Windowed X-Engine

Block: Windowed X-Engine (win_x_engine)
Block Author: Jason Manley, Aaron Parsons, Terry Filiba
Document Author: Jason Manley

Contents

Summary
Mask Parameters
Ports
Description

Summary

CASPER X engine with added internal valid data masking functionality. Based on Aaron Parsons’ design.

Mask Parameters

Parameter	Variable	Description
Number of antennas	n_ants	Number of antennas to process.
Bit width of samples in	n_bits	Bit width of each input sample number. Usually set to 4, resulting in 16 bit input numbers (2 polarizations, complex numbers).
Accumulation length	acc_len	Specified per antenna.
Adder latency	add_latency	Used to set the latency of internal adders.
Multiplier latency	mult_latency	Used to set the latency of internal multipliers.
BRAM latency	bram_latency	Used to set the latency of internal BRAMs.
Implementation: Multiplier type	use_ded_mult	Select the type of multipliers to use. Can be a single number or array - see below.
Implementation: Delay type	use_bram_delay	Selects the type of delays to implement. Single number configures all internal taps.

Ports

Port	Dir	Data Type	Description
ant	in	variable width. see below.	Input port for incoming antenna data.
sync_in	in	boolean	Synchronization pulse. New window begins clock cycle after sync received.
window_valid	in	boolean	Indicates incoming antenna data is valid. Must remain constant for acc_len*n_ants.
acc	out	variable width. see below.	Output data.
valid	out	boolean	Indicates data on acc is valid.
sync_out	out	boolean	Passthrough for sync pulses.

Description

Introduction

The CASPER X engine is a streaming architecture block where complex antenna data is input and accumulated products (for all cross-multiplications) are output in conjugated form. Because it is streaming with valid data expected on every clock cycle, data is logically divided into windows. These windows can either be valid (in which case the computation yields valid, outputted results) or invalid (in which case computation still occurs, but the results are ignored and not presented to the user).

Input format

Data is input serially: antenna A, antenna B, antenna C etc. Each antenna’s data consists of dual polarization, complex data. The bit width of each component number can be set as a parameter, n_bits. The X-engine thus expects these four numbers of n_bits to be concatenated into a single, unsigned number. CASPER convention dictates that complex numbers are represented with higher bits as real and lower bits as imaginary. The top half of the input number is polarization one and the lower half polarization two.

The internals of the block are reset with the reception of a sync pulse. A new window begins on the very next clock cycle. Each window is int_len * n_ants clock cycles long. The data for each antenna is input for acc_len clock cycles.

For example, for n_bits of 4 and acc_len of 2, the input to the X-engine would be 16 bits every clock cycle mapped as follows:

…	t₄	t₃	t₂	t₁	t₀	$rightarrow$
…	C_1real	B_1real	B_1real	A_1real	A_1real	most_sig 4b $rightarrow$
…	C_1imag	B_1imag	B_1imag	A_1imag	A_1imag	4b $rightarrow$
…	C_2real	B_2real	B_2real	A_2real	A_2real	4b $rightarrow$
…	C_2imag	B_2imag	B_2imag	A_2imag	A_2imag	least_sig 4b $rightarrow$

X-engine input with acc_len of 2.

The window_valid line is expected to remain constant for the duration of each window. If it is high, the output is considered valid and captured into the output FIFO buffer. With the close of that window, the output will be presented to the user as valid data on every second clock pulse. If window_valid was held low, the data is ignored.

With the close of one window, anther begins directly afterwards. Data can thus be streamed in and out continuously, while a sync pulse will force the start of a new window.

Output Format

The windowed X-engine will produce $num_baselines1$ valid outputs. The unwindowed x engine produces $num_baselines2$ results. The extra valids are a result of the algorithm employed and are masked out by the internal x_engine_mask.

Generally, the output of the X-engine configured for N antennas can be mapped into a table with $tfrac$ columns and N rows as follows:

1^st	0 $times$ 0	0 $times$ N	0 $times$ (N-1)	0 $times$ (N-2)	…	$rightarrow$
2^nd	1 $times$ 1	0 $times$ 1	1 $times$ N	1 $times$ (N-1)	…	$rightarrow$
3^rd	2 $times$ 2	1 $times$ 2	0 $times$ 2	2 $times$ N	…	$rightarrow$
4^th	3 $times$ 3	2 $times$ 3	1 $times$ 3	0 $times$ 3	…	$rightarrow$
5^th	4 $times$ 4	3 $times$ 4	2 $times$ 4	1 $times$ 4	…	$rightarrow$
6^th	5 $times$ 5	4 $times$ 5	3 $times$ 5	2 $times$ 5	…	$rightarrow$
…	…	…	…	…	…	$rightarrow$

Each table entry represents a valid output. Data is read out right to left, top to bottom. Bracketed values are from previous window.

As an example, consider the output for a 4 antenna system (with antennas numbered A through D):

1st	AA	prev win DA	prev win CA
2nd	BB	AB	prev win BD
3rd	CC	BC	AC
4th	next win AA	CD	BD
5th	next win BB	next win AB	DB

Boldfaced type represents current valid window of data. Data is read out right to left, top to bottom. Non-boldfaced data is masked.

Thanks to the inclusion of the x_engine_mask block, X-engine output duplicates (observed in rows 5 and 6 above) are automatically removed. The output of a 4 antenna windowed X-engine is thus AA, AB, BB, AC, BC, CC, BD, CD, DD, DA.