Combination or Configurable Logic: When to Use Which and How?

August 20, 2019 by Paul Shepard

Combination and configurable logic devices provide the flexibility to implement various logic functions with a reduced pin count. Combination and configurable logic can also lower system cost, simplify inventory & assembly, and improve the qualification process. Nexperia recently published the following blog posting to help readers find out how to design more with less:

Nexperia is aware of today's design needs and the goals of reducing system space, component count and avoiding board traces while maintaining affordability. When more functionality needs to be achieved in less space, combination and configurable logic are innovative yet proven solutions that will help designers achieve these goals.

Combination logic integrates at least two different logic functions in a single Mini Logic package. These functions can be internally cascaded or fully independent. Whatever the case, combination logic enables the use of one device instead of several. This reduces power dissipation, pin count and significantly saves space because the internal signals share power and ground.

Combination logic is particularly useful when a design requires dissimilar functions that need to be placed close to each other, for example an OR gate followed by an AND gate (see configuration figure below). In this case, combination logic offers an ideal integration opportunity that even reduces delay, as internally routed signals eradicate the delay associated with I/O buffers and no longer require package pins.

In a single-gate device that integrates two cascaded dissimilar functions, the output of one function is connected internally on the die to the input of the other function, helping improve performance and reduce pin count. In dual-gate cases, the two functions are fully independent and the I/O signals for each gate are made available outside the device but there are still pin count reduction benefits. As for triple-gate solutions, these integrate three different buffer and inverter functions which remain independent and aren't connected within the package.

At Nexperia, combination logic is available chiefly in our ultra-low power AUP technology, with multiple single, dual and triple-gate solutions, ranging from gates to buffers and inverters. Crystal drivers are also available.

Configurable logic to qualify just one device that can perform nine different functions

With configurable logic, the pin configuration determines the logic function the device will perform. One device can thus be used to perform multiple functions. Having fewer devices in inventory naturally helps reduce costs and simplify logistics, but configurable logic solutions equally deliver technical advantages.

To begin with, engineers benefit from maximal flexibility, allowing them to choose the function in the later stages of their design process. Using one configurable logic device instead of 9 discrete solutions also prevents qualifying more parts. In addition, configurable logic types can feature advanced options such as an integrated level-shifter option or an Output Enable function which can save further board space and add valuable functionality. Finally, all configurable logic types have Schmitt-trigger inputs to interface with slowly transitioning signals.

How to configure Nexperia's configurable logic types?

At Nexperia, configurable logic is available in extremely low-power AXP, ultra-low power AUP and low-voltage LVC technologies.

Configurable logic covers nine basic functions, with further options for complex variants of these. Nexperia's configurable portfolio includes multiple 2-input gates (AND, OR, NAND, NOR, XOR and XNOR), as well as buffers/inverters and also a multiplexer. A design engineer will need to configure the device to match his requirements and will have to follow a few simple steps, essentially selecting which pin is tied to GND or Vcc to choose the logic function, and which pins are the I/O of the function.

In the example of the 74LVC1G98 below, configurations are shown for producing either a 2-input NAND gate or a 2-input NOR gate. When pin 1 is connected to GND the device is configured as a 2-input NAND gate with inputs A and C and output Y. When pin 3 is connected to Vcc the device is configured as a 2-input NOR gate with inputs B and C and output Y.

Nexperia's combination and configurable logic solutions are available in small footprint leaded (PicoGate) and leadless (MicroPak) Mini Logic packages. Browse our combination gates and configurable gates range to find your space-saving control logic solutions and start saving board space and eliminating unnecessary inventory complexity.