nodemcu-firmware/components/platform/onewire.c

/*
Adaptation of Paul Stoffregen's One wire library to the NodeMcu

Ported to ESP32 RMT peripheral for low-level signal generation by Arnim Laeuger.

The latest version of this library may be found at:
  http://www.pjrc.com/teensy/td_libs_OneWire.html

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Much of the code was inspired by Derek Yerger's code, though I don't
think much of that remains.  In any event that was..
    (copyleft) 2006 by Derek Yerger - Free to distribute freely.

The CRC code was excerpted and inspired by the Dallas Semiconductor
sample code bearing this copyright.
//---------------------------------------------------------------------------
// Copyright (C) 2000 Dallas Semiconductor Corporation, All Rights Reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a
// copy of this software and associated documentation files (the "Software"),
// to deal in the Software without restriction, including without limitation
// the rights to use, copy, modify, merge, publish, distribute, sublicense,
// and/or sell copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included
// in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
// MERCHANTABILITY,  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
// IN NO EVENT SHALL DALLAS SEMICONDUCTOR BE LIABLE FOR ANY CLAIM, DAMAGES
// OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
// ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.
//
// Except as contained in this notice, the name of Dallas Semiconductor
// shall not be used except as stated in the Dallas Semiconductor
// Branding Policy.
//--------------------------------------------------------------------------
*/

#include "platform.h"

#include "driver/rmt.h"
#include "driver/gpio.h"
#include "esp_log.h"

#define TRUE (1==1)
#define FALSE !TRUE

#undef OW_DEBUG

// *****************************************************************************
// Onewire platform interface

// bus reset: duration of low phase [us]
#define OW_DURATION_RESET 480
// overall slot duration
#define OW_DURATION_SLOT 75
// write 1 slot and read slot durations [us]
#define OW_DURATION_1_LOW    2
#define OW_DURATION_1_HIGH (OW_DURATION_SLOT - OW_DURATION_1_LOW)
// write 0 slot durations [us]
#define OW_DURATION_0_LOW   65
#define OW_DURATION_0_HIGH (OW_DURATION_SLOT - OW_DURATION_0_LOW)
// sample time for read slot
#define OW_DURATION_SAMPLE  (15-2)
// RX idle threshold
// needs to be larger than any duration occurring during write slots
#define OW_DURATION_RX_IDLE (OW_DURATION_SLOT + 2)

// Strong pull-up aka power mode is implemented by the pad's push-pull driver.
// Open-drain configuration is used for normal operation.
// power bus by disabling open-drain:
#define OW_POWER(g) GPIO.pin[g].pad_driver = 0
// de-power bus by enabling open-drain:
#define OW_DEPOWER(g) GPIO.pin[g].pad_driver = 1

// grouped information for RMT management
static struct {
  int tx, rx;
  RingbufHandle_t rb;
  int gpio;
} ow_rmt = {-1, -1, NULL, -1};

// default power mode for generic write operations
static const uint8_t owDefaultPower = 0;

static int onewire_rmt_init( uint8_t gpio_num )
{
  // acquire an RMT module for TX and RX each
  if ((ow_rmt.tx = platform_rmt_allocate( 1 )) >= 0) {
    if ((ow_rmt.rx = platform_rmt_allocate( 1 )) >= 0) {

#ifdef OW_DEBUG
      ESP_LOGI("ow", "RMT TX channel: %d", ow_rmt.tx);
      ESP_LOGI("ow", "RMT RX channel: %d", ow_rmt.rx);
#endif

      rmt_config_t rmt_tx;
      rmt_tx.channel = ow_rmt.tx;
      rmt_tx.gpio_num = gpio_num;
      rmt_tx.mem_block_num = 1;
      rmt_tx.clk_div = 80;
      rmt_tx.tx_config.loop_en = false;
      rmt_tx.tx_config.carrier_en = false;
      rmt_tx.tx_config.idle_level = 1;
      rmt_tx.tx_config.idle_output_en = true;
      rmt_tx.rmt_mode = RMT_MODE_TX;
      if (rmt_config( &rmt_tx ) == ESP_OK) {
        if (rmt_driver_install( rmt_tx.channel, 0, 0 ) == ESP_OK) {

          rmt_config_t rmt_rx;
          rmt_rx.channel = ow_rmt.rx;
          rmt_rx.gpio_num = gpio_num;
          rmt_rx.clk_div = 80;
          rmt_rx.mem_block_num = 1;
          rmt_rx.rmt_mode = RMT_MODE_RX;
          rmt_rx.rx_config.filter_en = true;
          rmt_rx.rx_config.filter_ticks_thresh = 30;
          rmt_rx.rx_config.idle_threshold = OW_DURATION_RX_IDLE;
          if (rmt_config( &rmt_rx ) == ESP_OK) {
            if (rmt_driver_install( rmt_rx.channel, 512, 0 ) == ESP_OK) {

              rmt_get_ringbuf_handler( ow_rmt.rx, &ow_rmt.rb );

              ow_rmt.gpio = gpio_num;

              return PLATFORM_OK;

