DP83TC815-Q1 has an in-built PPM monitor to calculate frequency offset between any two internal clock sources or between an external clock source and any internal clock source. The PPM monitor is a continuous monitor which provides real-time frequency offset between the two clock sources.

PPM Monitor takes in a reference clock and compares the frequency offset with a monitor clock. Both reference clock and monitor clock can be selected from options of:

• 25MHz XI clock input
• Internal Leader transmit clock of 200MHz
• MDI Recovered clock of 200MHz
• External Clock input on GPIOs (LED_0, GPIO_5)
• PTP Trigger 0 Output
• SGMII Recovered Clock
• PLL clk of 250Mhz

Following software sequence can be used to read out frequency offset

• Step-1: Disable PPM monitor before configuring settings in below steps, by programming 0x01AF[13]= 0
• Step-2: Select Reference Clock and Monitor Clock by programming register fields 0x01AF[12:9], 0x01AF[8:5], 0x01AF[15:14]
• Step-3: Select refresh period of the ppm monitor (for example, 1ms, 10ms, 100ms, and so forth.) Monitor accuracy increases with refresh period. Refresh period must be Common Multiple of monitor clock period and reference clock period.
• Step-4: Program monitor clock count and reference clock count
  • {0x01A4, 0x01A3} = refresh period/monitor clock period
  • {0x01A6, 0x01A5} = refresh period/reference clock period
• Step-5: Enable PPM monitor by programming 0x01AF[13] = 1
• Step-6: Wait for at least one refresh period to complete, write 0x01AF[4] = 1 to latch the ppm monitor values and read status registers {0x01AE, 0x01AD}
  • If 0x01AE[15] = 0, ppm offset is negative, if 0x01AE[15] = 1, ppm offset is positive.
  • PPM offset of monitor clock = {0x01AE[14:0],0x01AD[15:0]}/ {0x01A4, 0x01A3}

Example - Configuring PPM Monitor:

Calculating PPM offset between 12.5MHz external clock (on GPIO5), XI input clock of 25MHz for a refresh period of 10ms

Configuring Interrupt for PPM Monitor Unlock:

PPM monitor can also be used to give an unlock interrupt if the PPM of the monitor clock goes beyond configured thresholds.

Following software sequence can be used to configure interrupt thresholds

• Step-1: Disable PPM monitor before configuring settings in below steps, by programming 0x01AF[13]= 0
• Step-2: Choose PPM thresholds beyond interrupt has to be indicated
• Step-3: Calculate counter register thresholds
  • Threshold = Monitor clock count (value loaded in register 0x01A4, 0x01A3) * PPM threshold
• Step-4: Load values into the registers
  • Load positive PPM threshold in registers {0x01A8, 0x01A7} = Monitor Clock Count * positive PPM beyond which interrupt is flagged
  • Load negative PPM threshold in registers {0x01AA,0x01A9} = Monitor clock count - (Monitor Clock Count * negative PPM beyond which interrupt is flagged
• Step-5: (Optional) Enable interrupt indication on INT_N pin by programming 0x0017 = 0x0008. Status is available in bit 11 of register 0x0017 irrespective of whether indication on INT_N is enabled
• Step-6: Enable PPM monitor by programming 0x01AF[13] = 1
• Step-7: Wait for at least 1 refresh period and read status from 0x0017[11]

Example - Configuring PPM Monitor Interrupt:

Enabling Interrupt for previous example in Table 7-9 with PPM threshold chosen as 200ppm