Merged in f5soh/librepilot/update_credits (pull request #529)

[librepilot.git] / ground / gcs / src / experimental / USB_UPLOAD_TOOL / SSP / ssp.cpp

/***********************************************************************************************************
*
*  NAME:          ssp.c
*  DESCRIPTION:   simple serial protocol - packet based serial transport layer.
*  AUTHOR:        Joe Hlebasko
*  HISTORY:       Created 1/1/2010
*
* Packet Formats
*   Format:
*    +------+----+------+---------------------------+--------+
*    | 225  | L1 | S#   | App Data (0-254 bytes)    | CRC 16 |
*    +------+----+------+---------------------------+--------+
*
* 225 = sync byte, indicates start of a packet
* L1 = 1 byte for size of data payload. (sequence number is part of data payload.)
* S# = 1 byte for sequence number.
*                       Seq of 0 = seq # synchronise request, forces other end to reset receive sequence number to 1.
*                               sender of synchronise request will reset the tx seq number to 1
*                       Seq # of 1..127 = normal data packets. Sequence number is incremented by for each transmitted
*                               packet.  Rolls over from 127 to 1.
*                       if most sig. bit is set then the packet is an ACK packet of data packet sequence number of the
*                               lower 7 bits (1..127)
* App Data may contain 0..254 bytes. The sequence number is consider part of the payload.
* CRC 16 - 16 bits of CRC values of Sequence # and data bytes.
*
* Protocol has two types of packets: data and ack packets. ACK packets have the most sig. bit set in the
* sequence number, this implies that valid sequence numbers are 1..127
*
* This protocol uses the concept of sequences numbers to determine if a given packet has been received. This
* requires both devices to be able to synchronize sequence numbers. This is accomplished by sending a packet
* length 1 and sequence number = 0. The receive then resets it's transmit sequence number to 1.
*
* ACTIVE_SYNCH is a version that will automatically send a synch request if it receives a synch packet. Only
* one device in the communication should do otherwise you end up with an endless loops of synchronization.
* Right now each side needs to manually issues a synch request.
*
* This protocol is best used in cases where one device is the master and the other is the slave, or a don't
* speak unless spoken to type of approach.
*
* The following are items are required to initialize a port for communications:
* 1. The number attempts for each packet
* 2. time to wait for an ack.
* 3. pointer to buffer to be used for receiving.
* 4. pointer to a buffer to be used for transmission
* 5. length of each buffer (rx and tx)
* 6. Four functions:
*       1. write byte = writes a byte out the serial port (or other comm device)
*       2. read byte = retrieves a byte from the serial port. Returns -1 if a byte is not available
*       3. callback = function to call when a valid data packet has been received. This function is responsible
*               to do what needs to be done with the data when it is received. The primary mission of this function
*               should be to copy the data to a private buffer out of the working receive buffer to prevent overrun.
*               processing should be kept to a minimum.
*       4. get time = function should return the current time. Note that time units are not specified it just
*               needs to be some measure of time that increments as time passes by.  The timeout values for a given
*               port should the units used/returned by the get time function.
*
* All of the state information of a communication port is contained in a Port_t structure. This allows this
* module to operature on multiple communication ports with a single code base.
*
* The ssp_ReceiveProcess and ssp_SendProcess functions need to be called to process data through the
* respective state machines. Typical implementation would have a serial ISR to pull bytes out of the UART
* and place into a circular buffer.  The serial read function would then pull bytes out this buffer
* processing. The TX side has the write function placing bytes into a circular buffer with the TX ISR
* pulling bytes out of the buffer and putting into the UART.  It is possible to run the receive process from
* the receive ISR but care must be taken on processing data when it is received to avoid holding up the ISR
* and sending ACK packets from the receive ISR.
*
***********************************************************************************************************/

/** INCLUDE FILES **/


#include <stdint.h>
#include <string.h>
#include <stdio.h>

#include "ssp.h"

/** PRIVATE DEFINITIONS **/
#define SYNC     225                     // Sync character used in Serial Protocol
#define ESC      224                     // ESC character used in Serial Protocol
#define ESC_SYNC 1 // ESC_SYNC character used in Serial Protocol
#define ACK_BIT  0x80                    // Ack bit, bit 7 of sequence number, 1 = Acknowledge, 0 =
                                         // new packet
// packet location definitions.
#define LENGTH   0
#define SEQNUM   1
#define DATA     2


