RED PID

Overview

Digital Servo.

• Hardware: RedPitaya
• Gateware: using Migen, Misoc, and some snippets from the RedPitaya Verilog.
• Software: basic CSR-over-SSH CLI interface.
• Test benches: see e.g. IIR transfer function.

Features

See gateware/chains.py for the full flow graph. In general there are two "fast" signal chains, roughly from each of the fast analog input to the corresponding analog output, and four "slow" chains from each of the XADC inputs to the DeltaSigma outputs:

* fast chain
  input: adc -> iir1_a (first order pipelined) -> demod -> iir2i_b (second order iterative)
         -> x: any of the previous
  process: (x or 0) + dx -> limit_a -> iir1_c -> iir2_d -> iir2i_e
         -> y: any of the previous
  output: y + dy + relock + sweep + modulate -> limit_b -> dac
  signal inputs from crossbar: dx, dy, relock_x
  signal outputs to crossbar: x, y
  state outputs to crossbar: x_saturated, x_railed, y_saturated, y_railed, unlocked
  state inputs from crossbar: x_hold, x_clear, y_hold, y_clear, y_relock
  speed: 125 MHz
  signal path width: 25 bit
  coefficient width: 18 bit (except iir2i: 36 bit)
  coefficient denominator: mostly 2 ** 16

* slow chain
  input: adc -> x
  process: (x or 0) + dx -> limit_x -> iir2i -> limit_y -> y
  output: y -> dac
  signal inputs from crossbar: dx
  signal outputs to crossbar: x, y
  state outputs to crossbar: saturated, railed
  state inputs from crossbar: hold, clear
  speed: 125 MHz * 8/120 = 8 1/3 MHz
  delta-sigma output: 250 MHz
  signal path width: 25 bit
  coefficient width: 18 bit (except iir2i: 36 bit)
  coefficient denominator: mostly 2 ** 16

* misc sources and sinks:
  signals:
      XORSHIFT pseudo random noise generator (period 2**25)
      two signal generator channels from standard RedPitaya gateware (web interface)
      two oscilloscope channels to standard RedPitaya gateware (web interface)
      force-0 signal
  states:
      8 digital inputs
      8 gitial outputs
      force-1 state

* crossbar switching matrices:
  state: any logical-or-combination of the 27 input states and one to each of the 26 outputs
  signal: any of the 16 input signals to each of the 12 output signals
  current/max/min signal value monitors: each input, with clear for each signal

• Propagation delay (single IIR) is about 150 ns, thus some 3 MHz loop bandwidth.
• All registers/states/values exposed on the CSR bus (see test/csrmap.py)

See Also

• NIST Ion Storage Digital Servo (open hardware, open software, open gateware, but Verilog) https://github.com/nist-ionstorage/digital-servo
• RedPitaya source code: http://redpitaya.com/