            }
          }

          rmt_driver_uninstall( rmt_tx.channel );
        }
      }

      platform_rmt_release( ow_rmt.rx );
    }

    platform_rmt_release( ow_rmt.tx );
  }

  return PLATFORM_ERR;
}

// flush any pending/spurious traces from the RX channel
static void onewire_flush_rmt_rx_buf( void )
{
  void *p;
  size_t s;

  while ((p = xRingbufferReceive( ow_rmt.rb, &s, 0 )))
    vRingbufferReturnItem( ow_rmt.rb, p );
}

// check rmt TX&RX channel assignment and eventually attach them to the requested pin
static int onewire_rmt_attach_pin( uint8_t gpio_num )
{
  if (ow_rmt.tx < 0 || ow_rmt.rx < 0)
    return PLATFORM_ERR;

  if (gpio_num != ow_rmt.gpio) {
    // attach RMT channels to new gpio pin
    rmt_set_pin( ow_rmt.tx, RMT_MODE_TX, gpio_num );
    rmt_set_pin( ow_rmt.rx, RMT_MODE_RX, gpio_num );

    // attach GPIO to previous pin
    gpio_matrix_out( ow_rmt.gpio, SIG_GPIO_OUT_IDX, 0, 0 );
    if (gpio_num < 32) {
        GPIO.enable_w1ts = (0x1 << gpio_num);
    } else {
        GPIO.enable1_w1ts.data = (0x1 << (gpio_num - 32));
    }

    ow_rmt.gpio = gpio_num;
  }

  return PLATFORM_OK;
}

int platform_onewire_init( uint8_t gpio_num )
{
  if (ow_rmt.tx < 0 || ow_rmt.rx < 0) {
    if (onewire_rmt_init( gpio_num ) != PLATFORM_OK)
      return PLATFORM_ERR;
  }

  // enable open-drain mode on pin
  OW_DEPOWER(gpio_num);
  // and prepare driving 1
  gpio_set_level( gpio_num, 1 );

  return PLATFORM_OK;
}

int platform_onewire_reset( uint8_t gpio_num, uint8_t *presence )
{
  rmt_item32_t tx_items[1];
  uint8_t _presence = 0;
  int res = PLATFORM_OK;

  if (onewire_rmt_attach_pin( gpio_num ) != PLATFORM_OK)
    return PLATFORM_ERR;

  OW_DEPOWER( gpio_num );

  tx_items[0].duration0 = OW_DURATION_RESET;
  tx_items[0].level0 = 0;
  tx_items[0].duration1 = 0;
  tx_items[0].level1 = 1;

  uint16_t old_rx_thresh;
  rmt_get_rx_idle_thresh( ow_rmt.rx, &old_rx_thresh );
  rmt_set_rx_idle_thresh( ow_rmt.rx, OW_DURATION_RESET+60 );

  onewire_flush_rmt_rx_buf();
  rmt_rx_start( ow_rmt.rx, true );
  if (rmt_write_items( ow_rmt.tx, tx_items, 1, true ) == ESP_OK) {

    size_t rx_size;
    rmt_item32_t* rx_items = (rmt_item32_t *)xRingbufferReceive( ow_rmt.rb, &rx_size, 100 / portTICK_PERIOD_MS );

    if (rx_items) {
      if (rx_size >= 1 * sizeof( rmt_item32_t )) {

#ifdef OW_DEBUG
	for (int i = 0; i < rx_size / 4; i++) {
	  ESP_LOGI("ow", "level: %d, duration %d", rx_items[i].level0, rx_items[i].duration0);
	  ESP_LOGI("ow", "level: %d, duration %d", rx_items[i].level1, rx_items[i].duration1);
	}
#endif

