Revisions Affected

0, A, B

Details

The Manchester decoding algorithm samples the Manchester bitstream with SYSCLK in a calibration window of 1024 SDx_Dy signal transitions. The derived clock from the Manchester bitstream is used to sample for data in the subsequent calibration window cycle.

There are several scenarios that can cause large errors in the filter results:

• Any single noise event on SDx_Dy can corrupt the decoded Manchester clock and cause subsequent data to be sampled at an incorrect frequency.
• If the Manchester bitstream clock rate is a near exact integer multiple of SYSCLK, then an occasional Manchester bit can be skipped when the phases of the Manchester stream and internal SYSCLK drift past each other in phase before the next 1024 transition calibration window becomes effective. Deviations in duty cycle from 50% of the Manchester clock also need to be accounted for to ensure the longer Manchester pulses are not an integer multiple of SYSCLK. This situation can be unavoidable if the clock sources for either the SD modulator or this device have a wide variation since a wide range of keep out frequencies become problematic
• If the Manchester edge delay variation between rising and falling (duty cycle of the bitstream) is greater than one SYSCLK, then the SDFM clock decode algorithm can incorrectly identify the clock period as shorter than it is.

Workarounds

The workarounds available are:

• Avoid using Manchester mode and consider using Mode 0, which provides the best filter performance under noisy conditions. This is the recommended workaround.
• Avoid any noise on the Manchester bitstream and avoid integer multiples of SYSCLK for the selected Manchester clock source. A precision clock source for the modulator and this device must be used.
• Ensure rising and falling edge delays (high and low pulses) are within one SYSCLK of each other in length.
• Design an application-level algorithm that is robust against occasional incorrect SDFM results.