Avoid sleeps to prevent low-frequency artifacts

Replace them with busy looks that do not lower the current draw.
It still prevent memory saturation as the loops do not touch memory.

src/main.c

@@ -25,7 +25,7 @@
 #define CLK_SYS_HZ (300 * MHZ)
 
 #define LO_PIN 9
-#define RX_PIN 13
+#define RX_PIN 8
 #define FB_PIN 5
 #define PSU_PIN 23
 
@@ -87,6 +87,14 @@ static queue_t iq_queue;
 static uint8_t iq_queue_buffer[IQ_QUEUE_LEN][IQ_BLOCK_LEN];
 static size_t iq_queue_pos = 0;
 
+static uint32_t rnd = 0;
+
+inline static __unused uint32_t rnd_next()
+{
+	rnd = rnd * 0x41c64e6d + 12345;
+	return rnd;
+}
+
 static void dma_channel_clear_chain_to(int ch)
 {
 	uint32_t ctrl = dma_hw->ch[ch].al1_ctrl;
@@ -128,7 +136,7 @@ static void init_lo()
 	sm_config_set_out_shift(&pc, false, true, 32);
 	pio_sm_init(PIO, LO_SM, prog.origin, &pc);
 
-	pio_sm_set_consecutive_pindirs(PIO, LO_SM, LO_PIN, 1, GPIO_OUT);
+	pio_sm_set_consecutive_pindirs(PIO, LO_SM, LO_PIN, 1, GPIO_IN);
 	pio_sm_exec_wait_blocking(PIO, LO_SM, pio_encode_set(pio_pins, 0));
 }
 
@@ -477,10 +485,12 @@ static void rf_rx(void)
 		int head = (dma_hw->ch[dma_ch_in_cos].write_addr - base) / 4;
 		int delta = (head < pos ? head + RX_WORDS : head) - pos;
 
-		sleep_us(10);
-
 		while (delta < RX_STRIDE) {
-			sleep_us(1);
+			int wait = rnd_next() & 63;
+
+			for (int i = 0; i < wait; i++)
+				asm volatile("nop");
+
 			head = (dma_hw->ch[dma_ch_in_cos].write_addr - base) / 4;
 			delta = (head < pos ? head + RX_WORDS : head) - pos;
 		}
@@ -616,7 +626,10 @@ static void do_rx()
 			fwrite(block, IQ_BLOCK_LEN, 1, stdout);
 			fflush(stdout);
 		} else {
-			sleep_us(25);
+			int wait = rnd_next() & 0x1fff;
+
+			for (int i = 0; i < wait; i++)
+				asm volatile("nop");
 		}
 	}