Module common

This document contains technical documentation for the common module.

addr_pkg.vhd

Collection of types/functions for working with address decode/matching.

attribute_pkg.vhd

Commonly used attributes for Xilinx Vivado, with description and their valid values. Information gathered from documents UG901 and UG912.

axi_stream_protocol_checker.vhd

View source code on gitlab.com.

$component axi_stream_protocol_checker is generic ( -- Assign a non-zero value in order to use the 'data'/'strobe' ports for protocol checking data_width : natural; -- Assign a non-zero value in order to use the 'id' port for protocol checking id_width : natural; logger_name_suffix : string; -- This can be used to essentially disable the -- "rule 4: Check failed for performance - tready active N clock cycles after tvalid." -- warning by setting a very high value for the limit. -- This warning is considered noise in most testbenches that exercise backpressure. -- Set to a lower value in order the enable the warning. rule_4_performance_check_max_waits : natural ); port ( clk : in std_ulogic; --# {{}} ready : in std_ulogic; valid : in std_ulogic; -- Optional to connect. last : in std_ulogic; -- Optional to connect. -- Must set a valid 'id_width' generic value in order to use these. id : in std_ulogic_vector; -- Optional to connect. -- Must set a valid 'data_width' generic value in order to use these. data : in std_ulogic_vector; strobe : in std_ulogic_vector ); end component;$

A wrapper around the VUnit AXI-Stream protocol checker. Has simpler interface, and can hence be included in synthesizable code with a generate guard:

if in_simulation generate

  axi_stream_protocol_checker_inst : common.axi_stream_protocol_checker
    generic map (
      ...
    );

end generate;

Without the generate guard, synthesis will fail. The file is placed in the “sim” folder, so it will not be included in synthesis projects by default when using tsfpga.

clean_packet_dropper.vhd

View source code on gitlab.com.

$component clean_packet_dropper is generic ( data_width : positive; fifo_depth : positive ); port ( clk : in std_ulogic; --# {{}} drop : in std_ulogic; --# {{}} input_ready : out std_ulogic; input_valid : in std_ulogic; input_last : in std_ulogic; input_data : in std_ulogic_vector; input_strobe : in std_ulogic_vector; --# {{}} result_ready : in std_ulogic; result_valid : out std_ulogic; result_last : out std_ulogic; result_data : out std_ulogic_vector; result_strobe : out std_ulogic_vector ); end component;$

An incoming packet will be dropped cleanly if drop is asserted for at least one clock cycle during the active packet.

Once drop has been asserted during an active packet, this entity will

Not pass anything of the current input packet on to the result side, including anything that was consumed before drop was asserted.

This means that only whole, non-corrupted, packets will be available on the result side.
Keep input_ready high until the whole packet has been consumed, so the upstream on the input side is not stalled.

Note

The fifo.vhd instance in this module is in packet mode, meaning that a whole packet has to be written to FIFO before any data is passed on to result side. Hence the fifo_depth generic has to be chosen so that it can hold the maximum possible packet length from the input side.

clock_counter.vhd

View source code on gitlab.com.

$component clock_counter is generic ( resolution_bits : positive; max_relation_bits : positive; -- The shift register length is device specific. -- For Xilinx Ultrascale and 7 series devices, it should be set to 33 shift_register_length : positive ); port ( target_clock : in std_ulogic; --# {{}} reference_clock : in std_ulogic; target_tick_count : out u_unsigned ); end component;$

Measure the switching rate of an unknown clock by using a free-running reference clock of known frequency.

Note

This entity instantiates a resync_counter.vhd block. See documentation of that entity for constraining details.

The frequency of target_clock is given by

target_tick_count * reference_clock_frequency / 2 ** resolution_bits

The target_tick_count value is updated every 2 ** resolution_bits cycles. It is invalid for 2 * 2 ** resolution_bits cycles in the beginning as reference_clock starts switching, but after that it is always valid.

