Improve bpsk test transmission timing

src/main.c

@@ -947,10 +947,21 @@ static void command(const char *cmd)
 				actual = rx_lo_init(actual - step_hz);
 				printf("Frequency: %.0f\n", actual);
 			} else if ((c >= '1') && (c <= '9')) {
-				for (int i = 0; i < 1000; i++) {
-					phase = !phase;
+				uint32_t t0 = time_us_32();
+				uint32_t tmax = 1000000 / (c - '0');
+				uint32_t step = 1000 / (c - '0') / 2;
+
+				while (true) {
+					uint32_t t1 = time_us_32();
+					uint32_t te = t1 - t0;
+
+					if (te >= tmax)
+						break;
+
+					phase = (te / step) & 1;
+
 					gpio_set_outover(n, phase);
-					sleep_us(1000 / (c - '0'));
+					sleep_us(1);
 				}
 			}
 		}