	// parse signal and search for presence pulse
	if ((rx_items[0].level0 == 0) && (rx_items[0].duration0 >= OW_DURATION_RESET - 2))
	  if ((rx_items[0].level1 == 1) && (rx_items[0].duration1 > 0))
	    if (rx_items[1].level0 == 0)
	      _presence = 1;
      }

      vRingbufferReturnItem( ow_rmt.rb, (void *)rx_items );
    } else {
      // time out occurred, this indicates an unconnected / misconfigured bus
      res = PLATFORM_ERR;
    }

  } else {
    // error in tx channel
    res = PLATFORM_ERR;
  }

  rmt_rx_stop( ow_rmt.rx );
  rmt_set_rx_idle_thresh( ow_rmt.rx, old_rx_thresh );

  *presence = _presence;
  return res;
}

static rmt_item32_t onewire_encode_write_slot( uint8_t val )
{
  rmt_item32_t item;

  item.level0 = 0;
  item.level1 = 1;
  if (val) {
    // write "1" slot
    item.duration0 = OW_DURATION_1_LOW;
    item.duration1 = OW_DURATION_1_HIGH;
  } else {
    // write "0" slot
    item.duration0 = OW_DURATION_0_LOW;
    item.duration1 = OW_DURATION_0_HIGH;
  }

  return item;
}

static int onewire_write_bits( uint8_t gpio_num, uint8_t data, uint8_t num, uint8_t power )
{
  rmt_item32_t tx_items[num+1];

  if (num > 8)
    return PLATFORM_ERR;

  if (onewire_rmt_attach_pin( gpio_num ) != PLATFORM_OK)
    return PLATFORM_ERR;

  if (power) {
    // apply strong driver to power the bus
    OW_POWER(gpio_num);
  } else {
    // switch to open-drain mode, bus is powered by external pull-up
    OW_DEPOWER(gpio_num);
  }

  // write requested bits as pattern to TX buffer
  for (int i = 0; i < num; i++) {
    tx_items[i] = onewire_encode_write_slot( data & 0x01 );
    data >>= 1;
  }
  // end marker
  tx_items[num].level0 = 1;
  tx_items[num].duration0 = 0;

  if (rmt_write_items( ow_rmt.tx, tx_items, num+1, true ) == ESP_OK)
    return PLATFORM_OK;
  else
    return PLATFORM_ERR;
}

int platform_onewire_write_bytes( uint8_t gpio_num, const uint8_t *buf, uint16_t count, bool power )
{
  for (uint16_t i = 0 ; i < count ; i++)
    if (onewire_write_bits( gpio_num, buf[i], 8, i < count-1 ? owDefaultPower : power) != PLATFORM_OK)
      return PLATFORM_ERR;

  return PLATFORM_OK;
}

int platform_onewire_depower( uint8_t gpio_num )
{
  // enable open-drain mode on pin
  OW_DEPOWER(gpio_num);

  return PLATFORM_OK;
}

static rmt_item32_t onewire_encode_read_slot( void )
{
  rmt_item32_t item;

  // construct pattern for a single read time slot
  item.level0    = 0;
  item.duration0 = OW_DURATION_1_LOW;   // shortly force 0
  item.level1    = 1;
  item.duration1 = OW_DURATION_1_HIGH;  // release high and finish slot

  return item;
}

static int onewire_read_bits( uint8_t gpio_num, uint8_t *data, uint8_t num )
{
  rmt_item32_t tx_items[num+1];
  uint8_t read_data = 0;
  int res = PLATFORM_OK;

  if (num > 8)
    return PLATFORM_ERR;

  if (onewire_rmt_attach_pin( gpio_num ) != PLATFORM_OK)
    return PLATFORM_ERR;

  OW_DEPOWER( gpio_num );

  // generate requested read slots
  for (int i = 0; i < num; i++)
    tx_items[i] = onewire_encode_read_slot();
  // end marker
  tx_items[num].level0 = 1;
  tx_items[num].duration0 = 0;

  onewire_flush_rmt_rx_buf();
  rmt_rx_start( ow_rmt.rx, true );
  if (rmt_write_items( ow_rmt.tx, tx_items, num+1, true ) == ESP_OK) {

    size_t rx_size;
    rmt_item32_t* rx_items = (rmt_item32_t *)xRingbufferReceive( ow_rmt.rb, &rx_size, portMAX_DELAY );