For the calculation to work, target_clock must be no more than 2 ** (max_relation_bits - 1) times faster than reference_clock.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for clock_counter.vhd netlist builds.
Generics	Total LUTs	SRLs	FFs
resolution_bits = 24 max_relation_bits = 6	84	5	185
resolution_bits = 10 max_relation_bits = 4	38	2	86

common_pkg.vhd

View source code on gitlab.com.

Package with common features that do not fit in anywhere else, and are not significant enough to warrant their own package.

debounce.vhd

View source code on gitlab.com.

$component debounce is generic ( -- Number of cycles the input must be stable for the value to propagate to the result side. stable_count : positive; -- Initial value for the stable result that will be set until the first real input -- value has propagated. default_value : std_ulogic ); port ( -- Input value that may be metastable and/or noisy noisy_input : in std_ulogic; --# {{}} clk : in std_ulogic; stable_result : out std_ulogic; -- Asserted for one clock cycle when stabilized value goes from '0' to '1'. stable_rising_edge : out std_ulogic; -- Asserted for one clock cycle when stabilized value goes from '1' to '0'. stable_falling_edge : out std_ulogic ); end component;$

Simple debounce mechanism to be used with e.g. the signal from a button or dip switch. It eliminates noise and metastability by requiring the input to have a stable value for a specified number of clock cycles before propagating the value.

Note

This entity instantiates a resync_level.vhd block (async_reg chain) to make sure the input is not metastable. The resync_level.vhd has a scoped constraint file that must be used.

handshake_mux.vhd

View source code on gitlab.com.

$component handshake_mux is generic ( num_inputs : positive; data_width : positive ); port ( clk : in std_ulogic; --# {{}} input_ready : out std_ulogic_vector; input_valid : in std_ulogic_vector; input_last : in std_ulogic_vector; input_data : in slv_vec_t; input_strobe : in slv_vec_t; --# {{}} result_ready : in std_ulogic; result_valid : out std_ulogic; result_last : out std_ulogic; result_data : out std_ulogic_vector; result_strobe : out std_ulogic_vector; -- The input port index where the packet originated result_id : out natural range 0 to num_inputs - 1 ); end component;$

Multiplex between many AXI-Stream-like inputs towards one output bus. Will lock onto one input and let its data through until a packet has passed, as indicated by the last signal.

The implementation is simple, which comes with a few limitations:

Warning

If there are holes in an input packet stream after valid has been asserted, this multiplexer will be unnecessarily stalled even if another input has data available. It is up to the user to make sure that this does not occur, using e.g. a fifo.vhd in packet mode, or calculate that the system throughput is still sufficient.

The arbitration is done in the most resource-efficient round-robin manner possible, which means that one input can starve out the others if it continuously sends data.

handshake_pipeline.vhd

View source code on gitlab.com.

$component handshake_pipeline is generic ( data_width : natural; -- Setting to false can save logic footprint, at the cost of lower throughput full_throughput : boolean; -- Ensures that there is no combinatorial path between valid and ready on input and output. -- Will result in higher logic footprint. pipeline_control_signals : boolean; -- Ensures that there is no combinatorial path from data, strobe and last on input to output. -- Will result in higher logic footprint. pipeline_data_signals : boolean; -- In the typical use case where we want a "byte strobe", this would be eight. -- In other cases, for example when the data is packed, we might use a higher value. -- Must assign a valid value if input/output strobe is to be used. strobe_unit_width : positive ); port ( clk : in std_ulogic; --# {{}} input_ready : out std_ulogic; input_valid : in std_ulogic; -- Optional to connect. input_last : in std_ulogic; input_data : in std_ulogic_vector; -- Optional to connect. Must set valid 'strobe_unit_width' generic value in order to use this. input_strobe : in std_ulogic_vector; --# {{}} output_ready : in std_ulogic; output_valid : out std_ulogic; output_last : out std_ulogic; output_data : out std_ulogic_vector; output_strobe : out std_ulogic_vector ); end component;$

Handshake pipeline. Is used to ease the timing of a streaming data interface by inserting register stages on the data and/or control signals.

