diff --git a/libraries/AP_IOMCU/AP_IOMCU.cpp b/libraries/AP_IOMCU/AP_IOMCU.cpp
index e5e371f870..021fa153f6 100644
--- a/libraries/AP_IOMCU/AP_IOMCU.cpp
+++ b/libraries/AP_IOMCU/AP_IOMCU.cpp
@@ -652,11 +652,10 @@ void AP_IOMCU::push(void)
 // set output frequency
 void AP_IOMCU::set_freq(uint16_t chmask, uint16_t freq)
 {
-    const uint8_t masks[] = { 0x03,0x0C,0xF0 };
     // ensure mask is legal for the timer layout
-    for (uint8_t i=0; i<ARRAY_SIZE(masks); i++) {
-        if (chmask & masks[i]) {
-            chmask |= masks[i];
+    for (uint8_t i=0; i<ARRAY_SIZE(ch_masks); i++) {
+        if (chmask & ch_masks[i]) {
+            chmask |= ch_masks[i];
         }
     }
     rate.freq = freq;
diff --git a/libraries/AP_IOMCU/AP_IOMCU.h b/libraries/AP_IOMCU/AP_IOMCU.h
index 4310fcc29a..e6fffc58ac 100644
--- a/libraries/AP_IOMCU/AP_IOMCU.h
+++ b/libraries/AP_IOMCU/AP_IOMCU.h
@@ -103,6 +103,9 @@ public:
     bool setup_mixing(RCMapper *rcmap, int8_t override_chan,
                       float mixing_gain, uint16_t manual_rc_mask);
     
+    // channel group masks
+    const uint8_t ch_masks[3] = { 0x03,0x0C,0xF0 };
+
 private:
     AP_HAL::UARTDriver &uart;