Diamond Support for Picoharp Fill Pattern Measurement
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README.rst

EPICS Driver for PicoHarp 300

This EPICS driver supports the use of the PicoQuant PicoHarp 300 for fill pattern measurement.

Dependencies

To build this support module edit configure/RELEASE for the following dependencies:

EPICS_BASE
This module has been developed with EPICS version 3.14.12.3, but has no particular version dependencies.
ASYN
Similary Asyn version 4.21 was used, but the version is unlikely to matter too much.
PHLIB
The PicoHarp interface library can be downloaded from the PicoQuant web site at PicoQuant [1] and the library used for development was downloaded from PHLib [2]. Some further configuration will be needed, see PicoHarp Library below.
EPICSDBBUILDER
This tool is used to build the database templates and can be downloaded at EPICSdbbuilder [3]. This symbol should be set to the directory containing the epicsdbbuilder module, this does not need to be installed separately.

PicoHarp Library

You will need to ensure that the PicoHarp 300 instrument is configured for remote USB operation, this is a separately purchased option. You will also need to configure your target system so that when the PicoHarp is connected its USB device nodes are user writeable: the corresponding USB id is 0e0d:0003.

The makefile for this module expects to find the three header files errorcodes.h, phdefin.h, and phlib.h in the directory $(PHLIB)/include and expects to find phlib.so with the name libphlib.so in $(PHLIB)/lib. I suggest that this library be installed manually in a suitable directory, and that you don't attempt to run the install script provided with the library.

Creating an IOC

The docs/example_ioc directory contains a simple IOC which implements support for a single PicoHarp. Creating the IOC is automated through the IOC_Builder [4] tool, but unfortunately this has too many Diamond specific dependencies for practical use elsewhere.

To configure a PicoHarp we must first perform some calculations. The two basic parameters are the bunch number (called buckets in the code) and the machine revolution frequency. At Diamond we have:

buckets &= 936 \\
f_{rev} &= 533820 \,\text{Hz}

From these numbers we need to compute the following three numbers to configure the driver:

bin_size

Control parameter (in range 0 to 7), determines PicoHarp sample size in nanseconds via equation:

sample\_time = 4 \times 2^{bin\_size} \,\text{ps}

Compute this as the smallest number in the range 0 to 7 such that valid\_samples < 65536.

valid_samples

Number of valid samples, must be no more than 65536, determined from bin_size as:

\frac{10^{12}}{f_{rev} \times sample\_time}
samples_per_bucket

Lower bound on valid samples per machine bunch, computed as

\lfloor{valid\_samples / buckets}\rfloor

From these values the startup script and substitution file can now be computed.

Startup Script

The following calls must be made before iocInit:

scanPicoDevices
This discovers all connected PicoHarp devices and will print out the serial string associated with each device. This must be called once before initialising any PicoHarp instance.
initPicoAsyn

This takes the following 9 arguments, which we have just determined:

asyn_port This is an arbitrary string used internally to identify PicoHarp instance: a single IOC can operate multiple PicoHarps if ports are assigned.
serial_string The serial code printed out by scanPicoDevices for the PicoHarp of interest should be passed here as a string.
event_fast, event_5s The driver uses two EPICS event codes internally, they must be unique to each PicoHarp instance. Event codes 100 and 101 seem to work ok.
buckets Number of bunches in the ring.
bin_size Number in range 0 to 7 corresponding to selected bin size, computed above
valid_samples Number of samples used for fill pattern measurement, computed above
samples_per_bucket Samples per bucket as computed above.
f_rev Machine revolution frequency in Hz.

Database Substitutions

One substitution instance for db/picoharp.db must be made for each PicoHarp controlled by this IOC. Note that the array sizes in the substitutions must be correct as otherwise a segmentation fault is quite likely to occur!

picoharp.db

Instantiate with the following parameters:

DEVICE Prefix used to name the PicoHarp instance.
PORT This must be identical to the asyn_port parameter passed to initPicoAsyn.
CURRENT This should name an EPICS PV which provides an up to date reading of the machine beam current in mA. This will be used to scale the computed fill pattern.
EVENT_FAST The event_fast value passed to initPicoAsyn
EVENT_5S The event_5s value passed to initPicoAsyn
BUCKETS Must be buckets, ie number of bunches per turn.
BUCKETS_1 Must be buckets-1.
PROFILE Must be samples_per_bucket.

EPICS Interface

The directory opi/sr contains three EDM screens which can be used to control and view the fill pattern computed by the PicoHarp. The top level screen is fill.edl, with fillconfig.edl used to edit configuration settings and fillwf.edl to provide a view on the raw PicoHarp data.

