L- and H-Tile Transceiver PHY User Guide

ID 683621
Date 1/30/2024
Document Table of Contents

3.1.3. CMU PLL

The clock multiplier unit (CMU) PLL resides locally within each transceiver channel. The channel PLL's primary function is to recover the receiver clock and data in the transceiver channel. In this case the PLL is used in clock and data recovery (CDR) mode.

When the channel PLL of channel 1 or channel 4 is configured in the CMU mode, the channel PLL can drive the local clock generation block (CGB) of its own channel. However, when the channel PLL is used as a CMU PLL, the channel can only be used as a transmitter channel because the CDR block is not available to recover the received clock and data.

The CMU PLL from transceiver channel 1 and channel 4 can also be used to drive other transceiver channels within the same transceiver bank. The CDR of channels 0, 2, 3, and 5 cannot be configured as a CMU PLL.

For data rates lower than 6 Gbps, the local CGB divider has to be engaged (TX local division factor in transceiver PHY IP under the TX PMA tab).

Figure 148. CMU PLL Block Diagram

Input Reference Clock

The input reference clock for a CMU PLL can be sourced from either the reference clock network or a receiver input pin. The input reference clock is a differential signal. The input reference clock must be stable and free-running at device power-up for proper PLL operation. If the reference clock is not available at device power-up, then you must recalibrate the PLL when the reference clock is available.

Note: The CMU PLL calibration is clocked by the OSC_CLK_1 clock which must be stable and available for calibration to proceed.

Reference Clock Multiplexer (Refclk Mux)

The reference clock (refclk) mux selects the input reference clock to the PLL from the various reference clock sources available.

N Counter

The N counter divides the refclk mux's output. The N counter division helps lower the loop bandwidth or reduce the frequency to within the phase frequency detector's (PFD) operating range. Possible divide ratios are 1 (bypass), 2, 4, and 8.

Phase Frequency Detector (PFD)

The reference clock (refclk) signal at the output of the N counter block and the feedback clock (fbclk) signal at the output of the M counter block is supplied as an input to the PFD. The PFD output is proportional to the phase difference between the two inputs. It aligns the input reference clock (refclk) to the feedback clock (fbclk). The PFD generates an "Up" signal when the reference clock's falling edge occurs before the feedback clock's falling edge. Conversely, the PFD generates a "Down" signal when feedback clock's falling edge occurs before the reference clock's falling edge.

Charge Pump and Loop Filter (CP + LF)

The PFD output is used by the charge pump and loop filter to generate a control voltage for the VCO. The charge pump translates the "Up"/"Down" pulses from the PFD into current pulses. The current pulses are filtered through a low pass filter into a control voltage which drives the VCO frequency.

Voltage Controlled Oscillator (VCO)

The CMU PLL has a ring oscillator based VCO.

L Counter

The L counter divides the differential clocks generated by the CMU PLL. The division factors supported are 1 and 2.

M Counter

The M counter is used in the PFD's feedback path. The output of the L counter is connected to the M counter. The combined division ratios of the L counter and the M counter determine the overall division factor in the PFD's feedback path.

Lock Detector (LD)

The lock detector indicates when the CMU PLL is locked to the desired output's phase and frequency. The lock detector XORs the Up/Down pulses and indicates when the M counter's output and N counter's output are phase-aligned.