There are many modes available, with different characteristics, that are enabled with different combinations of full_throughput, pipeline_control_signals and pipeline_data_signals. See the descriptions within the code for more details about throughput and fanout.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for handshake_pipeline.vhd netlist builds.
Generics	Total LUTs	FFs	Maximum logic level
data_width = 32 full_throughput = True pipeline_control_signals = True pipeline_data_signals = True	41	78	2
data_width = 32 full_throughput = True pipeline_control_signals = False pipeline_data_signals = True	1	38	2
data_width = 32 full_throughput = True pipeline_control_signals = False pipeline_data_signals = False	0	0	0
data_width = 32 full_throughput = False pipeline_control_signals = True pipeline_data_signals = True	1	39	2
data_width = 32 full_throughput = False pipeline_control_signals = True pipeline_data_signals = False	2	3	2
data_width = 32 full_throughput = False pipeline_control_signals = False pipeline_data_signals = True	2	38	2
data_width = 32 full_throughput = False pipeline_control_signals = False pipeline_data_signals = False	0	0	0

handshake_splitter.vhd

View source code on gitlab.com.

$component handshake_splitter is generic ( num_interfaces : positive ); port ( clk : in std_ulogic; --# {{}} input_ready : out std_ulogic; input_valid : in std_ulogic; --# {{}} output_ready : in std_ulogic_vector; output_valid : out std_ulogic_vector ); end component;$

Combinatorially split an AXI-Stream-like handshaking interface, for cases where many slaves are to receive the data. Maintains full throughput and is AXI-stream compliant in its handling of the handshake signals (valid does not wait for ready, valid does not fall unless a transaction has occurred).

This entity has no pipelining of the handshake signals, but instead connects them combinatorially. This increases the logic depth for handshake signals where this entity is used. If timing issues occur (on the input or one of the output s) a handshake_pipeline.vhd instance can be used.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for handshake_splitter.vhd netlist builds.
Generics	Total LUTs	FFs
num_interfaces = 2	4	2
num_interfaces = 4	9	4

keep_remover.vhd

View source code on gitlab.com.

$component keep_remover is generic ( data_width : positive; strobe_unit_width : positive ); port ( clk : in std_ulogic; --# {{}} input_ready : out std_ulogic; input_valid : in std_ulogic; input_last : in std_ulogic; input_data : in std_ulogic_vector; input_keep : in std_ulogic_vector; --# {{}} output_ready : in std_ulogic; output_valid : out std_ulogic; output_last : out std_ulogic; output_data : out std_ulogic_vector; output_strobe : out std_ulogic_vector ); end component;$

This entity removes strobe’d out lanes from the input, resulting in an output stream where all lanes are always strobed (except for the last beat, potentially). The strobe on input can be considered as the TKEEP signal in AXI-Stream terminology, and the output strobe would be TKEEP/TSTRB.

The entity works by continuously filling up a data buffer with data from the input. Only the lanes that are strobed will be saved to the buffer. Note that input words may have all their lanes strobed out (except for the last beat, see below). When enough lanes are saved to fill a whole word, data is passed to the output by asserting output_valid. When input_last is asserted for an input word, an output word will be sent out, with output_last asserted, even if a whole strobed word is not currently filled in the buffer.

The strobe unit data width is configurable via a generic. Most of the time it would be eight, i.e. a byte strobe. But in some cases the strobe represents a wider quanta, in which case the generic can be increased. Increasing the generic will drastically decrease the resource utilization, since that is the “atom” of data that is handled internally.

The handling of input_last presents a corner case. Lets assume that data_width is 16 and strobe_unit_width is 8. Furthermore, there is one atom of data available in the buffer, and input stream has both lanes strobed. In this case, one input word shall result in two output words. The first output word comes from a whole word being filled in the buffer. The second word comes from a half filled word in the buffer, but input_last being asserted. This is solved by having a small state machine that pads input data with an extra word when this corner case arises. The padding stage makes it possible to have a very simple data buffer stage, with low resource utilization.