// Make larger sized integers from smaller sized integers
#define MAKEWORD16(ub, lb)          ((uint16_t)0x0000 | ((uint16_t)(ub) << 8) | (uint16_t)(lb))
#define MAKEWORD32(uw, lw)          ((uint32_t)(0x0UL | ((uint32_t)(uw) << 16) | (uint32_t)(lw)))
#define MAKEWORD32B(b3, b2, b1, b0) ((uint32_t)((uint32_t)(b3) << 24) | ((uint32_t)(b2) << 16) | ((uint32_t)(b1) << 8) | ((uint32_t)(b0)))


// Used to extract smaller integers from larger sized intergers
#define LOWERBYTE(w)                (uint8_t)((w) & 0x00ff)
#define UPPERBYTE(w)                (uint8_t)(((w) & 0xff00) >> 8)
#define UPPERWORD(lw)               (uint16_t)(((lw) & 0xffff0000) >> 16)
#define LOWERWORD(lw)               (uint16_t)((lw) & 0x0000ffff)

// Macros to operate on a target and bitmask.
#define CLEARBIT(a, b)              ((a) = (a) & ~(b))
#define SETBIT(a, b)                ((a) = (a) | (b))
#define TOGGLEBIT(a, b)             ((a) = (a) ^ (b))

// test bit macros operate using a bit mask.
#define ISBITSET(a, b)              (((a) & (b)) == (b) ? TRUE : FALSE)
#define ISBITCLEAR(a, b)            ((~(a) & (b)) == (b) ? TRUE : FALSE)

/** PRIVATE FUNCTIONS **/
// static void          sf_SendSynchPacket( Port_t *thisport );
static uint16_t sf_crc16(uint16_t crc, uint8_t data);
static void     sf_write_byte(Port_t *thisport, uint8_t c);
static void     sf_SetSendTimeout(Port_t *thisport);
static uint16_t sf_CheckTimeout(Port_t *thisport);
static int16_t  sf_DecodeState(Port_t *thisport, uint8_t c);
static int16_t  sf_ReceiveState(Port_t *thisport, uint8_t c);

static void     sf_SendPacket(Port_t *thisport);
static void     sf_SendAckPacket(Port_t *thisport, uint8_t seqNumber);
static void     sf_MakePacket(uint8_t *buf, const uint8_t *pdata, uint16_t length, uint8_t seqNo);
static int16_t  sf_ReceivePacket(Port_t *thisport);

/* Flag bit masks...*/
#define SENT_SYNCH       (0x01)
#define ACK_RECEIVED     (0x02)
#define ACK_EXPECTED     (0x04)

#define SSP_AWAITING_ACK 0
#define SSP_ACKED        1
#define SSP_IDLE         2

/** PRIVATE DATA **/
static const uint16_t CRC_TABLE[] = {
    0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
    0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
    0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
    0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
    0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
    0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
    0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
    0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
    0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
    0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
    0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
    0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
    0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
    0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
    0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
    0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
    0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
    0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
    0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
    0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
    0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
    0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
    0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
    0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
    0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
    0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
    0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
    0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
    0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
    0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
    0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
    0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
};

/** EXTERNAL DATA **/

/** EXTERNAL FUNCTIONS **/

/** VERIFICATION FUNCTIONS **/


/***********************************************************************************************************/

/*!
 * \brief   Initializes the communication port for use
 * \param   thisport = pointer to port structure to initialize
 * \param       info = config struct with default values.
 * \return  None.
 *
 * \note
 * Must be called before calling the Send or REceive process functions.
 */

void ssp_Init(Port_t *thisport, const PortConfig_t *const info)
{
    thisport->pfCallBack    = info->pfCallBack;
    thisport->pfSerialRead  = info->pfSerialRead;
    thisport->pfSerialWrite = info->pfSerialWrite;
    thisport->pfGetTime     = info->pfGetTime;

    thisport->maxRetryCount = info->max_retry;
    thisport->timeoutLen    = info->timeoutLen;
    thisport->txBufSize     = info->txBufSize;
    thisport->rxBufSize     = info->rxBufSize;
    thisport->txBuf = info->txBuf;
    thisport->rxBuf = info->rxBuf;
    thisport->retryCount    = 0;
    thisport->sendSynch     = FALSE;                // TRUE;
    thisport->rxSeqNo = 255;
    thisport->txSeqNo = 255;
    thisport->SendState     = SSP_IDLE;
}

