microbit-dal/source/ble-services/MicroBitRadio.cpp

#include "MicroBit.h"

/**
  * Provides a simple broadcast radio abstraction, built upon the raw nrf51822 RADIO module.
  *
  * The nrf51822 RADIO module supports a number of proprietary modes of operation oher than the typical BLE usage.
  * This class uses one of these modes to enable simple, point to multipoint communication directly between micro:bits.
  *
  * TODO: The protocols implemented here do not currently perform any significant form of energy management, 
  * which means that they will consume far more energy than their BLE equivalent. Later versions of the protocol
  * should look to address this through energy efficient broadcast techbiques / sleep scheduling. In particular, the GLOSSY
  * approach to efficient rebroadcast and network synchronisation would likely provide an effective future step.
  *
  * TODO: Meshing should also be considered - again a GLOSSY approach may be effective here, and highly complementary to
  * the master/slave arachitecture of BLE.
  *
  * TODO: This implementation may only operated whilst the BLE stack is disabled. The nrf51822 provides a timeslot API to allow 
  * BLE to cohabit with other protocols. Future work to allow this colocation would be benefical, and would also allow for the
  * creation of wireless BLE bridges.
  *
  * NOTE: This API does not contain any form of encryption, authentication or authorisation. Its purpose is solely for use as a
  * teaching aid to demonstrate how simple communications operates, and to provide a sandpit through which learning can take place.
  * For serious applications, BLE should be considered a substantially more secure alternative.
  */

MicroBitRadio* MicroBitRadio::instance = NULL;

extern "C" void RADIO_IRQHandler(void)
{
    // Move on to the next buffer, if possible.
    MicroBitRadio::instance->queueRxBuf();
    NRF_RADIO->PACKETPTR = (uint32_t) MicroBitRadio::instance->getRxBuf();

    // Start listening for the next packet.
    NRF_RADIO->EVENTS_END = 0;
    NRF_RADIO->TASKS_START = 1;
}

/**
  * Constructor.
  *
  * Initialise the MicroBitRadio. Note that this class is demand activated, so most resources are only committed 
  * if send/recv or event registrations calls are made.
  */
MicroBitRadio::MicroBitRadio(uint16_t id) : datagram()
{
    this->id = id;
    this->status = 0;
	this->group = 0;
	this->queueDepth = 0;
    this->rxQueue = NULL;
    this->rxBuf = NULL;

    instance = this;
}

/**
 * Change the output power level of the transmitter to the given value.
 *
 * @param power a value in the range 0..7, where 0 is the lowest power and 7 is the highest. 
 * @return MICROBIT_OK on success, or MICROBIT_INVALID_PARAMETER if the value is out of range.
 *
 */
int MicroBitRadio::setTransmitPower(int power)
{
    if (power < 0 || power >= MICROBIT_BLE_POWER_LEVELS)
        return MICROBIT_INVALID_PARAMETER;

    NRF_RADIO->TXPOWER = (uint32_t)MICROBIT_BLE_POWER_LEVEL[power];

    return MICROBIT_OK;
}

/**
 * Change the transmission and reception band of the radio to the given channel
 *
 * @param band a frequency band in the range 0 - 100. Each step is 1MHz wide, based at 2400MHz.
 * @return MICROBIT_OK on success, or MICROBIT_INVALID_PARAMETER if the value is out of range, 
 * or MICROBIT_NOT_SUPPORTED if the BLE stack is running.
 *
 */
int MicroBitRadio::setFrequencyBand(int band)
{
    if (uBit.ble)
        return MICROBIT_NOT_SUPPORTED;

    if (band < 0 || band > 100)
        return MICROBIT_INVALID_PARAMETER;

    NRF_RADIO->FREQUENCY = (uint32_t)band;

    return MICROBIT_OK;
}

/**
 * Retrieve a pointer to the currently allocated receive buffer. This is the area of memory
 * actively being used by the radio hardware to store incoming data.
 *
 * @return a pointer to the current receive buffer 
 */
PacketBuffer* MicroBitRadio::getRxBuf()
{
    return rxBuf;
}

