The DELAY_TYPE attribute set to VARIABLE selects the variable tap delay line (Table 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 the following figure).
The RXTX_BITSLICE.EN_VTC pin should be held Low during the delay change command to ensure that any automatic adjustments are stopped.
To increment/decrement delay lines when using TIME mode:
- Deassert (Low) the RXTX_BITSLICE.EN_VTC pin.
- Wait a minimum of 10 clock cycles.
- Use the CE and INC ports to increment or decrement the delay line.
- Wait a minimum of 10 clock cycles.
- 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.
Notes on the preceding figure:
- At the rising edge of clock event 7, CE and INC are both High. The current tap is incremented by one because the INC signal is High for two clock cycles while CE only is High for a single cycle. UPDATE_MODE = ASYNC. The new setting is represented by Val_x+1.
- At the rising edge of clock event 13, CE is High and INC is Low. The delay line is decremented by one tap. The DATAOUT again has taken the position of the first loaded value.
- At the rising edge of clock event 19, CE and INC are both High. The current tap is incremented by one because the INC signal is High for two clock cycles while CE is High only for a single cycle. UPDATE_MODE = ASYNC. The new setting is represented by Val_x+1.