Attention
This documentation is a work in progress. Expect to see errors and unfinished things.
ph_acc
Description
Phase accumulator, to act as basis for DDS (direct digital synthesizer).
Tuned to allow 32-bit control, divided 20-bit high and 12-bit low,
which gets merged to 32-bit binary when modulo is zero.
But also supports non-binary frequencies: see the modulo input port.
Note that phase_step_h and phase_step_l combined fit in a 32-bit word.
This is intentional, to allow atomic updates of the two controls
in 32-bit systems. Indeed, when modulo==0, those 32 bits can be considered
a single phase step increment
The phase generation algorithm
0. The phase increments for dds are generated using a technique described
in these 2 places:
Section: PROGRAMMABLE MODULUS MODE in:
Basically, increment the phase step at a coarse resolution, accumulate the
error on the side, and when that error accumulates to the lowest bit of
the coarse counter, add an extra 1 to the following phase step.
1. phase_step_h is the coarse (20 bit) integer truncated phase increment for
the cordic. There is a 1-bit increment of phase that comes from
accumulating residue phase.
2. This residue phase is accumulating in steps of phase_step_l, in a 12-bit
counter.
3. However, there will be an extra residue even for this 12-bit counter,
which is the modulus, and this added as an offset when the counter crosses 0
12-bit modulo supports largest known periodicity in a suggested LLRF system,
1427 for JLab. For more normal periodicities, use a multiple to get finer
granularity.
Note that the downloaded modulo control is the 2’s complement of the
mathematical modulus.
e.g., SSRF IF/F_s ratio 8/11, use
modulo = 4096 - 372*11 = 4
phase_step_h = 2^20*8/11 = 762600
phase_step_l = (2^20*8%11)*372 = 2976
e.g., Argonne RIA test IF/F_s ratio 9/13, use
modulo = 4096 - 315*13 = 1
phase_step_h = 2^20*9/13 = 725937
phase_step_l = (2^20*9%13)*315 = 945
Pinout
Ports
Signal |
Direction |
Description |
|---|---|---|
clk |
Input |
Rising edge clock input; all logic is synchronous in this domain |
reset |
Input |
Active high, synchronous with clk |
en |
Input |
Enable |
phase_acc[18:0] |
Output |
Output phase word |
phase_step_h[19:0] |
Input |
High order (coarse, binary) phase step |
phase_step_l[11:0] |
Input |
Low order (fine, possibly non-binary) phase step |
modulo[11:0] |
Input |
Encoding of non-binary modulus; 0 means binary |
Implementation and use
The portable Verilog implementation can be found in ph_acc Source File