Phase-locked Loops (PLLs) in Clock Buffers - JTAG Boundary-scan Tip

PLLs contained in clock distribution ICs generally will not function correctly with a clock input that neither maintains a constant frequency nor operates in the correct frequency range. This applies to both the JTAG clock (TCK) and to synchronous device clock pins, such as those found on SDRAM.

However, all hope is not lost! Many buffers have a method of disabling or bypassing the PLL. For boundary-scan testing this mode should be used whenever possible, and the clock distribution device should use a transparent model in ScanExpress TPG. Some common methods for dealing with PLLs include:

• PLL disable pin, such as a test pin.


• Mode pins, which include a bypass mode. Sometimes this is stated by
  saying the “reference” is applied to the outputs.


• Applying a different voltage (sometimes no power) to a power pin,
  usually the PLL power pin.


• Please refer to the device data sheet to determine if and how the
  internal PLL can be disabled.

For example, compare the popular Cypress CY2305 & CY2309 clock buffers (data sheet available from the Cypress website at

http://www.cypress.com/?rID=13269). See table 2 of the referenced data sheet: CY2309 includes select input pins not available on the CY2305, adding a PLL shutdown mode in which the output source follows the reference clock, allowing the clock buffer to be treated as a transparent device during boundary-scan tests.

Source: JTAG Boundary-scan Tip: Phase-locked Loops (PLLs) in Clock Buffers