External ToD Interface Sub Block - 2.6 English

1G/10G/25G Switching Ethernet Subsystem Product Guide (PG292)

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2.6 English

This module is optional, it is present only in the example design when you select Enable Ext ToD Bus I/F option while generating the IP. Its function is to load the reference seconds value onto the PTP System Timer and synchronize the System Timer to an external 1PPS source. It is intended to interface with a high precision clock source/timing device.

Inputs to this module are 1PPS pulse, serial 1-bit data bus carrying reference ToD seconds time that is clocked by a serial clock input. The serial clock is expected to be less than or equal to the free-running clock (ts_clk) of the Timer Sync block. The process of loading the reference 48-bits second is shown in the following figure.

Figure 1. ToD Value (seconds) Interface

The External ToD I/F block stores serial input ToD seconds value in a holding register. At the next received positive edge of the 1PPS signal, the holding register’s value is incremented by +1 second, and the result be presented to the PTP System Timer sub-block. The positive edge of the 1PPS signal is re-timed within the External ToD I/F block to the ts_clk clock and output for use by the PTP System Timer block.