FIFO mode

A variation of the TCSPC technique does not build up photon distributions but stores information about each individual photon. This is called time tag , time stamp , list , or FIFO mode. The memory is configured as a FIFO buffer. For each photon, this method stores the time in the signal period ( micro time ), the time from the start of the experiment ( macro time ), and the data word at the channel input. During the measurement, the FIFO is continuously read, and the photon data are transferred into the main memory or to the hard disc of a computer. Advanced TCSPC devices often can be configured either to build up a multidimensional photon distribution or to store the individual photons. The FIFO mode is described in Sect. 3.6, page 43. 

Spectroscopy of single molecules is based on fluorescence correlation, photoncounting histograms, or burst-integrated-lifetime techniques. Each case requires recording not only the times of the photons in the laser period, but also their absolute time. Modem time-resolved single molecule techniques therefore use almost exclusively the FIFO (time-tag) mode of TCSPC. The FIFO mode records all information about each individual photon, i.e. the time in the laser pulse sequence (micro time), the time from the start of the experiment (macro time), and the number of the detector that detected the photon (see Sect. 3.6, page 43). 

Figure 5.127 eompares the file size for eontinuous flow data and FIFO mode data. The file size in the eontinuous flow mode depends on the time per sequence step the file size in the FIFO mode on the average count rate. 

Fig. 5.127 File size versus acquisition time for the continuous flow and the FIFO mode. For the FIFO mode the amount of data depends on the average count rate, for the continuous flow mode on the time per step of the sequence and on the ADC resolution...

Setting up the FIFO is done with two parameters the first is the option to size the FIFO to n (with n < 32), the second is the operation mode of the buffer. The latter can be either stream mode , in which the last n <32 samples are stored in the FIFO while discarding older samples, trigger mode , in which only the first n <32 samples are stored after a pre-defined event occurs, and FIFO mode , in which it stops collecting new samples after the FIFO is filled up. As a default however, the FIFO is set to bypass , in which it forwards its samples without logging in the buffer. The buffer option can be combined with a lower-power mode. 

The structure in the time-tag mode is shown in Fig. 3.15. It contains the channel register, the time-measurement block, a macro time" clock, and the FIFO buffer for a large number of photons. It has some similarity to the multidimensional TCSPC described in the paragraphs above. In fact, many advanced TCSPC modules have both the photon distribution and the time-tag mode implemented, and the configuration can be changed by a software command [25]. The sequencer then turns into the macrotime clock, and the memory turns into the FIFO buffer. 

The output of the FIFO is continuously read by the eomputer. Consequently, the time-tag mode delivers a continuous and virtually unlimited stream of photon data. It is, of eourse, imperative that the computer read the photon data at a rate higher than the average photon count rate. However, modem operation systems are multitask systems, and it is unlikely that the computer reads the FIFO eontinu-ously. Moreover, in typieal applications bursts of photons appear on a baekground... 

A mueh higher burst resolution ean be obtained by reeording the photons in the FIFO or time-tag mode. The time-tag mode is deseribed under Sect. 3.6, page 43. From the time-tag data, BIFL results with a burst resolution down to the laser pulse period ean be obtained. MCS traees are available, and FCS and PCHs can be ealeulated. Beeause the full information about all photons is recorded time-tag data are extremely flexible. Conformational dynamics, rotational relaxation, and intersystem erossing ean be investigated at almost any time scale [108, 154, 155, 295, 419, 500]. However, time-tag data are also voluminous. For each photon four or six bytes are reeorded, and file sizes of a gigabyte per measurement are not unusual. 

The afterpulsing probability can be measured by illuminating the detector by a source of continuous classic light, such as an incandescent lamp or an LED, and recording the detector pulses in the FIFO (time-tag) mode of a TCSPC module. 

Collection. Another functionality that appeared with the digital interface of MEMS accelerometers is on-sensor temporal storage in the form of an internal First In, First Out (FIFO) buffer for the data samples. This results in a more relaxed interface between sensor and the micro-controller as the latter can spend longer periods in sleep-mode The ADXL345 has for instance a buffer of 32 samples, meaning that for 10 ms samples, the micro controller needs to accept three larger data packages per second from the sensor, instead of 100 smaller ones. Meanwhile, the micro controller could spend time in a lower-power mode (idle or sleep) or handle other tasks. 

FIFO sampling. Since only the ADXL345 contains this feature, a second configuration uses this sensor only, in the FIFO buffer trigger mode with the maximum size (32 samples). All further parameters are set as in the baseline setup, i.e., the sampling is set to 10 ms and after the entire FIFO is sent to the... 

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