From e39d070b7880221a4e3f670efc5eb5ccb20f2928 Mon Sep 17 00:00:00 2001
From: Andrew Tridgell <andrew@tridgell.net>
Date: Mon, 30 Jul 2018 10:42:59 +1000
Subject: [PATCH] AP_Compass: clear DEV_ID for undetected compasses

this prevents user confusion with the number of compasses detected
---
 libraries/AP_Compass/AP_Compass.cpp | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/libraries/AP_Compass/AP_Compass.cpp b/libraries/AP_Compass/AP_Compass.cpp
index 2a1a870b52..77b72e510b 100644
--- a/libraries/AP_Compass/AP_Compass.cpp
+++ b/libraries/AP_Compass/AP_Compass.cpp
@@ -487,6 +487,14 @@ Compass::init()
         hal.scheduler->delay(100);
         read();
     }
+    // set the dev_id to 0 for undetected compasses, to make it easier
+    // for users to see how many compasses are detected. We don't do a
+    // set_and_save() as the user may have temporarily removed the
+    // compass, and we don't want to force a re-cal if they plug it
+    // back in again
+    for (uint8_t i=_compass_count; i<COMPASS_MAX_INSTANCES; i++) {
+        _state[i].dev_id.set(0);
+    }
     return true;
 }