Understanding I/O Blocks and FastCONNECT in CPLDs
I/O Block (IOB) Functionality
The I/O Block (IOB) serves as the interface between the internal logic of a CPLD and the device’s user I/O pins. Each IOB comprises several key components:
- Input buffer
- Output driver
- Output enable selection multiplexer
- User-programmable ground control
The input buffer is compatible with various signal levels, including standard 5V CMOS, 5V TTL, and 3.3V. It utilizes the internal 5V voltage supply (VCCINT) to maintain constant input thresholds, independent of the VCCIO voltage.
The output enable can be derived from multiple sources:
- A product term signal from the macrocell
- Any of the global OE signals
- Always “1” (enabled)
- Always “0” (disabled)
Devices with up to 144 macrocells feature two global output enables, while devices with 180 or more macrocells have four.
Both polarities of any of the global 3-state control (GTS) pins can be utilized within the device.
Purpose of the FastCONNECT Switch Matrix
The FastCONNECT Switch Matrix plays a crucial role in CPLD architecture:
- Efficiently routing signals between Function Blocks (FBs) and I/O Blocks (IOBs).
- Ensuring uniform delay for signals, irrespective of the routing path.
- Providing high-speed connectivity for internal logic.
Key Features of the FastCONNECT Switch Matrix
Full Programmability:
- Designers have complete control over how signals are routed within the CPLD.
- Any FB or IOB can be interconnected with any other FB or IOB.
Uniform Signal Delays:
- The matrix guarantees that all routing paths introduce similar delays.
- This simplifies timing analysis and enhances design predictability.
High Fan-In Capability:
- Allows multiple outputs (e.g., from FBs) to drive a single input with minimal timing delays.
- Includes a feature for creating internal wired-AND outputs, increasing effective logic capacity.
The FastCONNECT switch matrix connects signals to the FB inputs, as illustrated in the figure below. All IOB outputs (corresponding to user pin inputs) and all FB outputs drive the FastCONNECT matrix. Any of these (up to a FB fan-in limit of 36) may be selected, through user programming, to drive each FB with a uniform delay.
The FastCONNECT switch matrix can combine multiple internal connections into a single wired-AND output before driving the destination FB. This provides additional logic capability and increases the effective logic fan-in of the destination FB without any additional timing delay. This capability is available for internal connections originating from FB outputs only and is automatically invoked by the development software where applicable.
XC 4000 Features
CLB (Configurable Logic Block):
- Two flip-flops (FFs) per CLB and two per I/O cell.
- 25 gates per CLB for logic implementation.
- 32 bits of SRAM per CLB.
- Special fast carry logic between CLBs.
Interconnects:
- Direct and general-purpose wires have been replaced with more efficient single-length and double-length lines.
- Sufficient resources are provided for most applications.
Additional Features:
- Synchronous Single and Dual-Port RAM.
- Internal three-state buffers.
- System performance up to 80 MHz.
- 0.5µ SRAM Process Technology.
Function Blocks
Logic Blocks (CLB):
- Also known as: Configurable logic block, logic element, logic module, logic unit, or Logic array block.
- Used to implement combinational and sequential logic.
Interconnect:
- Wires used to connect inputs and outputs to logic blocks.
I/O Blocks:
- Special logic blocks located at the periphery of the device for external connections.
