The DELAY_TYPE attribute set to VARIABLE selects the variable tap delay line (Table: Control Pin when DELAY_TYPE = VARIABLE(1) ). In VARIABLE mode, the CE and INC pins are used to manually increment and decrement the delay line tap per tap (INC/DEC increments or decrements one tap at a time). The tap delay increments by setting CE = 1 and INC = 1, or decrements by CE = 1 and INC = 0. The increment/decrement operation depends on the UPDATE_MODE attribute (see This Figure).
The RXTX_BITSLICE.EN_VTC pin should be held Low during the delay change command to ensure that any automatic adjustments are stopped.
Note: The input and output delays for each bitslice share a delay ratio register (see Table: Register Bit Description (PQTR_RATIO, NQTR_RATIO, IODELAY_RATIO)). The delay ratio register ensures the input delays are correctly calibrated based on the reference clock frequency. Output delays must match input delays for optimal calibration accuracy for the output delays.
To increment/decrement delay lines when using TIME mode:
1.Deassert (Low) the RXTX_BITSLICE.EN_VTC pin.
3.Use the CE and INC ports to increment or decrement the delay line.
5.Assert the RXTX_BITSLICE.EN_VTC pin.
When using the delay line in COUNT mode, the RXTX_BITSLICE.EN_VTC port is always Low after the reset sequence has completed. Use the TIME mode procedure in step 2 through step 4 in this VARIABLE Mode section.