LAN9250 datasheet(312/421 Pages) MICROCHIP

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LAN9250

DS00001913A-page 312

 2015 Microchip Technology Inc.

In addition to adjusting the frequency of the 1588 Clock, the Host may directly set the 1588 Clock, make a one-time step

adjustment of the 1588 Clock or specify a temporary rate. The choice of method depends on needed adjustment. For

initial adjustments, direct or one-time step adjustments may be best. For on-going minor adjustments, the temporary

rate adjustment may be best. Ideally, the frequency will be matched and once the 1588 Clock is synchronized, no further

adjustments would be needed.

In order to perform a direct writing of the 1588 Clock, the desired value is written into the 1588 Clock Seconds Register

(1588_CLOCK_SEC), 1588 Clock NanoSeconds Register (1588_CLOCK_NS) and 1588 Clock Sub-NanoSeconds

ister (1588_CMD_CTL) is then set.

In order to perform a one-time positive or negative adjustment to the seconds portion of the 1588 Clock, the desired

change and direction are written into the 1588 Clock Step Adjustment Register (1588_CLOCK_STEP_ADJ). The Clock

Step Seconds (1588_CLOCK_STEP_SECONDS) bit in the 1588 Command and Control Register (1588_CMD_CTL) is

then set. The internal sub-nanoseconds counter and the nanoseconds portion of the 1588 Clock are not affected. If a

nanoseconds portion rollover coincides with the 1588 Clock adjustment, the 1588 Clock adjustment is applied in addi-

tion to the seconds increment.

In order to perform a one-time positive adjustment to the nanoseconds portion of the 1588 Clock, the desired change

is written into the 1588 Clock Step Adjustment Register (1588_CLOCK_STEP_ADJ). The Clock Step NanoSeconds

(1588_CLOCK_STEP_NANOSECONDS) bit in the 1588 Command and Control Register (1588_CMD_CTL) is then

set. If the addition to the nanoseconds portion results in a rollover past zero, then the seconds portion of the 1588 Clock

is incremented. The normal (9, 10 or 11 ns) increment to the nanoseconds portion is suppressed for one clock. This can

be compensated for by specifying an addition value 10ns higher. A side benefit is that using an addition value of 0 effec-

tively pauses the 1588 Clock for 10ns while a value less than 10 slows the clock down just briefly. The internal sub-

nanoseconds counter of the 1588 Clock is not affected by the adjustment, however, if a sub-nanoseconds counter roll-

over coincides with the 1588 Clock adjustment it will be missed.

In order to perform a temporary rate adjustment of the 1588 Clock, the desired temporary rate and direction are written

into the 1588 Clock Temporary Rate Adjustment Register (1588_CLOCK_TEMP_RATE_ADJ) and the duration of the

temporary rate, specified in reference clock cycles, is written into the 1588 Clock Temporary Rate Duration Register

(1588_CLOCK_TEMP_RATE_DURATION). The Clock Temporary Rate (1588_CLOCK_TEMP_RATE) bit in the 1588

Command and Control Register (1588_CMD_CTL) is then set. Once the temporary rate duration expires, the Clock

Temporary Rate (1588_CLOCK_TEMP_RATE) bit will self-clear and the 1588 Clock Rate Adjustment Register

(1588_CLOCK_RATE_ADJ) will once again control the 1588 Clock rate. This method of adjusting the 1588 Clock may

be preferred since it avoids large discrete changes in the 1588 Clock value. For a maximum setting in both the 1588

Clock Temporary Rate Adjustment Register (1588_CLOCK_TEMP_RATE_ADJ) and 1588 Clock Temporary Rate Dura-

tion Register (1588_CLOCK_TEMP_RATE_DURATION), the 1588 Clock can be adjusted by approximately 1 second.

LAN9250 Datasheet(PDF) 312 Page - Microchip Technology