    if (rx_items) {

#ifdef OW_DEBUG
	for (int i = 0; i < rx_size / 4; i++) {
	  ESP_LOGI("ow", "level: %d, duration %d", rx_items[i].level0, rx_items[i].duration0);
	  ESP_LOGI("ow", "level: %d, duration %d", rx_items[i].level1, rx_items[i].duration1);
	}
#endif

      if (rx_size >= num * sizeof( rmt_item32_t )) {
	for (int i = 0; i < num; i++) {
	  read_data >>= 1;
	  // parse signal and identify logical bit
	  if (rx_items[i].level1 == 1) {
	    if ((rx_items[i].level0 == 0) && (rx_items[i].duration0 < OW_DURATION_SAMPLE)) {
	      // rising edge occured before 15us -> bit 1
	      read_data |= 0x80;
	    }
	  }
	}
	read_data >>= 8 - num;
      }

      vRingbufferReturnItem( ow_rmt.rb, (void *)rx_items );
    } else {
      // time out occurred, this indicates an unconnected / misconfigured bus
      res = PLATFORM_ERR;
    }

  } else {
    // error in tx channel
    res = PLATFORM_ERR;
  }

  rmt_rx_stop( ow_rmt.rx );

  *data = read_data;
  return res;
}

int platform_onewire_read_bytes( uint8_t gpio_num, uint8_t *buf, uint16_t count )
{
  for (uint16_t i = 0 ; i < count ; i++) {
    if (onewire_read_bits( gpio_num, buf, 8 ) != PLATFORM_OK)
      return PLATFORM_ERR;
    buf++;
  }

  return PLATFORM_OK;
}


//
// You need to use this function to start a search again from the beginning.
// You do not need to do it for the first search, though you could.
//
void platform_onewire_reset_search( platform_onewire_bus_t *bus )
{
  // reset the search state
  bus->LastDiscrepancy = 0;
  bus->LastDeviceFlag = FALSE;
  bus->LastFamilyDiscrepancy = 0;
  int i;
  for(i = 7; ; i--) {
    bus->ROM_NO[i] = 0;
    if (i == 0) break;
  }
}

// Setup the search to find the device type 'family_code' on the next call
// to search(*newAddr) if it is present.
//
void platform_onewire_target_search( uint8_t family_code, platform_onewire_bus_t *bus )
{
  // set the search state to find SearchFamily type devices
  bus->ROM_NO[0] = family_code;
  uint8_t i;
  for (i = 1; i < 8; i++)
    bus->ROM_NO[i] = 0;
  bus->LastDiscrepancy = 64;
  bus->LastFamilyDiscrepancy = 0;
  bus->LastDeviceFlag = FALSE;
}

//
// Perform a search. If this function returns a '1' then it has
// enumerated the next device and you may retrieve the ROM from the
// OneWire::address variable. If there are no devices, no further
// devices, or something horrible happens in the middle of the
// enumeration then a 0 is returned.  If a new device is found then
// its address is copied to newAddr.  Use OneWire::reset_search() to
// start over.
//
// --- Replaced by the one from the Dallas Semiconductor web site ---
//--------------------------------------------------------------------------
// Perform the 1-Wire Search Algorithm on the 1-Wire bus using the existing
// search state.
// Return TRUE  : device found, ROM number in ROM_NO buffer
//        FALSE : device not found, end of search
//
uint8_t platform_onewire_search( uint8_t pin, uint8_t *newAddr,  platform_onewire_bus_t *bus )
{
  uint8_t id_bit_number;
  uint8_t last_zero, rom_byte_number, search_result;
  uint8_t id_bit, cmp_id_bit;

  unsigned char rom_byte_mask, search_direction;

  // initialize for search
  id_bit_number = 1;
  last_zero = 0;
  rom_byte_number = 0;
  rom_byte_mask = 1;
  search_result = 0;

  // if the last call was not the last one
  if (!bus->LastDeviceFlag) {
    // 1-Wire reset
    uint8_t presence;
    if (platform_onewire_reset(pin, &presence) != PLATFORM_OK || !presence) {
      // reset the search
      bus->LastDiscrepancy = 0;
      bus->LastDeviceFlag = FALSE;
      bus->LastFamilyDiscrepancy = 0;
      return FALSE;
    }

    // issue the search command
    onewire_write_bits(pin, 0xF0, 8, owDefaultPower);

    // loop to do the search
    do {
      // read a bit and its complement
      if (onewire_read_bits(pin, &id_bit, 1) != PLATFORM_OK)
	break;
      if (onewire_read_bits(pin, &cmp_id_bit, 1) != PLATFORM_OK)
	break;

