Settings

Each FPGA has 32-bit registers accessible by the TinySh shell running on their PowerPC processesors through serial port connections. These registers allow run-time control over the spectrometer without requiring hardware recompiles. Each register is numbered and can be set using the Octave serial script with the command

Available Registers

• DDC
  • 0: Input selection
• PFB
  • 0: Input selection
  • 1: FFT downshift control
• Corner Turner
  • 0: Input selection
• FFT
  • 0: Input selection
  • 1: FFT downshift control
• Thresholder
  • 0: Input selection
  • 1: Threshold scaler
  • 2: FIFO reset
  • 3: Event limit

Input Selection

For debugging purposes, each module can generate its own sync pulses and test vectors. The input selection register on each FPGA controls a multiplexor that switches between internal and external sources for sync and data. Only the 3 lowest bits of the register are used, with selector[1:0] choosing between the 4 sync options (numbered 0 to 3), and selector[2] choosing between the 2 data options (numbered 0 and 1). The register value to be supplied is thus

Sync options
0	External sync
1	Internal sync, periodic
2	Internal sync, toggled one-shot
3	No sync

Data options
0	External data
1	Internal test data

FFT Downshifts

The FFT blocks in the PFB and FFT modules have dynamically selectable downshifts per stage. Each bit of the 32-bit register controls whether or not a stage in the FFT downshifts, with the LSB corresponding to the first stage. For example, shifting every other stage could be done one of two ways:

register_value = x55555555 = 0b01010101010101010101010101010101 (shift 1st, 3rd, 5th,... 31st stage)
register_value = xAAAAAAAA = 0b10101010101010101010101010101010 (shift 2nd, 4th, 6th,... 32nd stage)

Thresholder Settings

Threshold scaler
The thresholder takes an 18.9 unsigned number that it uses to scale the baseline average to compute a threshold. When manually writing to the register through the serial port, the desired scaler should be multiplied by 2^9 to account for the binary representation. For example, the system defaults to a scaler of 0.09375, so manually setting that register would be the command:

The shell also provides a function setscaler <scaler> to set this parameter.

FIFO reset
The reporting FIFOs can be manually reset and emptied on the next sync pulse by setting the LSB of the register to 1'b1. The FIFOs will continue to be reset by each sync pulse until the LSB of that register is reset to 1'b0.

Event limit
The thresholder hardware can limit the number of above-threshold hits reported in each PFB bin according to the value in this register. The limit can be in the range [1,32768], but limits greater than ~100 may require more bandwidth than is available, and cause data to be dropped.

The shell also provides a function seteventlimit <event_limit> to set this parameter.