Output Data Format

Data read from xtdc4_read() is provided as packets. The data layout of a packet is crono_packet.

The following code snippets shows the principal of how to unpack a crono_packet.

A complete coding example can be found on github.com/cronologic-de/xtdc_babel or in Section Code Example.

uint32_t binsize_ps = param_info->packet_binsize;

int64_t absolute_time_bins = packet->timestamp;
double absolute_time_ps = absolute_time_bins * binsize_ps;

// 2 TDC hits are in each packet->data word.
int hit_count = 2 * packet->length;
// If an odd number of hits were recorded, the high 32 bits of the last data word
// do not contain valid data
if ((packet->flags & XTDC4_PACKET_FLAG_ODD_HITS) != 0)
    hit_count -= 1;

uint32_t *packet_data = (uint32_t*)(packet->data);

// keep track of possible timestamp rollovers
uint32_t rollover_counter = 0;
uint64_t rollover_period_bins = static_info->rollover_period;

for(int i=0; i<hit_count; ++i)
{
    uint32_t channel = packet_data[i]      & 0xF;
    uint32_t flags   = packet_data[i] >> 4 & 0xF;

    if ((flags & XTDC4_HIT_FLAG_TIME_OVERFLOW) != 0)
    {
        rollover_counter += 1;
    }
    else
    {
        uint32_t timestamp_bins = packet_data[i] >> 8 & 0xFFFFFF;
        double timestamp_ps = binsize * (
            timestamp_bins + rollover_counter * rollover_period_bins);
        double absolute_timestamp = absolute_time_ps + timestamp_ps;
    }
}

crono_packet

struct crono_packet

uint8_t channel

Always 0 for the xTDC4.

The channel source of a hit are encoded in data.

uint8_t card

Identifies the source card of the data, in case multiple xTDC4 are installed in the system.

Defaults to 0 if no value is assigned to xtdc4_init_parameters.board_id.

uint8_t type

The type of crono_packet.data.

In case of the xTDC4, this is always an 32-bit unsigned integer, corresponding to CRONO_PACKET_TYPE_32_BIT_UNSIGNED.

uint8_t flags

Bit field of packet flags.

Each bit corresponds to one of the following:

XTDC4_PACKET_FLAG_ODD_HITS

0x1.

If this bit is set, the last word in the data array consists of one timestamp only, which means that only the lower 32 bits (little endian) of the 64-bit data word are valid.

XTDC4_PACKET_FLAG_SLOW_SYNC

0x2.

If this bit is set, the timestamp of a hit is above the range of the 8-bit rollover number and the 24-bit hit timestamp. The group is closed and all other hits are ignored.

XTDC4_PACKET_FLAG_START_MISSED

0x4.

If this bit is set, packets were discarded due to a full FIFO potentially caused by a data hit rate that is too high.

Starts were missed and stops are potentially in wrong groups.

XTDC4_PACKET_FLAG_SHORTENED

0x8.

If this bit is set, packets were shortened due to a full pipeline of the FIFO potentially caused by a data hit rate that is too high.

Stops are missing in the current packet.

XTDC4_PACKET_FLAG_DMA_FIFO_FULL

0x10.

If this bit is set, the internal FIFO was full, potentially caused by a hit rate that is too high.

XTDC4_PACKET_FLAG_HOST_BUFFER_FULL

0x20.

If this bit is set, the host buffer was full. This might result in dropped packets.

This is caused either by a hit rate that is too high or by data not being retrieved and acknowledged fast enough from the buffer.

Solutions are increasing the buffer size (see xtdc4_init_parameters.buffer_size) or avoiding or optimizing any additional processing in the application reading the data.

uint32_t length

Length of data in multiples of 64 bit.

Each 64-bit data word contains up to two TDC hits.

The number of TDC hits can be calculated as

2 \(\times\) length – ((flags & XTDC4_PACKET_FLAG_ODD_HITS) ? 1 : 0)

uint64_t timestamp

Coarse timestamp of the Start trigger hit.

Values are given in multiples xtdc4_param_info.packet_binsize.

uint64_t data[1]

Contains the TDC hits as variable length array.

The length is given by length and can be zero.

Note

To directly operate on the TDC hit, cast this array to uint32_t.

The data layout of a 32-bit TDC hit is as follows:

Bits	31 – 8	7 – 4	3 – 0
Content	24-bit timestamp	Flags	Channel

That is, one can extract the timestamp, flags, and channel number as follows:

uint32_t timestamp = (hit >> 8) & 0xFFFFFF;
uint32_t flags     = (hit >> 4) & 0xF;
uint32_t channel   =  hit       & 0xF;

The 24-bit timestamp is given in multiples of xtdc4_param_info.packet_binsize.

Channel value 0–3 correspond to Stop channels A–D.

The flag bits correspond to the following:

XTDC4_HIT_FLAG_FPGA_MISSING

0x8, corresponding to bit 7 of the data word.

See also XTDC4_HIT_FLAG_COARSE_TIMESTAMP.

XTDC4_HIT_FLAG_COARSE_TIMESTAMP

0x4, corresponding to bit 6 of the data word.

Bits 6 and 7 encode the measurement resolution according to the following table:

bit 7	bit 6	Measurement Type
0	0	Normal, full resolution measurement.
0	1	Measurement performed with the Delay-Line TDC at about 150 ps resolution.
1	0	Full resolution measurement that be might in the wrong place in the data stream.
1	1	Measurement with only 5000 ps / 6 ≈ 833.3 ps resolution.

XTDC4_HIT_FLAG_TIME_OVERFLOW

0x2.

If this bit is set, the data word indicates a rollover and does not encode a TDC hit.

The time since the Start trigger hit exceeded the 24-bit range that can be encoded in one 32-bit TDC hit.

To correctly determine the timestamp of the following TDC hits, the application needs to increment a rollover counter if the XTDC4_HIT_FLAG_TIME_OVERFLOW bit is set:

int rollover_counter = 0;
if ((flags & XTDC4_HIT_FLAG_TIME_OVERFLOW) != 0)
{
    rollover_counter += 1;
}
timestamp += rollover_counter * rollover_period

rollover_period is given by xtdc4_static_info.rollover_period.

Attention

The rollover counter needs to be reset for each crono_packet.

XTDC4_HIT_FLAG_RISING

0x1.

If this bit is set, this hit corresponds to a signal’s rising edge, otherwise to a falling edge.