/fw/tk1/proto.h
index 105d5ad..4bf225f 100644
--- a/hw/application_fpga/fw/tk1/proto.h
+++ b/hw/application_fpga/fw/tk1/proto.h
@@ -51,7 +51,7 @@ struct frame_header {
 };
 
 void writebyte(uint8_t b);
-uint8_t readbyte();
+uint8_t readbyte(void);
 void fwreply(struct frame_header hdr, enum fwcmd rspcode, uint8_t *buf);
 int readcommand(struct frame_header *hdr, uint8_t *cmd, int state);
 #endif