3.17.1. Extended Delay Measurement for Soft Internal Buffers
The CPRI Intel® FPGA IP uses a dedicated clock, ex_delay_clk, to measure the delay through the RX and TX internal buffers to your desired precision. The extended delay process is identical for the two directions of flow through the IP core; the TX_EX_DELAY and RX_EX_DELAY registers hold the same information for the two directions.
The tx_msrm_period field of the TX_EX_DELAY register contains the value N, such that N clock periods of the ex_delay_clk clock are equal to some whole number M of cpri_clkout periods. For example, N may be a multiple of M, or the M/N frequency ratio may be slightly greater than 1, such as 64/63 or 128/127. The application layer specifies N to ensure the accuracy your application requires. The accuracy of the Tx buffer delay measurement is N/least_common_multiple(N,M) cpri_clkout periods.
Similarly, the rx_msrm_period field of the RX_EX_DELAY register contains the value N, such that N clock periods of the ex_delay_clk clock are equal to some whole number M of cpri_clkout periods.
If your application does not require this precision, drive the ex_delay_clk input port with the cpri_clkout signal. In this case, the M/N ratio is 1 because the frequencies are the same.
The tx_buf_delay field of the TX_DELAY register indicates the number of 32-bit words currently in the Tx buffer. After you program the tx_msrm_period field of the TX_EX_DELAY register with the value of N, the tx_ex_delay field of the TX_EX_DELAY register holds the current measured delay through the Tx buffer. The unit of measurement is cpri_clkout periods. The tx_ex_delay_valid field indicates that a new measurement has been written to the tx_ex_delay field since the previous register read. The following sections explain how you set and use these register values to derive the extended Tx delay measurement information.
M/N Ratio Selection
As your selected M/N ratio approaches 1, the accuracy provided by the extended delay measurement increases.
|M||N||cpri_clkout Period||ex_delay_clk Period||Resolution|
|128||127||13.02 ns (1/76.80 MHz)||13.12 ns||±100 ps|
|64||63||13.22 ns||±200 ps|
|1||4||3.25 ns||±3.25 ns|
Extended Delay Measurement Calculation Example
This section walks you through an example that shows you how to calculate the frequency at which to run ex_delay_clk, and how to program and use the registers to determine the delay through the CPRI Receive Buffer.
For example, assume your CPRI Intel® FPGA IP runs at CPRI line bit rate 3.072 Gbps. In this case, the cpri_clkout frequency is 76.80, so a cpri_clkout cycle is 1/76.80 MHz.
If your accuracy resolution requirements are satisfied by an M/N ratio of 128/127, perform the following steps:
- Program the value N=127 in the rx_msrm_period field of the RX_EX_DELAY register at offset 0x54.
- Perform the following calculation to determine the ex_delay_clk frequency that supports your desired accuracy resolution:
ex_delay_clk period = (M/N) cpri_clkout period = (128/127)(1/(76.80 MHz)= 13.123356 ns.
Based on this calculation, the frequency of ex_delay_clk is 1/(13.123356 ns)
The following steps assume that you run ex_delay_clk at this frequency.
- Read the value of the RX_EX_DELAY register at offset 0x54.
If the rx_ex_delay_valid field of the register is set to 1, the value in the rx_ex_delay field has been updated, and you can use it in the following calculations. For this example, assume the value read from the rx_ex_delay field is 0x107D, which is decimal 4221.
- Perform the following calculation to determine the delay through the Rx buffer:
Delay through Rx buffer = (rx_ex_delay x cpri_clkout period) / N = (4221 x 13.02083 ns) / 127 = 432.7632 ns.
This delay comprises (432.7632ns / 13 .02083 ns) = 33.236 cpri_clkout clock cycles.
These numbers provide you the result for this particular example. For illustration, the preceding calculation shows the result in nanoseconds. You can derive the result in cpri_clkout clock cycles by dividing the preceding result by the cpri_clkout clock period. Alternatively, you can calculate the number of cpri_clkout clock cycles of delay through the Rx buffer directly, as rx_msrm_period/N.
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