Throughput

The entity achieves full throughput, except for the corner case mentioned above, where it might stall one cycle on the input.

Limitations

input_last may not be asserted on an input word that has all lanes strobed out.
There may never be a ‘1’ above a ‘0’ in the input strobe. E.g. “0111” is allowed, but “1100” is not.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for keep_remover.vhd netlist builds.
Generics	Total LUTs	FFs	Maximum logic level
data_width = 32 strobe_unit_width = 16	88	79	3
data_width = 64 strobe_unit_width = 8	410	175	6
data_width = 128 strobe_unit_width = 32	414	282	5

periodic_pulser.vhd

View source code on gitlab.com.

$component periodic_pulser is generic ( -- The period between pulses period : positive range 2 to integer'high; -- The shift register length is device specific. -- For Xilinx UltraScale and 7 series devices, it should be set to 33. shift_register_length : positive ); port ( clk : in std_ulogic; --# {{}} count_enable : in std_ulogic; pulse : out std_ulogic ); end component;$

Outputs a one cycle pulse after a generic number of assertions of count_enable.

Shift registers are used as far as possible to create the pulse. This makes the implementation resource efficient on devices with cheap shift registers (such as SRLs in Xilinx devices). In the worst case a single counter is created.

The period is broken down into factors that are represented using shift registers, with the shift register length being the factor value. By rotating the shift register on each count_enable, a fixed period is created. When possible, multiple shift registers are AND-gated to create a longer period. For example a period of 30 can be achieved by gating two registers of length 10 and 3. This method only works if the lengths are mutual primes (i.e. the greatest common divisor is 1).

If the remaining factor is not 1 after the shift registers have been added, a new instance of this module is added through recursion.

If period cannot be factorized into one or more shift registers, recursion ends with either a simple counter or a longer shift register (depending on the size of the factor).

Example

Let’s say that the maximum shift register length is 16. A period of 510 = 10 * 3 * 17 can then be achieved using two shift registers of length 10 and 3, and then instantiating a new periodic_pulser.vhd

[0][0][0][0][0][0][0][0][0][1]
                              \
                               [AND] -> pulse -> [periodic_pulser with period 17]
                              /
                     [0][0][1]

The next stage will create a counter, because 17 is a prime larger than the maximum shift register length.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for periodic_pulser.vhd netlist builds.
Generics	Total LUTs	SRLs	FFs
period = 33 shift_register_length = 33	2	1	1
period = 33 shift_register_length = 1	6	0	6
period = 37 shift_register_length = 33	3	2	1
period = 37 shift_register_length = 1	6	0	6
period = 100 shift_register_length = 33	3	2	2
period = 100 shift_register_length = 1	8	0	7
period = 125 shift_register_length = 33	4	2	2
period = 125 shift_register_length = 1	7	0	7
period = 127 shift_register_length = 33	5	4	1
period = 127 shift_register_length = 1	8	0	7
period = 4625 shift_register_length = 33	7	4	3
period = 4625 shift_register_length = 1	2	0	13
period = 311000000 shift_register_length = 33	15	4	15
period = 311000000 shift_register_length = 1	2	0	29

strobe_on_last.vhd

View source code on gitlab.com.

$component strobe_on_last is generic ( data_width : positive ); port ( clk : in std_ulogic; --# {{}} input_ready : out std_ulogic; input_valid : in std_ulogic; input_last : in std_ulogic; input_data : in std_ulogic_vector; input_strobe : in std_ulogic_vector; --# {{}} output_ready : in std_ulogic; output_valid : out std_ulogic; output_last : out std_ulogic; output_data : out std_ulogic_vector; output_strobe : out std_ulogic_vector ); end component;$

The goal of this entity is to process an AXI-Stream so that packets where last is asserted on a word that is completely strobed out are modified so that last is instead asserted on the last word which does have a strobe.