The PVs provided by the PicoHarp are documented below. All of the PVs have names of the form device:pv where device is the DEVICE name configured in the database instance. In the documentation below the device: part is omitted from each name.

Status PVs

MAX_BIN
Returns the maximum number of samples in any one sampling bin observed by the PicoHarp in the most recent data acquisition.
COUNT_RATE_0
This should be equal to f_{rev}, if not the triggering configuration for the PicoHarp is not set correctly.
COUNT_RATE_1
Ideally this needs to be close to COUNT_RATE_0 -- this counts the data rate of PicoHarp samples.
RESOLUTION
Should be equal to 4\times2^{bin\_size}.
RESET_TIME
This records the exact time when RESET_ACCUM was last processed.
ERROR
Returns error string associated with last PicoHarp interface action.

Configuration PVs

The following PVs are used to control the PicoHarp configuration.

TIME
Controls the acquisition rate in ms, should be set to a fraction of 5 seconds. The FAST periodic PVs will update at intervals determined by this setting.
SHIFT
This is a circular shift in bins applied to the computed fill pattern used to correct for trigger timing offsets.
SAMPLE_WIDTH
Determines how many bins around the centre of the computed PROFILE_rate will be used to compute the number of samples in each bunch.
OFFSET
This value is passed to the PH_SetOffset function of the PicoHarp library, but I have been unable to determine what it does. Leave this setting at zero.
CFDZEROX0, CFDZEROX1, CFDLEVEL0, CFDLEVEL1
These control the zero crossing and input detect levels for the two input triggers for the PicoHarp. Input 0 should be connected to the machine revolution clock.
SYNCDIV
This value is passed to the PH_SetSyncDiv function of the PicoHarp library to control synchronisation to an external source. Should probably be left at its default setting of 1.
RANGE

This needs to be left at the bin_size parameter determined above. This can be changed in order to capture raw data with differing PicoHarp bin sizes, but the computed fill pattern will be wrong.

The picoharp bin size is equal to 4\times2^{RANGE} ps.

DEADTIME
This should be set to the PicoHarp's dead time, measured in machine buckets, approximately 42 at 500 MHz. This is used by the fill pattern pileup correction algorithm.
RESET_ACCUM
Processing this PV will reset the history accumulated into the ALL PVs.

Periodic PVs

All of the PVs listed here have names of the form device:pv_rate where rate is one of FAST, 5, 60, 180, or ALL. The PVs with a named rate represent a rolling history as follows:

FAST PVs with this suffix update at the rate determined by the TIME PV. This should be set to an integral fraction of 5 seconds.
5, 60, 180 These PVs update at 5 second intervals and deliver data integrated over the corresponding number of seconds.
ALL These PVs also update at 5 seconds, delivering data integrated since the PV RESET_ACCUM was last processed.
BUCKETS_rate
This is the computed fill pattern, after pileup correction and fill rotation.
SAMPLES_rate
This is the raw histogram captured from the PicoHarp, scaled to counts per turn, and accumulated over the corresponding period.
RAW_BUCKETS_rate
This is the corresponding computed fill pattern after fill shift correction but before fill pattern pileup correction.
FIXUP_rate
This is the computed pileup correction factor for each bucket.
MAX_FIXUP_rate
This is the maximum pileup correction factor. If this number is much larger than 1 then the data capture rate is too high.
SOCS_rate
Sum Of Charges Squared, in nC^2.
TURNS_rate
Total number of turns captured in this sample. The calculation of this value is rather approximate.
PROFILE_rate

This waveform shows the pattern of PicoHarp bins within a single bunch, averaged over the entire fill. The peak of the bunch should be centred, as bucket measurement will only occur within this region.

If possible fine tuning of the PicoHarp trigger should be done to keep the centre of this profile away from the edges.

FLUX_rate
Counts per turn: number of captured events per machine revolution. If this number is too much larger than 1 then pileup will distort the captured data beyond correction abilities of FIXUP_rate.
TOTAL_COUNT_rate
Total number of captured events in the sample period.
PEAK_rate
Offset of peak in PROFILE_rate, used for computing fill pattern from raw sample data.

References

[1]http://www.picoquant.com/products/category/tcspc-and-time-tagging-modules/picoharp-300-stand-alone-tcspc-module-with-usb-interface
[2]http://www.picoquant.com/dl_software/PicoHarp300/PicoHarp300_SW_and_DLL_v3_0_0_1.zip
[3]http://controls.diamond.ac.uk/downloads/python/epicsdbbuilder/
[4]http://controls.diamond.ac.uk/downloads/python/iocbuilder/