/*!
 * \brief   Runs the send process, checks for receipt of ack, timeouts and resends if needed.
 * \param   thisport = which port to use
 * \return  SSP_TX_WAITING - waiting for a valid ACK to arrive
 * \return  SSP_TX_TIMEOUT - failed to receive a valid ACK in the timeout period, after retrying.
 * \return  SSP_TX_IDLE    - not expecting a ACK packet (no current transmissions in progress)
 * \return  SSP_TX_ACKED   - valid ACK received before timeout period.
 *
 * \note
 *
 */
int16_t ssp_SendProcess(Port_t *thisport)
{
    int16_t value = SSP_TX_WAITING;

    if (thisport->SendState == SSP_AWAITING_ACK) {
        if (sf_CheckTimeout(thisport) == TRUE) {
            if (thisport->retryCount < thisport->maxRetryCount) {
                // Try again
                sf_SendPacket(thisport);
                sf_SetSendTimeout(thisport);
                value = SSP_TX_WAITING;
            } else {
                // Give up, # of trys has exceded the limit
                value = SSP_TX_TIMEOUT;
                CLEARBIT(thisport->flags, ACK_RECEIVED);
                thisport->SendState = SSP_IDLE;
            }
        } else {
            value = SSP_TX_WAITING;
        }
    } else if (thisport->SendState == SSP_ACKED) {
        SETBIT(thisport->flags, ACK_RECEIVED);
        value = SSP_TX_ACKED;
        thisport->SendState = SSP_IDLE;
    } else {
        thisport->SendState = SSP_IDLE;
        value = SSP_TX_IDLE;
    }
    return value;
}

/*!
 * \brief   Runs the receive process. fetches a byte at a time and runs the byte through the protocol receive state machine.
 * \param   thisport - which port to use.
 * \return  receive status.
 *
 * \note
 *
 */
int16_t ssp_ReceiveProcess(Port_t *thisport)
{
    int16_t b;
    int16_t packet_status = SSP_RX_IDLE;

    do {
        b = thisport->pfSerialRead(); // attempt to read a char from the serial buffer
        if (b != -1) {
            packet_status = sf_ReceiveState(thisport, b); // process the newly received byte in the receive state machine
        }
        // keep going until either we received a full packet or there are no more bytes to process
    } while (packet_status != SSP_RX_COMPLETE && b != -1);
    return packet_status;
}

/*!
 * \brief   processes a single byte through the receive state machine.
 * \param   thisport = which port to use
 * \return  current receive status
 *
 * \note
 *
 */

int16_t ssp_ReceiveByte(Port_t *thisport)
{
    int16_t b;
    int16_t packet_status = SSP_RX_IDLE;

    b = thisport->pfSerialRead();
    if (b != -1) {
        packet_status = sf_ReceiveState(thisport, b);
    }
    return packet_status;
}

/*!
 * \brief   Sends a data packet and blocks until timeout or ack is received.
 * \param   thisport = which port to use
 * \param       data = pointer to data to send
 * \param       length = number of data bytes to send. Must be less than 254
 * \return  true = ack was received within number of retries
 * \return      false = ack was not received.
 *
 * \note
 *
 */
uint16_t   ssp_SendDataBlock(Port_t *thisport, uint8_t *data, uint16_t length)
{
    int16_t packet_status = SSP_TX_WAITING;
    uint16_t retval = FALSE;

    packet_status = ssp_SendData(thisport, data, length); // send the data
    while (packet_status == SSP_TX_WAITING) { // check the status
        (void)ssp_ReceiveProcess(thisport); // process any bytes received.
        packet_status = ssp_SendProcess(thisport); // check the send status
    }
    if (packet_status == SSP_TX_ACKED) { // figure out what happened to the packet
        retval = TRUE;
    } else {
        retval = FALSE;
    }
    return retval;
}

