AP_RCProtocol: added separate soft serial decoder

this will replace the version in each protocol

libraries/AP_RCProtocol/SoftSerial.cpp

@@ -0,0 +1,114 @@
+/*
+ * This file is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+/*
+  soft serial receive implementation, based on pulse width inputs
+ */
+
+#include "SoftSerial.h"
+#include <stdio.h>
+
+SoftSerial::SoftSerial(uint32_t _baudrate, serial_config _config) :
+    baudrate(_baudrate),
+    config(_config),
+    half_bit((1000000U / baudrate)/2)
+{
+    switch (config) {
+    case SERIAL_CONFIG_8N1:
+        data_width = 8;
+        byte_width = 10;
+        stop_mask = 0x200;
+        break;
+    case SERIAL_CONFIG_8E2I:
+        data_width = 9;
+        byte_width = 12;
+        stop_mask = 0xC00;
+        break;
+    }
+}
+
+/*
+  process a pulse made up of a width of values at high voltage
+  followed by a width at low voltage
+ */
+bool SoftSerial::process_pulse(uint32_t width_high, uint32_t width_low, uint8_t &byte)
+{
+    // convert to bit widths, allowing for a half bit error
+    uint16_t bits_high = ((width_high+half_bit)*baudrate) / 1000000;
+    uint16_t bits_low = ((width_low+half_bit)*baudrate) / 1000000;
+
+    byte_timestamp_us = timestamp_us;
+    timestamp_us += (width_high + width_low);
+
+    if (bits_high == 0 || bits_low == 0) {
+        // invalid data
+        goto reset;
+    }
+
+    if (bits_high >= byte_width) {
+        // if we have a start bit and a stop bit then we can have at
+        // most 9 bits in high state for data. The rest must be idle
+        // bits
+        bits_high = byte_width-1;
+    }
+
+    if (state.bit_ofs == 0) {
+        // we are in idle state, waiting for first low bit. swallow
+        // the high bits
+        bits_high = 0;
+    }
+
+    state.byte |= ((1U<<bits_high)-1) << state.bit_ofs;
+
+    state.bit_ofs += bits_high + bits_low;
+
+    if (state.bit_ofs >= byte_width) {
+        // check start bit
+        if ((state.byte & 1) != 0) {
+            goto reset;
+        }
+        // check stop bits
+        if ((state.byte & stop_mask) != stop_mask) {
+            goto reset;
+        }
+        if (config == SERIAL_CONFIG_8E2I) {
+            // check parity
+            if (__builtin_parity((state.byte>>1)&0xFF) != (state.byte&0x200)>>9) {
+                goto reset;
+            }
+        }
+
+        byte = ((state.byte>>1) & 0xFF);
+        state.byte >>= byte_width;
+        state.bit_ofs -= byte_width;
+        if (state.bit_ofs > byte_width) {
+            state.byte = 0;
+            state.bit_ofs = bits_low;
+        }
+        // swallow idle bits
+        while (state.bit_ofs > 0 && (state.byte & 1)) {
+            state.bit_ofs--;
+            state.byte >>= 1;
+        }
+        return true;
+    }
+    return false;
+
+reset:
+    state.byte = 0;
+    state.bit_ofs = 0;
+
+    return false;
+}
+

libraries/AP_RCProtocol/SoftSerial.h

@@ -0,0 +1,50 @@
+/*
+ * This file is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This file is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include "AP_RCProtocol.h"
+
+class SoftSerial {
+public:
+    enum serial_config {
+        SERIAL_CONFIG_8N1,  // DSM, SRXL etc, 8 bit, no parity, 1 stop bit
+        SERIAL_CONFIG_8E2I, // SBUS, 8 bit, even parity, 2 stop bits, inverted
+    };
+
+    SoftSerial(uint32_t baudrate, enum serial_config config);
+    bool process_pulse(uint32_t width_s0, uint32_t width_s1, uint8_t &b);
+
+    // get timestamp of the last byte
+    uint32_t get_byte_timestamp_us(void) const {
+        return byte_timestamp_us;
+    }
+
+private:
+    const uint32_t baudrate;
+    const uint8_t half_bit; // width of half a bit in microseconds
+    const enum serial_config config;
+
+    uint8_t data_width;
+    uint8_t byte_width;
+    uint16_t stop_mask;
+    uint32_t timestamp_us;
+    uint32_t byte_timestamp_us;
+
+    struct {
+        uint32_t byte;
+        uint16_t bit_ofs;
+    } state;
+};