/**
 * Attempt to queue a buffer received by the radio hardware, if sufficient space is available.
 *
 * @return MICROBIT_OK on success, or MICROBIT_NO_RESOURCES if a replacement receiver buffer
 * could not be allocated (either by policy or memory exhaustion).
 */
int MicroBitRadio::queueRxBuf()
{
    if (rxBuf == NULL)
        return MICROBIT_INVALID_PARAMETER;

    if (queueDepth >= MICROBIT_RADIO_MAXIMUM_RX_BUFFERS)
        return MICROBIT_NO_RESOURCES;

    // Ensure that a replacement buffer is available before queuing.
    PacketBuffer *newRxBuf = new PacketBuffer();

    if (newRxBuf == NULL)
        return MICROBIT_NO_RESOURCES;

    // We add to the tail of the queue to preserve causal ordering.
    rxBuf->next = NULL;

    if (rxQueue == NULL)
    {
        rxQueue = rxBuf;
    }
    else
    {
        PacketBuffer *p = rxQueue;
        while (p->next != NULL)
            p = p->next;

        p->next = rxBuf;
    }

    // Increase our received packet count
    queueDepth++;

    // Allocate a new buffer for the receiver hardware to use. the old on will be passed on to higher layer protocols/apps.
    rxBuf = newRxBuf;

    return MICROBIT_OK;
}

/**
  * Initialises the radio for use as a multipoint sender/receiver.
  * This is currently only possible if the BLE stack (Soft Device) is disabled.
  *
  * @return MICROBIT_OK on success, MICROBIT_NOT_SUPPORTED if SoftDevice is enabled.
  */
int MicroBitRadio::enable()
{
    // If the device is already initialised, then there's nothing to do.
    if (status & MICROBIT_RADIO_STATUS_INITIALISED)
        return MICROBIT_OK;

    // Only attempt to enable this radio mode if BLE is disabled.
    if (uBit.ble)
        return MICROBIT_NOT_SUPPORTED;

    // If this is the first time we've been enable, allocate out receive buffers.
    if (rxBuf == NULL)
        rxBuf = new PacketBuffer();

    if (rxBuf == NULL)
        return MICROBIT_NO_RESOURCES;

    // Enable the High Frequency clock on the processor. This is a pre-requisite for
    // the RADIO module. Without this clock, no communication is possible.
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART = 1;
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);

    // Bring up the nrf51822 RADIO module in Nordic's proprietary 1MBps packet radio mode.
    setTransmitPower(MICROBIT_RADIO_DEFAULT_TX_POWER);
    setFrequencyBand(MICROBIT_RADIO_DEFAULT_FREQUENCY);

    // Configure for 1Mbps throughput. 
    // This may sound excessive, but running a high data rates reduces the chances of collisions...
    NRF_RADIO->MODE = RADIO_MODE_MODE_Nrf_1Mbit;

    // Configure the addresses we use for this protocol. We run ANONYMOUSLY at the core.
    // A 40 bit addresses is used. The first 32 bits match the ASCII character code for "uBit". 
    // Statistically, this provides assurance to avoid other similar 2.4GHz protocols that may be in the vicinity.
    // We also map the assigned 8-bit GROUP id into the PREFIX field. This allows the RADIO hardware to perform 
    // address matching for us, and only generate an interrupt when a packet matching our group is received. 
    NRF_RADIO->BASE0 = MICROBIT_RADIO_BASE_ADDRESS;

    // Join the default group. This will configure the remaining byte in the RADIO hardware module.
    setGroup(MICROBIT_RADIO_DEFAULT_GROUP);

    // The RADIO hardware module supports the use of multiple addresses, but as we're running anonymously, we only need one.
    // Configure the RADIO module to use the default address (address 0) for both send and receive operations.
    NRF_RADIO->TXADDRESS = 0;      
    NRF_RADIO->RXADDRESSES = 1;    

