Introduction / Context:
In Complex Programmable Logic Devices (CPLDs) such as the MAX7000S family, each user pin typically connects to an Output Logic Macrocell (OLMC). Designers often ask what happens to that macrocell when the corresponding pin is configured strictly as an input. Understanding this frees up valuable on-chip resources and enables more efficient logic packing.
Given Data / Assumptions:
- Target device family: MAX7000S class CPLD.
- A specific I/O pin is configured as an input-only pad.
- Each I/O pin normally pairs with an OLMC (macrocell) that can drive the pin or be fed back internally.
- The question: how the macrocell can be repurposed when the pin is input-only.
Concept / Approach:
A macrocell can be used either to drive an external pin or for internal (buried) logic. If the pin is set as input-only, the output driver is disabled, allowing the OLMC to be used internally without consuming an additional external pin. This is commonly described as using the macrocell as “buried logic.”
Step-by-Step Solution:
Identify OLMC roles → can implement sum-of-products logic and optionally a register.Configure pin as input → disables external drive; the OLMC is no longer obligated to feed an output buffer.Freed OLMC → can be allocated for additional equations/registers inside the device (buried).Therefore, the correct reuse is “buried logic.”
Verification / Alternative check:
Vendor handbooks and fitter reports typically label such macrocells as “buried” when not bonded to outputs.
Why Other Options Are Wrong:
another output: Not possible because the pin is configured as input-only; no external drive is used.extra speed: Speed is a timing characteristic, not a functional repurpose of the macrocell.in-system testing: Boundary scan or JTAG are separate features; this is not the macrocell’s repurpose.
Common Pitfalls:
Assuming each macrocell must always drive a pin; in CPLDs, macrocells can be internal.Confusing input buffer configuration with macrocell availability.
Final Answer:
buried logic
Discussion & Comments