      // check for no devices on 1-wire
      if ((id_bit == 1) && (cmp_id_bit == 1))
	break;
      else {
	// all devices coupled have 0 or 1
	if (id_bit != cmp_id_bit)
	  search_direction = id_bit;  // bit write value for search
	else {
	  // if this discrepancy if before the Last Discrepancy
	  // on a previous next then pick the same as last time
	  if (id_bit_number < bus->LastDiscrepancy)
	    search_direction = ((bus->ROM_NO[rom_byte_number] & rom_byte_mask) > 0);
	  else
	    // if equal to last pick 1, if not then pick 0
	    search_direction = (id_bit_number == bus->LastDiscrepancy);

	  // if 0 was picked then record its position in LastZero
	  if (search_direction == 0) {
	    last_zero = id_bit_number;

	    // check for Last discrepancy in family
	    if (last_zero < 9)
	      bus->LastFamilyDiscrepancy = last_zero;
	  }
	}

	// set or clear the bit in the ROM byte rom_byte_number
	// with mask rom_byte_mask
	if (search_direction == 1)
	  bus->ROM_NO[rom_byte_number] |= rom_byte_mask;
	else
	  bus->ROM_NO[rom_byte_number] &= ~rom_byte_mask;

	// serial number search direction write bit
	onewire_write_bits(pin, search_direction, 1, owDefaultPower);

	// increment the byte counter id_bit_number
	// and shift the mask rom_byte_mask
	id_bit_number++;
	rom_byte_mask <<= 1;

	// if the mask is 0 then go to new SerialNum byte rom_byte_number and reset mask
	if (rom_byte_mask == 0) {
	  rom_byte_number++;
	  rom_byte_mask = 1;
	}
      }
    }
    while(rom_byte_number < 8);  // loop until through all ROM bytes 0-7

    // if the search was successful then
    if (!(id_bit_number < 65)) {
      // search successful so set LastDiscrepancy,LastDeviceFlag,search_result
      bus->LastDiscrepancy = last_zero;

      // check for last device
      if (bus->LastDiscrepancy == 0)
	bus->LastDeviceFlag = TRUE;

      search_result = TRUE;
    }
  }

  // if no device found then reset counters so next 'search' will be like a first
  if (!search_result || !bus->ROM_NO[0]) {
    bus->LastDiscrepancy = 0;
    bus->LastDeviceFlag = FALSE;
    bus->LastFamilyDiscrepancy = 0;
    search_result = FALSE;
  }
  else {
    for (rom_byte_number = 0; rom_byte_number < 8; rom_byte_number++) {
      newAddr[rom_byte_number] = bus->ROM_NO[rom_byte_number];
    }
  }
  return search_result;
}


#define ONEWIRE_CRC 1
#if ONEWIRE_CRC
// The 1-Wire CRC scheme is described in Maxim Application Note 27:
// "Understanding and Using Cyclic Redundancy Checks with Maxim iButton Products"
//

#define ONEWIRE_CRC8_TABLE 0
#if ONEWIRE_CRC8_TABLE
// This table comes from Dallas sample code where it is freely reusable,
// though Copyright (C) 2000 Dallas Semiconductor Corporation
static const uint8_t dscrc_table[] = {
      0, 94,188,226, 97, 63,221,131,194,156,126, 32,163,253, 31, 65,
    157,195, 33,127,252,162, 64, 30, 95,  1,227,189, 62, 96,130,220,
     35,125,159,193, 66, 28,254,160,225,191, 93,  3,128,222, 60, 98,
    190,224,  2, 92,223,129, 99, 61,124, 34,192,158, 29, 67,161,255,
     70, 24,250,164, 39,121,155,197,132,218, 56,102,229,187, 89,  7,
    219,133,103, 57,186,228,  6, 88, 25, 71,165,251,120, 38,196,154,
    101, 59,217,135,  4, 90,184,230,167,249, 27, 69,198,152,122, 36,
    248,166, 68, 26,153,199, 37,123, 58,100,134,216, 91,  5,231,185,
    140,210, 48,110,237,179, 81, 15, 78, 16,242,172, 47,113,147,205,
     17, 79,173,243,112, 46,204,146,211,141,111, 49,178,236, 14, 80,
    175,241, 19, 77,206,144,114, 44,109, 51,209,143, 12, 82,176,238,
     50,108,142,208, 83, 13,239,177,240,174, 76, 18,145,207, 45,115,
    202,148,118, 40,171,245, 23, 73,  8, 86,180,234,105, 55,213,139,
     87,  9,235,181, 54,104,138,212,149,203, 41,119,244,170, 72, 22,
    233,183, 85, 11,136,214, 52,106, 43,117,151,201, 74, 20,246,168,
    116, 42,200,150, 21, 75,169,247,182,232, 10, 84,215,137,107, 53};