    // Packet layout configuration. The nrf51822 has a highly capable and flexible RADIO module that, in addition to transmission
    // and reception of data, also contains a LENGTH field, two optional additional 1 byte fields (S0 and S1) and a CRC calculation.
    // Configure the packet format for a simple 8 bit length field and no additional fields.
    NRF_RADIO->PCNF0 = 0x00000008;
    NRF_RADIO->PCNF1 = 0x02040000 | MICROBIT_RADIO_MAX_PACKET_SIZE;

    // Most communication channels contain some form of checksum - a mathematical calculation taken based on all the data
    // in a packet, that is also sent as part of the packet. When received, this calculation can be repeated, and the results
    // from the sender and receiver compared. If they are different, then some corruption of the data ahas happened in transit,
    // and we know we can't trust it. The nrf51822 RADIO uses a CRC for this - a very effective checksum calculation.
    //
    // Enable automatic 16bit CRC generation and checking, and configure how the CRC is calculated.
    NRF_RADIO->CRCCNF = RADIO_CRCCNF_LEN_Two; 
    NRF_RADIO->CRCINIT = 0xFFFF;            
    NRF_RADIO->CRCPOLY = 0x11021;     

    // Set the start random value of the data whitening algorithm. This can be any non zero number.
    NRF_RADIO->DATAWHITEIV = 0x18;     

    // Set up the RADIO module to read and write from our internal buffer. 
    NRF_RADIO->PACKETPTR = (uint32_t)rxBuf;  

    // Configure the hardware to issue an interrupt whenever a task is complete (e.g. send/receive).
    NRF_RADIO->INTENSET = 0x00000008;
    NVIC_ClearPendingIRQ(RADIO_IRQn);
    NVIC_EnableIRQ(RADIO_IRQn);
     
    // Start listening for the next packet
    NRF_RADIO->EVENTS_READY = 0;
    NRF_RADIO->TASKS_RXEN = 1;
    while(NRF_RADIO->EVENTS_READY == 0);

    NRF_RADIO->EVENTS_END = 0;
    NRF_RADIO->TASKS_START = 1;

    // register ourselves for a callback event, in order to empty the receive queue.
    uBit.addIdleComponent(this);

    // Done. Record that our RADIO is configured.
    status |= MICROBIT_RADIO_STATUS_INITIALISED;

    return MICROBIT_OK;
}

/**
  * Disables the radio for use as a multipoint sender/receiver.
  * @return MICROBIT_OK on success, MICROBIT_NOT_SUPPORTED if SoftDevice is enabled.
  */
int MicroBitRadio::disable()
{
    // Only attempt to enable.disable the radio if the protocol is alreayd running.
    if (uBit.ble)
        return MICROBIT_NOT_SUPPORTED;
   
    if (!(status & MICROBIT_RADIO_STATUS_INITIALISED))
        return MICROBIT_OK;

    // Disable interrupts and STOP any ongoing packet reception.
    NVIC_DisableIRQ(RADIO_IRQn);

    NRF_RADIO->EVENTS_DISABLED = 0;
    NRF_RADIO->TASKS_DISABLE = 1;
    while(NRF_RADIO->EVENTS_DISABLED == 0);

    // deregister ourselves from the callback event used to empty the receive queue.
    uBit.removeIdleComponent(this);

    return MICROBIT_OK;
}

/**
  * Sets the radio to listen to packets sent with the given group id.
  *
  * @param group The group to join. A micro:bit can only listen to one group ID at any time.
  * @return MICROBIT_OK on success, or MICROBIT_NOT_SUPPORTED if the BLE stack is running.
  */
int MicroBitRadio::setGroup(uint8_t group)
{
    if (uBit.ble)
        return MICROBIT_NOT_SUPPORTED;

    // Record our group id locally
    this->group = group;

    // Also append it to the address of this device, to allow the RADIO module to filter for us.
    NRF_RADIO->PREFIX0 = (uint32_t)group;

    return MICROBIT_OK;
}

/**
  * A background, low priority callback that is triggered whenever the processor is idle.
  * Here, we empty our queue of received packets, and pass them onto higher level protocol handlers.
  *
  * We provide optimised handling of well known, simple protocols and events on the MicroBitMessageBus 
  * to provide extensibility to other protocols that may be written in the future.
  */
void MicroBitRadio::idleTick()
{
    // Walk the list of packets and process each one.
    while(rxQueue)
    {
        PacketBuffer *p = rxQueue;

        switch (p->protocol)
        {
            case MICROBIT_RADIO_PROTOCOL_DATAGRAM:
                datagram.packetReceived();
                break;

            case MICROBIT_RADIO_PROTOCOL_EVENTBUS:
                event.packetReceived();
                break;

            default: 
                MicroBitEvent(MICROBIT_ID_RADIO_DATA_READY, p->protocol);
        }