As a consequence of this, all words in the stream that are completely strobed out are dropped by this entity.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for strobe_on_last.vhd netlist builds.
Generics	Total LUTs	FFs	Maximum logic level
data_width = 8	7	12	3
data_width = 32	8	39	3
data_width = 64	9	75	3

time_pkg.vhd

View source code on gitlab.com.

Contains a couple of methods for working with the VHDL time type.

The time type can be tricky sometimes because its precision is implementation dependent, just like the integer and universal_integer types:

integer'high is

2147483647 in GHDL 3.0.0-dev, corresponding to a 32 bit signed integer.
2147483647 in Vivado 2021.2, corresponding to a 32 bit signed integer.

time'high is

9223372036854775807 fs in GHDL 3.0.0-dev, corresponding to a 64 bit signed integer. Time values greater than this will result in an error.
2147483647 fs in Vivado 2021.2, corresponding to a 32 bit signed integer. However, Vivado 2021.2 can represent time values greater than this since it uses a dynamic secondary unit for time, as outlined in IEEE Std 1076-2008, page 39. Precision is never greater than 32 bits though.

In the standard library, the following functions are available for working with time values (IEEE Std 1076-2008, page 260):

function "=" (anonymous, anonymous: TIME) return BOOLEAN;
function "/=" (anonymous, anonymous: TIME) return BOOLEAN;
function "<" (anonymous, anonymous: TIME) return BOOLEAN;
function "<=" (anonymous, anonymous: TIME) return BOOLEAN;
function ">" (anonymous, anonymous: TIME) return BOOLEAN;
function ">=" (anonymous, anonymous: TIME) return BOOLEAN;
function "+" (anonymous: TIME) return TIME;
function "- (anonymous: TIME) return TIME;
function "abs" (anonymous: TIME) return TIME;
function "+" (anonymous, anonymous: TIME) return TIME;
function "-" (anonymous, anonymous: TIME) return TIME;
function "*" (anonymous: TIME; anonymous: INTEGER) return TIME;
function "*" (anonymous: TIME; anonymous: REAL) return TIME;
function "*" (anonymous: INTEGER; anonymous: TIME) return TIME;
function "*" (anonymous: REAL; anonymous: TIME) return TIME;
function "/" (anonymous: TIME; anonymous: INTEGER) return TIME;
function "/" (anonymous: TIME; anonymous: REAL) return TIME;
function "/" (anonymous, anonymous: TIME) return universal_integer;
function "mod" (anonymous, anonymous: TIME) return TIME;
function "rem" (anonymous, anonymous: TIME) return TIME;
function MINIMUM (L, R: TIME) return TIME;
function MAXIMUM (L, R: TIME) return TIME;

Notably missing is a convenient and accurate way of converting a time value to real or integer. So that is most of the complexity in the conversion functions below.

types_pkg.vhd

View source code on gitlab.com.

Some basic types that make it easier to work with VHDL. Also some basic functions operating on these types.

width_conversion.vhd

View source code on gitlab.com.

$component width_conversion is generic ( input_width : positive; output_width : positive; -- Enable usage of the 'input_last' and 'output_last' ports. -- Will increase the logic footprint. enable_last : boolean; -- Enable usage of the 'input_strobe' and 'output_strobe' ports. -- Will increase the logic footprint. enable_strobe : boolean; -- In the typical use case where we want a "byte strobe", this would be eight. -- In other cases, for example when the data is packed, we might use a higher value. -- Must assign a valid value if 'enable_strobe' is true. strobe_unit_width : positive; -- Enable if 'input' packet lengths are not a multiple of the 'output' width. -- Must set 'enable_strobe' and 'enable_last' as well to use this. -- See header for details about how this works. -- Will increase the logic footprint. support_unaligned_packet_length : boolean ); port ( clk : in std_ulogic; --# {{}} input_ready : out std_ulogic; input_valid : in std_ulogic; -- Optional packet 'last' signalling. Must set 'enable_last' generic in order to use this. input_last : in std_ulogic; input_data : in std_ulogic_vector; -- Optional word strobe. Must set 'enable_strobe' generic in order to use this. input_strobe : in std_ulogic_vector; --# {{}} output_ready : in std_ulogic; output_valid : out std_ulogic; -- Optional packet 'last' signalling. Must set 'enable_last' generic in order to use this. output_last : out std_ulogic; output_data : out std_ulogic_vector; -- Optional word strobe. Must set 'enable_strobe' generic in order to use this. output_strobe : out std_ulogic_vector ); end component;$