/*!
 * \brief   sends a chunk of data and does not block
 * \param   thisport = which port to use
 * \param       data = pointer to data to send
 * \param       length = number of bytes to send
 * \return      SSP_TX_BUFOVERRUN = tried to send too much data
 * \return      SSP_TX_WAITING = data sent and waiting for an ack to arrive
 * \return      SSP_TX_BUSY = a packet has already been sent, but not yet acked
 *
 * \note
 *
 */
int16_t ssp_SendData(Port_t *thisport, const uint8_t *data, const uint16_t length)
{
    int16_t value = SSP_TX_WAITING;

    if ((length + 2) > thisport->txBufSize) {
        // TRYING to send too much data.
        value = SSP_TX_BUFOVERRUN;
    } else if (thisport->SendState == SSP_IDLE) {
#ifdef ACTIVE_SYNCH
        if (thisport->sendSynch == TRUE) {
            sf_SendSynchPacket(thisport);
        }
#endif

#ifdef SYNCH_SEND
        if (length == 0) {
            // TODO this method could allow a task/user to start a synchronisation step if a zero is mistakenly passed to this function.
            // could add a check for a NULL data pointer, or use some sort of static flag that can only be accessed by a static function
            // that must be called before calling this function.
            // we are attempting to send a synch packet
            thisport->txSeqNo = 0; // make this zero to cause the other end to re-synch with us
            SETBIT(thisport->flags, SENT_SYNCH);
        } else {
            // we are sending a data packet
            CLEARBIT(thisport->txSeqNo, ACK_BIT); // make sure we are not sending a ACK packet
            thisport->txSeqNo++; // update the sequence number.
            if (thisport->txSeqNo > 0x7F) { // check for sequence number rollover
                thisport->txSeqNo = 1; // if we do have rollover then reset to 1 not zero,
                                       // zero is reserviced for synchronization requests
            }
        }

#else
        CLEARBIT(thisport->txSeqNo, ACK_BIT); // make sure we are not sending a ACK packet
        thisport->txSeqNo++; // update the sequence number.
        if (thisport->txSeqNo > 0x7F) { // check for sequence number rollover
            thisport->txSeqNo = 1; // if we do have rollover then reset to 1 not zero,
                                   // zero is reserved for synchronization requests
        }
#endif // ifdef SYNCH_SEND
        CLEARBIT(thisport->flags, ACK_RECEIVED);
        thisport->SendState  = SSP_AWAITING_ACK;
        value = SSP_TX_WAITING;
        thisport->retryCount = 0; // zero out the retry counter for this transmission
        sf_MakePacket(thisport->txBuf, data, length, thisport->txSeqNo);
        sf_SendPacket(thisport); // punch out the packet to the serial port
        sf_SetSendTimeout(thisport); // do the timeout values
    } else {
        // error we are already sending a packet. Need to wait for the current packet to be acked or timeout.
        value = SSP_TX_BUSY;
    }
    return value;
}

/*!
 * \brief   Attempts to synchronize the sequence numbers with the other end of the connectin.
 * \param   thisport = which port to use
 * \return  true = success
 * \return      false = failed to receive an ACK to our synch request
 *
 * \note
 * A. send a packet with a sequence number equal to zero
 * B. if timed out then:
 *              send synch packet again
 *              increment try counter
 *              if number of tries exceed maximum try limit then exit
 * C. goto A
 */
uint16_t ssp_Synchronise(Port_t *thisport)
{
    int16_t packet_status;
    uint16_t retval = FALSE;

#ifndef USE_SENDPACKET_DATA
    thisport->txSeqNo = 0; // make this zero to cause the other end to re-synch with us
    SETBIT(thisport->flags, SENT_SYNCH);
    // TODO - should this be using ssp_SendPacketData()??
    sf_MakePacket(thisport->txBuf, NULL, 0, thisport->txSeqNo); // construct the packet
    sf_SendPacket(thisport);
    sf_SetSendTimeout(thisport);
    thisport->SendState = SSP_AWAITING_ACK;
    packet_status = SSP_TX_WAITING;
#else
    packet_status = ssp_SendData(thisport, NULL, 0);
#endif
    while (packet_status == SSP_TX_WAITING) { // we loop until we time out.
        (void)ssp_ReceiveProcess(thisport); // do the receive process
        packet_status = ssp_SendProcess(thisport); // do the send process
    }
    thisport->sendSynch = FALSE;
    switch (packet_status) {
    case SSP_TX_ACKED:
        retval = TRUE;
        break;
    case SSP_TX_BUSY: // intentional fall through.
    case SSP_TX_TIMEOUT: // intentional fall through.
    case SSP_TX_BUFOVERRUN:
        retval = FALSE;
        break;
    default:
        retval = FALSE;
        break;
    }
    ;
    return retval;
}


