GTS Transceiver PHY User Guide

ID 817660
Date 4/01/2024
Public
Document Table of Contents

3.6.4. PMA Fractional Mode

For a given data rate, the drop-down menu lists the supported integer mode reference clock frequencies. For a given data rate, if the required reference clock frequency is not listed in the drop-down, you can either select one of the supported integer mode reference clock frequencies or enable fractional mode.

  • VCO frequency (MHz) = (M + k/2^22) * refclk frequency (MHz) * ( mul_div / N).

    Output frequency (MHz) = VCO frequency (MHz) / L

    When the fractional PLL reference clock frequency is entered, the IP GUI displays the VCO, M, N, L, k, and mul_div values in the System Messages tab as shown in the figure below.

  • The TX PLL consists of three PLLs: slow, medium and fast. The PLL that is automatically selected is also displayed in the System Messages tab as shown in the figure below.
  • For a given data rate, you must enable fractional mode if you need to dynamically configure the K value during run time. When you enable fractional mode, you must enter the TX PLL fractional mode reference clock frequency.
Figure 48. System Message for PLL Fractional Settings

GTS PMA supports fractional mode in following PMA modes:

Table 43.  PMA Support for Fractional Mode
PMA Mode Fractional Mode Support
TX simplex Each TX PLL supports fractional mode in TX simplex. To enable, select TX simplex option for PMA mode, and turn on the Enable TX PLL fractional mode in the parameter editor. The TX PLL fractional counter values automatically calculate for the selected reference clock frequency. You can place TX Simplex fractional mode on any of the TX PMAs.
Note: GTS PMA does not support fractional mode for RX simplex.
Duplex Each GTS PMA in Duplex PMA mode supports fractional mode. To enable fractional mode in duplex PMA mode, select the Duplex option for PMA mode, select up to 8 for the Number of PMA lanes, and turn on Enable TX PLL fractional mode option in the parameter editor.
  • In Duplex fractional mode, the output of each TX PLL is used as the reference clock for corresponding RX CDR. Each TX PLL is configured as fractional mode.
  • A separate reference clock is not required for RX CDR. TX PLL fractional counter values, and RX CDR reference clock frequency, automatically calculate for the selected reference clock frequency.
  • You can place duplex fractional mode on any of the PMAs.
Primary PLL configuration To enable fractional mode with the primary PLL configuration, select the Duplex option for PMA mode, select 4 for the Number of PMA lanes, and turn on Enable TX PLL fractional mode and Enable TX PLL cascade mode options in the parameter editor.
  • In Primary PLL configuration, the TX PLL of lane 0 is in fractional mode and acts as the reference clock source for the local CDR and TX PLLs of lanes 1-3 (configured in integer mode) within the transceiver bank.
  • The TX PLLs in each lane provides the reference clock to the local RX CDR in its own lane.
  • Primary PLL configuration is only supported when you select 4 for the number of PMA lanes.

Tuning the k Counter Value in Fractional Mode

You can configure the GTS PMA in fractional mode to adjust the frequency and data rate by +/- 1000ppm for rate matching purposes.

Each GTS PMA has an Avalon® memory-mapped interface register containing the K value. The K value / 2^22 gives the fractional value of the feedback counter. The fractional value plus the M counter value provides the total feedback counter and determines how much PPM each bit in the K value represents.

The procedure to change the K value is:

  1. Change the K value to the new value.
  2. Pulse the strobe bit 0 -> 1-> 0 to lock in the new K value.

Each GTS PMA contains 3 PLLs; slow, medium and fast. The K value and strobe bit Avalon® memory-mapped interface register addresses depend on the location of the transceiver in the bank and which PLL is used (slow, medium, fast) as shown in the table below.

Table 44.  GTS PMA Fractional k Counter and Strobe Register Addresses
Channel Location in Bank PLL Fractional K Value Register Strobe Register
0 Slow 0x094000[30:9] 0x09400C[17]
Medium 0x094100[30:9] 0x09410C[17]
Fast 0x094200[30:9] 0x09420C[17]
1 Slow 0x194000[30:9] 0x19400C[17]
Medium 0x194100[30:9] 0x19410C[17]
Fast 0x194200[30:9] 0x19420C[17]
2 Slow 0x294000[30:9] 0x29400C[17]
Medium 0x294100[30:9] 0x29410C[17]
Fast 0x294200[30:9] 0x29420C[17]
3 Slow 0x394000[30:9] 0x39400C[17]
Medium 0x394100[30:9] 0x39410C[17]
Fast 0x394200[30:9] 0x39420C[17]