Frequency meter

ENTITY FREQUENCY_METER IS

The component FREQUENCY_METER measures frequency of a (Clock) signal using the Equal Precision algorithm. For this, it needs a reference (Clock) signal of known frequency.

The “Measured frequency” is measured within an Interval, which is configurable via the MI bus (see the MI Address Space below). Start the measurement by a Write to the Command register. During this Interval, two counters are enabled, one running on the Reference Clock, the other on the Measured Clock. After it has passed, the counters’ data will be ready to be fetched (=> loaded to MI registers). Check the Status register for information about whether the measurement is running or has completed, if the data are ready, or if any errors occurred (and which one(s)).

Note

In the case of an overflow error (status(6-7)), either make a new design with wider counter(s) or just lower the duration of the measurement by writing a new value to the Interval length register. Be aware that lower lengths of the Interval (slightly) decrease the accuracy of the measurement.

Note

Issuing a MI reset command (cmd(1)) during an in-progress measurement without stopping it (re)starts the measurement anew, so there is no need to start it again by writing to the command register (cmd(0)).

The component cannot calculate the final frequency (is not yet implemented). At this time, it can simply be done in software using formula: Fmeas = Fref * Nmeas/Nref, where F<meas/ref> is the Measured or Reference frequency and N<meas/ref> are the data from the counters. It is recommended to use the provided python tool to calculate the final value of the measured frequency(ies).

Other yet-to-be-implemented features include storing the counters’ values from multiple measurements and a histogram of these values.

MI Address Space

Address offset	MI Register description
0x00	Command register (Write-only): cmd(0) - start (1) / stop (0) measuring the frequency (see Status(1)) cmd(1) - reset the measurement logic (except the interval length!) cmd(2) - fetch measured data when ready (see Status(2))
0x04	Status register (Read-only): status(0) - ready to measure (is able to accept the command to start measuring) status(1) - measuring in process (write in Command reg) status(2) - measurement done (measuring interval ended) status(3) - counters’ data are ready to be fetched status(4) - fetched data are ready to be read status(5) - an error occurred during the measurement (Counters’ data are invalid) status(6) - the Reference Frequency Counter overflowed status(7) - at least one Measured Frequency Counter overflowed status(8) - the Reference Clock was reset during the measurement status(9) - at least one of the Measured Clocks were reset during the measurement
0x08	Interval length register (Read and Write) default: 2**(INTERVAL_LEN_WIDTH/2)
0x0C	The Reference Clock frequency in Hz (Read-only)
0x10	Measured Frequencies register (Read-only): the number of measured frequencies
0x14	Reference Frequency Counter data register (Read-only): issue multiple Read requests when the Counter is wider than the MI bus
0x18	Measured Frequency Counter data register(s) (Read-only): issue one Read request to this register for each Counters’ data starts from index 0 and increments after each read to this address, resets at max current index can be read from the Read Pointer register
0x1C	Read Pointer register (Read-only): index of the next Measured Frequency Counter data register that will be read

Generics

Generic	Type	Default	Description
MI_DATA_WIDTH	natural	32
MI_ADDR_WIDTH	natural	32
INTERVAL_LEN_WIDTH	natural	32	Maximum width of the Interval length signal => the highest possible value is 2**INTERVAL_LEN_WIDTH-1. This value of the Interval length signal can be set over the MI. Defines the length of the Interval during which the frequency measurement takes place. Counts by 1 to its maximum which is when the “frequency counters” are sampled. Values over 32 (=MI_DATA_WIDTH) are currently not supported!
REFERENCE_CNTR_WIDTH	natural	31	Width of the “Reference Frequency Counter”. Watch out for overflow (indicated by a bit in the Status register). Must not be over 32!
MEASURED_CNTR_WIDTH	natural	31	Width of the “Measured Frequency Counter” (all of them if there is more than one). Watch out for overflow (indicated by a bit in the Status register). Must not be over 32!
MEASURED_FREQUENCIES	natural	10	Maximum number of measured frequencies.
REFERENCE_CLK_FREQ	natural	200_000_000	Frequency of the reference clock signal in Hz. Used in the final calculation when the CALCULATE_FREQ generic is True. Recommended to read before the SW calculation (when the CALCULATE_FREQ generic is False).
DSP_CNTR_EN	boolean	False	Utilize DSPs for both “frequency counters”.
CALCULATE_FREQ	boolean	False	Calculate the unknown frequency in the FPGA. Uses extra resources. Not supported yet!
STORE_DATA_EN	boolean	False	Store measured data and/or calculated frequencies in a FIFO to read out later. Could be useful for further analysis. Not supported yet! (Also leads to a generic for FIFO_SIZE)
HISTOGRAM_EN	boolean	False	Store measured data and/or calculated frequencies in a histogram. Could be useful for further analysis. Not supported yet!
DEVICE	string	“AGILEX”	Target device.

Ports

Port	Type	Mode	Description
REFERENCE_CLK	std_logic	in
REFERENCE_RESET	std_logic	in
MEASURED_CLK	std_logic_vector(MEASURED_FREQUENCIES-1 downto 0)	in
MEASURED_RESET	std_logic_vector(MEASURED_FREQUENCIES-1 downto 0)	in
MI_CLK	std_logic	in	MI interface
MI_RESET	std_logic	in
MI_DWR	std_logic_vector(MI_DATA_WIDTH-1 downto 0)	in
MI_ADDR	std_logic_vector(MI_ADDR_WIDTH-1 downto 0)	in
MI_RD	std_logic	in
MI_WR	std_logic	in
MI_ARDY	std_logic	out
MI_DRD	std_logic_vector(MI_DATA_WIDTH-1 downto 0)	out
MI_DRDY	std_logic	out