        // If the packet was processed, it will have been recv'd, and taken from the queue. 
        // If this was a packet for an unknown protocol, it will still be there, so simply free it.
        if (p == rxQueue)
        {
            recv();
            delete p;
        }
    }
}

/**
  * Determines the number of packets ready to be processed.
  * @return The number of packets in the receive buffer.
  */
int MicroBitRadio::dataReady()
{
    return queueDepth;
}

/**
 * Retrieves the next packet from the receive buffer.
 * If a data packet is available, then it will be returned immediately to
 * the caller. This call will also dequeue the buffer. 
 *
 * NOTE: Once recv() has been called, it is the callers resposibility to 
 * delete the buffer when appropriate.
 *
 * @return The buffer containing the the packet. If no data is available, NULL is returned.
 */
PacketBuffer* MicroBitRadio::recv()
{
    PacketBuffer *p = rxQueue;

    if (p)
    {
        rxQueue = rxQueue->next;
        queueDepth--;
    }

    return p;
}

/**
 * Transmits the given buffer onto the broadcast radio.
 * The call will wait until the transmission of the packet has completed before returning.
 *
 * @param data The packet contents to transmit.
 * @return MICROBIT_OK on success, or MICROBIT_NOT_SUPPORTED if the BLE stack is running. 
 */
int MicroBitRadio::send(PacketBuffer *buffer)
{
    if (uBit.ble)
        return MICROBIT_NOT_SUPPORTED;

    if (buffer == NULL)
        return MICROBIT_INVALID_PARAMETER;

    if (buffer->length > MICROBIT_RADIO_MAX_PACKET_SIZE + MICROBIT_RADIO_HEADER_SIZE - 1)
        return MICROBIT_INVALID_PARAMETER;

    // Firstly, disable the Radio interrupt. We want to wait until the trasmission completes.
    NVIC_DisableIRQ(RADIO_IRQn);

    // Turn off the transceiver.
    NRF_RADIO->EVENTS_DISABLED = 0;
    NRF_RADIO->TASKS_DISABLE = 1;
    while(NRF_RADIO->EVENTS_DISABLED == 0);

    // Configure the radio to send the buffer provided.
    NRF_RADIO->PACKETPTR = (uint32_t) buffer;

    // Turn on the transmitter, and wait for it to signal that it's ready to use.
    NRF_RADIO->EVENTS_READY = 0;
    NRF_RADIO->TASKS_TXEN = 1;
    while (NRF_RADIO->EVENTS_READY == 0);

    // Start transmission and wait for end of packet.
    NRF_RADIO->TASKS_START = 1;
    NRF_RADIO->EVENTS_END = 0;
    while(NRF_RADIO->EVENTS_END == 0);

    // Return the radio to using the default receive buffer
    NRF_RADIO->PACKETPTR = (uint32_t) rxBuf;

    // Turn off the transmitter.
    NRF_RADIO->EVENTS_DISABLED = 0;
    NRF_RADIO->TASKS_DISABLE = 1;
    while(NRF_RADIO->EVENTS_DISABLED == 0);

    // Start listening for the next packet
    NRF_RADIO->EVENTS_READY = 0;
    NRF_RADIO->TASKS_RXEN = 1;
    while(NRF_RADIO->EVENTS_READY == 0);

    NRF_RADIO->EVENTS_END = 0;
    NRF_RADIO->TASKS_START = 1;

    // Re-enable the Radio interrupt.
    NVIC_ClearPendingIRQ(RADIO_IRQn);
    NVIC_EnableIRQ(RADIO_IRQn);

    return MICROBIT_OK;
}