AP_HAL: always choose high for dshot prescaler calculation

libraries/AP_HAL/RCOutput.cpp

@@ -86,15 +86,23 @@ uint32_t AP_HAL::RCOutput::calculate_bitrate_prescaler(uint32_t timer_clock, uin
         }
     }
 
-    // find the closest value
     const uint32_t freq = timer_clock / prescaler;
-    const float delta = fabsf(float(freq) - target_frequency);
-    if (freq > target_frequency
-        && delta > fabsf(float(timer_clock / (prescaler+1)) - target_frequency)) {
-        prescaler++;
-    } else if (freq < target_frequency
-        && delta > fabsf(float(timer_clock / (prescaler-1)) - target_frequency)) {
-        prescaler--;
+    // if using dshot then always pick the high value. choosing low seems to not agree with some
+    // ESCs despite the fact that BLHeli32 is supposed not to care what the bitrate is
+    if (is_dshot) {
+        if (freq < target_frequency) {
+            prescaler--;
+        }
+    } else {
+        // find the closest value
+        const float delta = fabsf(float(freq) - target_frequency);
+        if (freq > target_frequency
+            && delta > fabsf(float(timer_clock / (prescaler+1)) - target_frequency)) {
+            prescaler++;
+        } else if (freq < target_frequency
+            && delta > fabsf(float(timer_clock / (prescaler-1)) - target_frequency)) {
+            prescaler--;
+        }
     }
 
     return prescaler;

libraries/AP_HAL/tests/test_prescaler.cpp

@@ -11,7 +11,7 @@ protected:
         const uint32_t prescaler = AP_HAL::RCOutput::calculate_bitrate_prescaler(clock, target_rate, is_dshot);
         // we would like at most a 1% discrepancy in target versus actual
         const float rate_delta = fabsf(float(clock / prescaler) - target_rate) / target_rate;
-        EXPECT_TRUE(rate_delta < 0.13f);
+        EXPECT_TRUE(rate_delta < 0.20f);
     }
 };