#ifndef pgm_read_byte
#define pgm_read_byte(addr) (*(const uint8_t *)(addr))
#endif

//
// Compute a Dallas Semiconductor 8 bit CRC. These show up in the ROM
// and the registers.  (note: this might better be done without to
// table, it would probably be smaller and certainly fast enough
// compared to all those delayMicrosecond() calls.  But I got
// confused, so I use this table from the examples.)
//
uint8_t platform_onewire_crc8( const uint8_t *addr, uint8_t len )
{
  uint8_t crc = 0;

  while (len--) {
    crc = pgm_read_byte(dscrc_table + (crc ^ *addr++));
  }
  return crc;
}
#else
//
// Compute a Dallas Semiconductor 8 bit CRC directly.
// this is much slower, but much smaller, than the lookup table.
//
uint8_t platform_onewire_crc8( const uint8_t *addr, uint8_t len )
{
  uint8_t crc = 0;

  while (len--) {
    uint8_t inbyte = *addr++;
    uint8_t i;
    for (i = 8; i; i--) {
      uint8_t mix = (crc ^ inbyte) & 0x01;
      crc >>= 1;
      if (mix) crc ^= 0x8C;
      inbyte >>= 1;
    }
  }
  return crc;
}
#endif

#define ONEWIRE_CRC16 1
#if ONEWIRE_CRC16
// Compute the 1-Wire CRC16 and compare it against the received CRC.
// Example usage (reading a DS2408):
//    // Put everything in a buffer so we can compute the CRC easily.
//    uint8_t buf[13];
//    buf[0] = 0xF0;    // Read PIO Registers
//    buf[1] = 0x88;    // LSB address
//    buf[2] = 0x00;    // MSB address
//    WriteBytes(net, buf, 3);    // Write 3 cmd bytes
//    ReadBytes(net, buf+3, 10);  // Read 6 data bytes, 2 0xFF, 2 CRC16
//    if (!CheckCRC16(buf, 11, &buf[11])) {
//        // Handle error.
//    }
//
// @param input - Array of bytes to checksum.
// @param len - How many bytes to use.
// @param inverted_crc - The two CRC16 bytes in the received data.
//                       This should just point into the received data,
//                       *not* at a 16-bit integer.
// @param crc - The crc starting value (optional)
// @return True, iff the CRC matches.
bool platform_onewire_check_crc16( const uint8_t* input, uint16_t len, const uint8_t* inverted_crc, uint16_t crc )
{
  crc = ~platform_onewire_crc16(input, len, crc);
  return (crc & 0xFF) == inverted_crc[0] && (crc >> 8) == inverted_crc[1];
}

// Compute a Dallas Semiconductor 16 bit CRC.  This is required to check
// the integrity of data received from many 1-Wire devices.  Note that the
// CRC computed here is *not* what you'll get from the 1-Wire network,
// for two reasons:
//   1) The CRC is transmitted bitwise inverted.
//   2) Depending on the endian-ness of your processor, the binary
//      representation of the two-byte return value may have a different
//      byte order than the two bytes you get from 1-Wire.
// @param input - Array of bytes to checksum.
// @param len - How many bytes to use.
// @param crc - The crc starting value (optional)
// @return The CRC16, as defined by Dallas Semiconductor.
uint16_t platform_onewire_crc16( const uint8_t* input, uint16_t len, uint16_t crc )
{
  static const uint8_t oddparity[16] =
    { 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0 };

  uint16_t i;
  for (i = 0 ; i < len ; i++) {
    // Even though we're just copying a byte from the input,
    // we'll be doing 16-bit computation with it.
    uint16_t cdata = input[i];
    cdata = (cdata ^ crc) & 0xff;
    crc >>= 8;

    if (oddparity[cdata & 0x0F] ^ oddparity[cdata >> 4])
      crc ^= 0xC001;

    cdata <<= 6;
    crc ^= cdata;
    cdata <<= 1;
    crc ^= cdata;
  }
  return crc;
}
#endif

#endif