From f51229ef9a0be16079360641843edf219e0ad112 Mon Sep 17 00:00:00 2001
From: jgromes <jan.gromes@gmail.com>
Date: Sun, 27 Nov 2022 20:47:01 +0100
Subject: [PATCH] [Pager] Added option to invert frequencies (#7)

---
 src/protocols/Pager/Pager.cpp | 22 ++++++++++++++++------
 src/protocols/Pager/Pager.h   |  7 ++++++-
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/src/protocols/Pager/Pager.cpp b/src/protocols/Pager/Pager.cpp
index 90ddaa59..144903a2 100644
--- a/src/protocols/Pager/Pager.cpp
+++ b/src/protocols/Pager/Pager.cpp
@@ -19,7 +19,7 @@ PagerClient::PagerClient(PhysicalLayer* phy) {
   _readBitInstance = _phy;
 }
 
-int16_t PagerClient::begin(float base, uint16_t speed, uint16_t shift) {
+int16_t PagerClient::begin(float base, uint16_t speed, bool invert, uint16_t shift) {
   // calculate duration of 1 bit in us
   _speed = (float)speed/1000.0f;
   _bitDuration = (uint32_t)1000000/speed;
@@ -34,6 +34,7 @@ int16_t PagerClient::begin(float base, uint16_t speed, uint16_t shift) {
   // calculate raw frequency shift
   _shiftHz = shift;
   _shift = _shiftHz/step;
+  inv = invert;
 
   // initialize BCH encoder
   encoderInit();
@@ -433,14 +434,23 @@ void PagerClient::write(uint32_t codeWord) {
   for(int8_t i = 31; i >= 0; i--) {
     uint32_t mask = (uint32_t)0x01 << i;
     uint32_t start = mod->micros();
+
+    // figure out the shift direction - start by assuming the bit is 0
+    int16_t change = _shift;
+
+    // now check if it's actually 1
     if(codeWord & mask) {
-      // send 1
-      _phy->transmitDirect(_baseRaw + _shift);
-    } else {
-      // send 0
-      _phy->transmitDirect(_baseRaw - _shift);
+      change = -_shift;
     }
 
+    // finally, check if inversion is enabled
+    if(inv) {
+      change = -change;
+    }
+
+    // now transmit the shifted frequency
+    _phy->transmitDirect(_baseRaw + change);
+
     // this is pretty silly, while(mod->micros() ... ) would be enough
     // but for some reason, MegaCore throws a linker error on it
     // "relocation truncated to fit: R_AVR_7_PCREL against `no symbol'"
diff --git a/src/protocols/Pager/Pager.h b/src/protocols/Pager/Pager.h
index 6a831b9e..eb828fbb 100644
--- a/src/protocols/Pager/Pager.h
+++ b/src/protocols/Pager/Pager.h
@@ -84,9 +84,13 @@ class PagerClient {
 
       \param speed Bit rate to use in bps. Common POCSAG decoders can receive 512, 1200 and 2400 bps.
 
+      \param invert Enable frequency inversion. Disabled by default (high frequency is digital 0).
+
+      \param shift Set custom frequency shift, defaults to 4500 Hz.
+
       \returns \ref status_codes
     */
-    int16_t begin(float base, uint16_t speed, uint16_t shift = RADIOLIB_PAGER_FREQ_SHIFT_HZ);
+    int16_t begin(float base, uint16_t speed, bool invert = false, uint16_t shift = RADIOLIB_PAGER_FREQ_SHIFT_HZ);
 
     /*!
       \brief Method to send a tone-only alert to a destination pager.
@@ -201,6 +205,7 @@ class PagerClient {
     uint32_t _readBatchPos;
     uint32_t _filterAddr;
     uint32_t _filterMask;
+    bool inv = false;
 
     // BCH encoder
     int32_t _bchAlphaTo[RADIOLIB_PAGER_BCH_N + 1];