/*!
 * \brief   sends out a preformatted packet for a give port
 * \param   thisport = which port to use.
 * \return  none.
 *
 * \note
 * Packet should be formed through the use of sf_MakePacket before calling this function.
 */
static void sf_SendPacket(Port_t *thisport)
{
    // add 3 to packet data length for: 1 length + 2 CRC (packet overhead)
    uint8_t packetLen = thisport->txBuf[LENGTH] + 3;

    // use the raw serial write function so the SYNC byte does not get 'escaped'
    thisport->pfSerialWrite(SYNC);
    for (uint8_t x = 0; x < packetLen; x++) {
        sf_write_byte(thisport, thisport->txBuf[x]);
    }
    thisport->retryCount++;
}


/*!
 * \brief   converts data to transport layer protocol packet format.
 * \param   txbuf = buffer to use when forming the packet
 * \param       pdata = pointer to data to use
 * \param       length = number of bytes to use
 * \param       seqNo = sequence number of this packet
 * \return  none.
 *
 * \note
 *  1. This function does not try to interpret ACK or SYNCH packets.  This should
 *      be done by the caller of this function.
 *  2. This function will attempt to format all data upto the size of the tx buffer.
 *      Any extra data beyond that will be ignored.
 *  3. TODO: Should this function return an error if data length to be sent is greater th tx buffer size?
 *
 */
void sf_MakePacket(uint8_t *txBuf, const uint8_t *pdata, uint16_t length, uint8_t seqNo)
{
    uint16_t crc    = 0xffff;
    uint16_t bufPos = 0;
    uint8_t b;

    // add 1 for the seq. number
    txBuf[LENGTH] = length + 1;
    txBuf[SEQNUM] = seqNo;
    crc    = sf_crc16(crc, seqNo);

    length = length + 2; // add two for the length and seqno bytes which are added before the loop.
    for (bufPos = 2; bufPos < length; bufPos++) {
        b   = *pdata++;
        txBuf[bufPos] = b;
        crc = sf_crc16(crc, b); // update CRC value
    }
    txBuf[bufPos++] = LOWERBYTE(crc);
    txBuf[bufPos]   = UPPERBYTE(crc);
}

/*!
 * \brief   sends out an ack packet to given sequence number
 * \param   thisport = which port to use
 * \param       seqNumber = sequence number of the packet we would like to ack
 * \return  none.
 *
 * \note
 *
 */

static void sf_SendAckPacket(Port_t *thisport, uint8_t seqNumber)
{
    uint8_t AckSeqNumber = SETBIT(seqNumber, ACK_BIT);

    // create the packet, note we pass AckSequenceNumber directly
    sf_MakePacket(thisport->txBuf, NULL, 0, AckSeqNumber);
    sf_SendPacket(thisport);
    // we don't set the timeout for an ACK because we don't ACK our ACKs in this protocol
}

/*!
 * \brief   writes a byte out the output channel. Adds escape byte where needed
 * \param   thisport = which port to use
 * \param       c = byte to send
 * \return  none.
 *
 * \note
 *
 */
static void sf_write_byte(Port_t *thisport, uint8_t c)
{
    if (c == SYNC) { // check for SYNC byte
        thisport->pfSerialWrite(ESC); // since we are not starting a packet we must ESCAPE the SYNCH byte
        thisport->pfSerialWrite(ESC_SYNC); // now send the escaped synch char
    } else if (c == ESC) { // Check for ESC character
        thisport->pfSerialWrite(ESC); // if it is, we need to send it twice
        thisport->pfSerialWrite(ESC);
    } else {
        thisport->pfSerialWrite(c); // otherwise write the byte to serial port
    }
}

