voxl_esc: Limit frequency of UART passthru writes to 20Hz

src/drivers/actuators/voxl_esc/voxl_esc.cpp

@@ -1336,6 +1336,11 @@ bool VoxlEsc::updateOutputs(uint16_t outputs[MAX_ACTUATORS],
 
 	uint8_t num_writes = 0;
 
+	// Don't do these faster than 20Hz
+	if (hrt_elapsed_time(&_last_uart_passthru) > 50_ms) {
+		_last_uart_passthru = hrt_absolute_time();
+
+		// Don't do more than a few writes each check
 		while (_esc_serial_passthru_sub.updated() && (num_writes < 4)) {
 			mavlink_tunnel_s uart_passthru{};
 			_esc_serial_passthru_sub.copy(&uart_passthru);
@@ -1347,6 +1352,7 @@ bool VoxlEsc::updateOutputs(uint16_t outputs[MAX_ACTUATORS],
 
 			num_writes++;
 		}
+	}
 
 	perf_count(_output_update_perf);
 

src/drivers/actuators/voxl_esc/voxl_esc.hpp

@@ -257,6 +257,7 @@ private:
 	Battery			_battery;
 	static constexpr unsigned _battery_report_interval{100_ms};
 	hrt_abstime		_last_battery_report_time;
+	hrt_abstime		_last_uart_passthru{0};
 
 	bool			_device_initialized{false};