Trip-T Front End Board Documentation Home

Trigger Primitive Generation Functionality

Disclaimer

Please note that the implementation described here is documented here as a call for feedback from those involved in the trigger algorithm specification. At present there is no deployed version of this algorithm.

Overview

The trigger primitive generation falls into two groups: the off-axis and Ingrid.

Ingrid requirements are:
The preliminary off-axis TFB cosmic trigger functionality requirements were I'll go through the approach I've used to address each of these points, hopefully delimiting the scope and highlighting some of the advantages and disadvantages present. Additionally, I'll explain any programmable parameters associated with the module.

The following subsections occur in the same order as the data flow through the system, starting with the discriminators and finishing at the cosmic accept output sent to the CTM. Ingrid is discussed first, then the off-axis module.


Ingrid

Channel Mask

The all 64 discriminator inputs are passed through a logical AND with a configuration bit as a way of enabling the channel. Although the same configuration space registers are used as for the off-axis module, the mechanism is slightly different. Once this stage has been passed the AND array output is passed to the logical OR.

Logic OR and Channel Division.

The channels are divided up into two groups, those from TripT A and TripT B (group AB) and those from TripT C and TripT D (group CD). These channels are arranged on opposite sides of the TFB, see TFB_V2_photo_top.pdf.

A cascade of 4-input OR gates is used (the s3 uses 4 input LUTs, so this modularity is convenient).

The first stage is formed of 2 arrays of 8 of such gates, ORing the 2 groups of 32 signals into 2 groups of 8 signals.

The second stage has 2 pairs of the same type of OR gate, producing 2 outputs for each pair.

A standard 2 input or gate pair produces the final output for each pair.

Gate Generation

Each output is then fed into a separate instance of the same type of gate generator as is used at the input stage of the Off-Axis algorithm. In addition, the same register is used to set the gate length as that for the Off-Axis detector.

The outputs of these gate generators are passed through a logical AND, the output of which is the accept signal that is transferred to the CTM, see the section below on "Algorithm Selection".


Off-Axis

Gate Generation and Channel Masking

Once triggered, the TripT discriminator outputs remain latched until the preamplifier reset is asserted. It is thus necessary to use the rising edge to generate a coincidence gate for each channel. This is implemented using a variable length shift register and some glue logic to refer the input to the delayed output. See Figure 1.


Note that this module has been verified to work using the geometrical cosmic trigger used at IC. One of these modules is instantiated per TripT discriminator output, there are, thus 64 per board. The length parameter is exposed to the user as a global parameter, that is, there is one gate length specified per TFB and that is applied to all 64 inputs.

The other user accessible control present is the Enable signal. This is present on a channel by channel basis and provides a route through which an individual channel can be masked out. The masking here is shown schematically as a multiplexer connected to the output of the glue logic.

The module is clocked at 100MHz, permitting the gate duration to be varied from 0ns to 160ns.

Channel Pairing

TFBs instrumenting the SMRD will be arranged to readout the SiPMs from both ends of the scintillator bars. Requiring paired hits for channels not involved in this kind of geometrical arrangement is clearly nonsensical. A programmable mapping is thus implemented to permit the user to switch between unpaired active channel counting and two types of channel pairings.

The basic logic requires that both paired channels be active before either will count (once) to the global count of active channels. This functionality is global, i.e. once a given TFB has been set up to perform channel pairing, it will affect every channel on the board.

There are two mappings, termed (1) Opposite and (2) Adjacent. Explicitly, these pair like-numbered channels as follows:

Mapping (1): Opposite
TripT [Channel Index] TripT [Channel Index]
A[0]D[0]
A[1]D[1]
A[2]D[2]
A[3]D[3]
A[4]D[4]
A[5]D[5]
A[6]D[6]
A[7]D[7]
A[8]D[8]
A[9]D[9]
A[10]D[10]
A[11]D[11]
A[12]D[12]
A[13]D[13]
A[14]D[14]
A[15]D[15]
B[0]C[0]
B[1]C[1]
B[2]C[2]
B[3]C[3]
B[4]C[4]
B[5]C[5]
B[6]C[6]
B[7]C[7]
B[8]C[8]
B[9]C[9]
B[10]C[10]
B[11]C[11]
B[12]C[12]
B[13]C[13]
B[14]C[14]
B[15]C[15]
and:
Mapping (2): Adjacent
TripT [Channel Index] TripT [Channel Index]
A[0]B[0]
A[1]B[1]
A[2]B[2]
A[3]B[3]
A[4]B[4]
A[5]B[5]
A[6]B[6]
A[7]B[7]
A[8]B[8]
A[9]B[9]
A[10]B[10]
A[11]B[11]
A[12]B[12]
A[13]B[13]
A[14]B[14]
A[15]B[15]
C[0]D[0]
C[1]D[1]
C[2]D[2]
C[3]D[3]
C[4]D[4]
C[5]D[5]
C[6]D[6]
C[7]D[7]
C[8]D[8]
C[9]D[9]
C[10]D[10]
C[11]D[11]
C[12]D[12]
C[13]D[13]
C[14]D[14]
C[15]D[15]


The data are latched at the output of this stage.

Active Channel Summation

The output from the pairing multiplexer is fed into the input of the summation module. In order to avoid a time consuming loop over the input channels, a divide and conquer approach was used. This is implemented as a multi-stage cascade of 2-input adders, of increasing width. The 1st stage uses 32 x 2-input, single bit wide adders that output a 2-bit summation. The 64 channels are paired and assigned. In the paired trigger mode, this stage simply has half the inputs permanently set to zero. The output of this stage feeds the next, which comprises an array of 16 x 2-input, dual bit wide adders. This second array produces 8 x 3-bit summations of the input data.

The cascade continues until there is one 2-input, 6-bit wide adder that produces the 7-bit total.

A register stage has been placed between the output of stage 2 and input of stage 3, where there are 8 x 4-bit results. I don't yet know what the performance of this part of the system will be.

Note that in either of the Paired modes, the maximum output is 32, whilst in un-paired mode, it is 64.

Accept Threshold

The registered output of the summation module is compared with a 7-bit user programmable threshold. If it is found to be greater than this number, the event is considered a cosmic accept and the primitive is generated.


Algorithm Selection

Both the Off-Axis and Ingrid algorithms are present in the firmware and reside side by side. Additionally, both are active at all times. The accept signals from each are fed into a multiplexer used to select between the two. The output of this mux drives the primitive signaling engine described in the next section.

Global Trigger Primitive Signaling

A local accept is transferred to the CTM via a 100ns long pulse. The TFB provides the ability to set this pulse length in steps of 10ns with 8-bit range, i.e. [0,255] = 0,2550ns. Additionally, this signal has programmable polarity.