/************************************************************************************************************
 *
 *  NAME:          uint16_t ssp_crc16( uint16_t crc, uint16_t data )
 *  DESCRIPTION:   Uses crc_table to calculate new crc
 *  ARGUMENTS:
 *        arg1:    crc
 *        arg2:    data - byte to calculate into CRC
 *  RETURN:        New crc
 *  CREATED:    5/8/02
 *
 *************************************************************************************************************/
/*!
 * \brief   calculates the new CRC value for 'data'
 * \param   crc = current CRC value
 * \param       data = new byte
 * \return  updated CRC value
 *
 * \note
 *
 */

static uint16_t sf_crc16(uint16_t crc, uint8_t data)
{
    return (crc >> 8) ^ CRC_TABLE[(crc ^ data) & 0x00FF];
}


/*!
 * \brief   sets the timeout for the given packet
 * \param   thisport = which port to use
 * \return  none.
 *
 * \note
 *
 */

static void sf_SetSendTimeout(Port_t *thisport)
{
    uint32_t timeout;

    timeout = thisport->pfGetTime() + thisport->timeoutLen;
    thisport->timeout = timeout;
}

/*!
 * \brief   checks to see if a timeout occured
 * \param   thisport = which port to use
 * \return  true = a timeout has occurred
 * \return      false = has not timed out
 *
 * \note
 *
 */
static uint16_t   sf_CheckTimeout(Port_t *thisport)
{
    uint16_t retval = FALSE;
    uint32_t current_time;

    current_time = thisport->pfGetTime();
    if (current_time > thisport->timeout) {
        retval = TRUE;
    }
    return retval;
}


/****************************************************************************
*  NAME:   sf_ReceiveState
*  DESC:   Implements the receive state handling code for escaped and unescaped data
*  ARGS:        thisport - which port to operate on
*               c - incoming byte
*  RETURN:
*  CREATED:
*  NOTES:
*  1. change from using pointer to functions.
****************************************************************************/
/*!
 * \brief   implements the receive state handling code for escaped and unescaped data
 * \param   thisport = which port to use
 * \param       c = byte to process through the receive state machine
 * \return  receive status
 *
 * \note
 *
 */
static int16_t sf_ReceiveState(Port_t *thisport, uint8_t c)
{
    int16_t retval = SSP_RX_RECEIVING;

    switch (thisport->InputState) {
    case state_unescaped_e:
        if (c == SYNC) {
            thisport->DecodeState = decode_len1_e;
        } else if (c == ESC) {
            thisport->InputState = state_escaped_e;
        } else {
            retval = sf_DecodeState(thisport, c);
        }
        break; // end of unescaped state.
    case state_escaped_e:
        thisport->InputState = state_unescaped_e;
        if (c == SYNC) {
            thisport->DecodeState = decode_len1_e;
        } else if (c == ESC_SYNC) {
            retval = sf_DecodeState(thisport, SYNC);
        } else {
            retval = sf_DecodeState(thisport, c);
        }
        break; // end of the escaped state.
    default:
        break;
    }
    return retval;
}

/****************************************************************************
*  NAME:   sf_DecodeState
*  DESC:   Implements the receive state finite state machine
*  ARGS:        thisport - which port to operate on
*               c - incoming byte
*  RETURN:
*  CREATED:
*  NOTES:
*  1. change from using pointer to functions.
****************************************************************************/

/*!
 * \brief   implements the receiving decoding state machine
 * \param   thisport = which port to use
 * \param       c = byte to process
 * \return  receive status
 *
 * \note
 *
 */
static int16_t sf_DecodeState(Port_t *thisport, uint8_t c)
{
    int16_t retval;

    switch (thisport->DecodeState) {
    case decode_idle_e:
        // 'c' is ignored in this state as the only way to leave the idle state is
        // recognition of the SYNC byte in the sf_ReceiveState function.
        retval = SSP_RX_IDLE;
        break;
    case decode_len1_e:
        thisport->rxBuf[LENGTH] = c;
        thisport->rxBufLen = c;
        if (thisport->rxBufLen <= thisport->rxBufSize) {
            thisport->DecodeState = decode_seqNo_e;
            retval = SSP_RX_RECEIVING;
        } else {
            thisport->DecodeState = decode_idle_e;
            retval = SSP_RX_IDLE;
        }
        break;
    case decode_seqNo_e:
        thisport->rxBuf[SEQNUM] = c;
        thisport->crc = 0xffff;
        thisport->rxBufLen--; // subtract 1 for the seq. no.
        thisport->rxBufPos = 2;

