--- Serial protocol definition for arduino-oscope --

 This is protocol definition for version 2.X. For 1.X versions, please
 read README.protocol.v1

 Arduino and display device (PC) communicate over a serial link, using a 
 simple packet-oriented protocol. The protocol is the same for both 
 directions.
 
 Packet description:
 
 
 +--------------+----------+---------------------+----------+
 | Data    Size | Command  | Payload             | Checksum |
 | (1-2 bytes)  | (1 byte) | (0 to payload size) |  1 byte  |
 +--------------+----------+---------------------+----------+
 
 * Data size
   - By data size we understand size of payload plus size of command
     field. For a packet with no payload at all, data size is 1.
     
     Data size field can be 1 byte or two bytes long. For data size
     less than 128 it is depicted as one-byte. If the most significant
     bit is set, then another byte will follow. Data size will then 
     be DATASIZE & 0x7FFF, you'll have to clear the MSbit.

     Examples:
       
       payload size 3, data size is one byte, 0x04
       payload size 512, data size is two bytes, 0x82 0x01
 
 * Command
   - Command or reply. 1 byte long. See list below for supported commands.
 
 * Payload
   - Variable size payload for this packet. Size is depicted by Payload 
     Size. If Payload Size is zero, this field is not present in packet.
     
 * Checksum
   - Packet checksum, using XOR with 0 as start value. To compute checksum
     for an outgoing packet, XOR all values of the packet except for the
     checksum itself. To validate checksum of a packet, XOR all bytes 
     received including checksum. If result is zero, then packet has no 
     errors.
   
 Packets received by arduino have a size limit, due to memory size constraints.
 Any packet received with size greater than this limit is ignored.
 
 Reset procedure
 
 Before sending out commands, you should reset the state machines by issuing
 at few number of zeroes. The minimum number of zeroes to send is 
 arduino packet size limit + 1. This will ensure proper reset of all 
 states.
 
 
 Supported commands (version 1.2):
 
  * COMMAND_PING           0x3E
    > Payload size: variable
    > Since: v1.1
    
      Request a ping from arduino. Arduino will reply with COMMAND_PONG and
      same payload.
   
  * COMMAND_GET_VERSION    0x40
    > Payload size: 0
    > Since: v1.1
    
      Request arduino oscope version. Arduino will reply with 
      COMMAND_VERSION_REPLY.
      
  * COMMAND_START_SAMPLING 0x41
    > Payload size: 0
    > Since: v1.1
    
      Request arduino to start sampling. Arduino will reply with 
      COMMAND_BUFFER_SEG once sampling is done.
      
  * COMMAND_SET_TRIGGER    0x42
    > Payload size: 1
    > Since: v1.1
    
      Set trigger level. Payload byte 0 depicts the desired trigger level.
      
  * COMMAND_SET_HOLDOFF    0x43
    > Payload size: 1
    > Since: v1.2
    
      Set holdoff value. Payload byte 0 depicts the desired holdoff samples.
    
  * COMMAND_SET_TRIGINVERT 0x44
    > Payload size: 1
    > Since: v1.2
    > Removed in v1.4. See CAPTURE_FLAGS.
     
      Set invert trigger flag. Payload byte 0 depicts if trigger is not 
      inverted (a 0 value) or if it is inverted (a 1 value).
      
  * COMMAND_SET_VREF       0x45
    > Payload size: 1
    > Since: v1.2
    
      Set VREF source for Arduino. Payload byte 0 defines VREF source value.
      Values can be 0 (AREF), 1 (AVcc) or 3 (Internal 1.1v).
      
  * COMMAND_SET_PRESCALER  0x46
    > Payload size: 1
    > Since: v1.2
     
      Set ADC prescaler value. Payload byte 0 defines the log2 of the 
      desired prescaler value (7 gives prescaler of 128, 6 prescaler of 64,
      and so on). Minimum prescaler value is 2.
      
  * COMMAND_GET_PARAMETERS 0x47
    > Payload size: 0
    > Since: v1.2
    
      Request configured parameters from arduino oscope. Arduino will reply
      with COMMAND_PARAMETERS_REPLY
      
  * COMMAND_VERSION_REPLY  0x80
    > Payload size: 2
    > Since: v1.1
    
      Arduino reply with version. Payload byte 0 depicts upper version, 
      and byte 1 lower version.
      
  * COMMAND_BUFFER_SEG     0x81
    > Payload size: NUM_SAMPLES
    > Since: v1.1
    
      Arduino reply with sampled data. Packet size may vary depending on 
      number of samples configured. This data is unsigned 8-bit (higher
      ADC sampled values).
      
  * COMMAND_PARAMETERS_REPLY 0x87
    > Payload size: 6 (v1.2), 7 (v1.4)
    > Since: v1.2
    
      Current configured values. Payload will contain the following values
      at byte offset:
      
        0 - Trigger level
        1 - Holdoff samples
        2 - ADC reference
        3 - ACD prescaler
        4,5 - Number of samples (NUM_SAMPLES). Big-endian.
        6 (v1.4) - Capture flags
        7 (v2.2) - Number of channels
        
  * COMMAND_PONG           0xE3
    > Payload size: variable
    > Since: v1.1
    
      Arduino reply to PING command. Payload is the same as ping request.
      
  * COMMAND_ERROR          0xFF
    > Payload size: 0
    > Since: v1.1
    
      Arduino reply to some unknown command.
      
  * COMMAND_SET_SAMPLES    0x48
    > Payload size: 2
    > Since: v1.3
    
      Set number of arduino samples. Arduino will reply with COMMAND_PARAMETERS_REPLY.

  * COMMAND_SET_FLAGS    0x50
    > Payload size: 1
    > Since: v1.1
    
      Set capture flags. Will reply with COMMAND_PARAMETERS_REPLY.
      Bitmap of following values:
        bit 0 - Invert trigger
        bit 1 - Set dual-channel (deprecated in v2.2 - see COMMAND_SET_CHANNELS)

  * COMMAND_SET_CHANNELS    0x51
    > Payload size: 1
    > Since: v2.2
    
      Set number of channels (1 to 4). Will reply with COMMAND_PARAMETERS_REPLY.