6

ATF1508ASV(L)

1408H–PLD–7/05

Extra Feedback

The ATF15xxSE Family macrocell output can be selected as registered or combinato-

rial. The extra buried feedback signal can be either combinatorial or a registered signal

regardless of whether the output is combinatorial or registered. (This enhancement

function is automatically implemented by the fitter software.) Feedback of a buried com-

binatorial output allows the creation of a second latch within a macrocell.

I/O Control

The output enable multiplexer (MOE) controls the output enable signal. Each I/O can be

individually configured as an input, output or for bi-directional operation. The output

enable for each macrocell can be selected from the true or compliment of the two output

enable pins, a subset of the I/O pins, or a subset of the I/O macrocells. This selection is

automatically done by the fitter software when the I/O is configured as an input, all mac-

rocell resources are still available, including the buried feedback, expander and cascade

logic.

Global Bus/Switch Matrix

The global bus contains all input and I/O pin signals as well as the buried feedback sig-

nal from all 128 macrocells. The switch matrix in each logic block receives as its inputs

all signals from the global bus. Under software control, up to 40 of these signals can be

selected as inputs to the logic block.

Foldback Bus

Each macrocell also generates a foldback product term. This signal goes to the regional

bus and is available to 16 macrocells. The foldback is an inverse polarity of one of the

macrocell’s product terms. The 16 foldback terms in each region allow generation of

high fan-in sum terms (up to 21 product terms) with little additional delay.

Open-collector Output Option

This option enables the device output to provide control signals such as an interrupt that

can be asserted by any of the several devices.