This section describes how to translate the incoming data rate and reference clock frequency into a valid NIDRU configuration.
The user configuration defines specifications for:
- Incoming data rate with associated tolerance (f_DIN ± PPM)
- Available reference clock frequency with associated tolerance (f_REFCLK ± PPM)
While f_DIN is given, f_REFCLK can be selected inside a valid range. The range upper limit comes from the necessity to close timing in the target device, and is thus device and speed grade dependent. The lower limit to f_REFCLK might be imposed by the PHY. For example, a SerDes typically specifies a minimum reference clock frequency. A SelectIO interface does not typically impose a lower limit to f_REFCLK . The maximum f_DIN is typically limited by the oversampling rate OR as defined in the following equation.
CENTER_F, G1 and G2 should be set according to the following equations:
Using the equal value guarantees that the NIDRU operates in the lock-in region over the full PPM range of both the incoming data and the reference clock. Further reducing G_2 increases the NIDRU bandwidth. In general, increasing G_2 is not recommended, as the NIDRU would operate in pull-in region, where the automatic lock is not always guaranteed.
In general, G_(1_P) should be evaluated using the spreadsheet in the folder /excel_plots. The value should be increased up to when the ringing effect on the output phase becomes negligible. When G_1= G_2, setting G_(1_P)=16 always guarantees a negligible ringing effect. Consequently, G_(1_P)=16 is good for most of the cases.
The resulting transfer function can be obtained by using the excel file nidru_transfer_function_v_1_0.xls in the folder /excel_plots.
The usual three cases are considered as examples, yielding the following results:
- Fast Ethernet (fDIN
=125 Mbit/s ± 100 PPM) with 125
MHz refclk (± 100 PPM):
CENTER_F=
b0000100000000000000000000000000000000
G1 = G2 ≤ 11
- Fast Ethernet (fDIN
=125 Mbit/s ± 100 PPM) with
155.52 MHz refclk (± 20 PPM):
CENTER_F=
b0000011001101110000101110010110101001
G1 = G2 ≤ 11
- OC3 (fDIN
=155.52 Mbit/s ± 20 PPM) with 125 MHz
refclk (± 100 PPM):
CENTER_F=
b0000100111110100000010100010100001110
G1 = G2 ≤ 11