Width conversion of an AXI-Stream-like data bus. Can handle downconversion (wide to thin) or upconversion (thin to wide). The data widths must be a power-of-two multiple of each other. E.g. 10->40 is supported while 8->24 is not.

There is a generic to enable usage of the last signal. The last indicator will be passed along with the data from the input to output side as-is. Note that enabling the support_unaligned_packet_length generic will enable further processing of last, but in barebone configuration the signal is merely passed on.

There is a generic to enable strobing of data. The strobe will be passed on from input to output side as-is. Note that enabling support_unaligned_packet_length generic will enable further processing of strobe, but in barebone configuration the signal is merely passed on. This means, for example, that there might be output words where all strobe lanes are zero when downconverting.

There are some limitations, and possible remedies, concerning packet length alignment, depending on if we are doing upconversion or downconversion. See below.

Downconversion

When doing downconversion, one input beat will result in two or more output beats, depending on width configuration. This means that the output packet length is always aligned with the input data width. This is not always desirable when working with the strobe and last signals. Say for example that we are converting a bus from 16 to 8, and input_last is asserted on a beat where the lowest byte is strobed but the highest is not. In this case, we would want output_last to be asserted on the second to last byte, and the last byte (which is strobed out) to be removed. This is achieved by enabling the support_unaligned_packet_length generic. If the generic is not set, output_last will be asserted on the very last byte, which will be strobed out.

Upconversion

When upconverting, two or more input beats result in one output beat, depending on width configuration. This means that the input packet length must be aligned with the output data width, so that each packet fills up a whole number of output words. If this can not be guaranteed, then the support_unaligned_packet_length mode can be used. When that is enabled, the input stream will be padded upon last indication so that a whole output word is filled. Consider the example of converting a bus from 8 to 16, and input last is asserted on the third input beat. If support_unaligned_packet_length is disabled, there will be one output beat sent and half an output beat left in the converter. If the mode is enabled however, the input stream will be padded with another byte so that an output beat can be sent. The padded parts will have strobe set to zero.

Note that the handling of unaligned packet lengths is highly dependent on the input stream being well behaved. Specifically

There may never be input beats where input_strobe is all zeros.

For all beats except the one where input_last is asserted, input_strobe must be asserted on all lanes.

Resource utilization

This entity has netlist builds set up with automatic size checkers in module_common.py. The following table lists the resource utilization for the entity, depending on generic configuration.

Resource utilization for width_conversion.vhd netlist builds.
Generics	Total LUTs	FFs	Maximum logic level
input_width = 32 output_width = 16 enable_strobe = False enable_last = False support_unaligned_packet_length = False	20	51	2
input_width = 32 output_width = 16 enable_strobe = True enable_last = True support_unaligned_packet_length = False	23	59	2
input_width = 32 output_width = 16 enable_strobe = True enable_last = True support_unaligned_packet_length = True	30	63	3
input_width = 16 output_width = 32 enable_strobe = False enable_last = False support_unaligned_packet_length = False	35	51	2
input_width = 16 output_width = 32 enable_strobe = True enable_last = True support_unaligned_packet_length = False	40	60	2
input_width = 16 output_width = 32 enable_strobe = True enable_last = True support_unaligned_packet_length = True	45	62	3