        thisport->crc = sf_crc16(thisport->crc, c);
        if (thisport->rxBufLen > 0) {
            thisport->DecodeState = decode_data_e;
        } else {
            thisport->DecodeState = decode_crc1_e;
        }
        retval = SSP_RX_RECEIVING;
        break;
    case decode_data_e:
        thisport->rxBuf[(thisport->rxBufPos)++] = c;
        thisport->crc = sf_crc16(thisport->crc, c);
        if (thisport->rxBufPos == (thisport->rxBufLen + 2)) {
            thisport->DecodeState = decode_crc1_e;
        }
        retval = SSP_RX_RECEIVING;
        break;
    case decode_crc1_e:
        thisport->crc = sf_crc16(thisport->crc, c);
        thisport->DecodeState = decode_crc2_e;
        retval = SSP_RX_RECEIVING;
        break;
    case decode_crc2_e:
        thisport->DecodeState = decode_idle_e;
        // verify the CRC value for the packet
        if (sf_crc16(thisport->crc, c) == 0) {
            // TODO shouldn't the return value of sf_ReceivePacket() be checked?
            sf_ReceivePacket(thisport);
            retval = SSP_RX_COMPLETE;
        } else {
            thisport->RxError++;
            retval = SSP_RX_IDLE;
        }
        break;
    default:
        thisport->DecodeState = decode_idle_e; // unknown state so reset to idle state and wait for the next start of a packet.
        retval = SSP_RX_IDLE;
        break;
    }
    return retval;
}

/************************************************************************************************************
 *
 *  NAME:             int16_t sf_ReceivePacket( )
 *  DESCRIPTION:      Receive one packet, assumed that data is in rec.buff[]
 *   ARGUMENTS:
 *   RETURN:          0 . no new packet was received, could be ack or same packet
 *                    1 . new packet received
 *                    SSP_PACKET_?
 *                    SSP_PACKET_COMPLETE
 *                    SSP_PACKET_ACK
 *   CREATED:         5/8/02
 *
 *************************************************************************************************************/
/*!
 * \brief   receive one packet. calls the callback function if needed.
 * \param   thisport = which port to use
 * \return  true = valid data packet received.
 * \return      false = otherwise
 *
 * \note
 *
 *      Created: Oct 7, 2010 12:07:22 AM by joe
 */

static int16_t sf_ReceivePacket(Port_t *thisport)
{
    int16_t value = FALSE;

    if (ISBITSET(thisport->rxBuf[SEQNUM], ACK_BIT)) {
        // Received an ACK packet, need to check if it matches the previous sent packet
        if ((thisport->rxBuf[SEQNUM] & 0x7F) == (thisport->txSeqNo & 0x7f)) {
            // It matches the last packet sent by us
            SETBIT(thisport->txSeqNo, ACK_BIT);
            thisport->SendState = SSP_ACKED;
            value = FALSE;
        }
        // else ignore the ACK packet
    } else {
        // Received a 'data' packet, figure out what type of packet we received...
        if (thisport->rxBuf[SEQNUM] == 0) {
            // Synchronize sequence number with host
#ifdef ACTIVE_SYNCH
            thisport->sendSynch = TRUE;
#endif
            sf_SendAckPacket(thisport, thisport->rxBuf[SEQNUM]);
            thisport->rxSeqNo   = 0;
            value = FALSE;
        } else if (thisport->rxBuf[SEQNUM] == thisport->rxSeqNo) {
            // Already seen this packet, just ack it, don't act on the packet.
            sf_SendAckPacket(thisport, thisport->rxBuf[SEQNUM]);
            value = FALSE;
        } else {
            // New Packet
            thisport->rxSeqNo = thisport->rxBuf[SEQNUM];
            // Let the application do something with the data/packet.
            if (thisport->pfCallBack != NULL) {
                // skip the first two bytes (length and seq. no.) in the buffer.
                thisport->pfCallBack(&(thisport->rxBuf[2]), thisport->rxBufLen);
            }
            // after we send the ACK, it is possible for the host to send a new packet.
            // Thus the application needs to copy the data and reset the receive buffer
            // inside of thisport->pfCallBack()
            sf_SendAckPacket(thisport, thisport->rxBuf[SEQNUM]);
            value = TRUE;
        }
    }
    return value;
}