Programmable logic device - has remotely switchable delay element between input gating circuitry and macro cell pin driver

摘要

The modified programmable logic device (PLD) comprises input gating circuitry (11, 12), a macro cell (24) and switchable delay elements, formed here from a delay circuit (14), two switches (15, 18) and a differentiator (20). The delay removes glitches by comparing a direct output from the input gating circuit OR-gate with a version of the signal that has been passed through a series of NOT-gates (13) and stores the signal on an R/S flip-flop. Either the output from the latter or the original input gating output are selected by a remotely controlled switch (16) and passed to the second switching circuit (18) where a second remotely controlled switch (19) selects between the instantaneous output of the first switching circuit (15) and a latched version of the same. This signal is then differentiated by NOR-gating the output of the second switching circuit with NOT-gated version of the same output and passed to the parallel drive elements (22, 23) of the macro cell. ADVANTAGE - Switchable delay unit enables new-technology programmable logic devices to be fully compatible with previous designs.