Xilinx Configuration Flash: XCF Series PROM Guide

Anyone who’s worked with older Xilinx FPGA designs knows the frustration of dealing with configuration storage. You’ve got a perfectly working bitstream, but every power cycle means reconfiguration—unless you’ve got the right PROM. The XCF series Platform Flash PROMs have been the go-to solution for non-volatile FPGA configuration storage for years, and devices like the XCF01SVOG20C and XCF04SVOG20C remain in countless production designs. This guide covers everything you need to know about these configuration memories, from pinouts to programming procedures.

Understanding the XCF Platform Flash Family

Xilinx introduced the Platform Flash series to provide in-system programmable configuration PROMs specifically designed for their FPGAs. Unlike generic flash memories that require external logic for configuration sequencing, the XCF series handles all the timing and protocol requirements internally.

XCF Series Family Overview

The Platform Flash family splits into two main branches based on operating voltage:

The “S” series (XCF01S, XCF02S, XCF04S) targets simpler applications where serial configuration suffices. The “P” series adds parallel configuration support, design revisioning, and built-in data decompression—features essential for larger FPGAs with multi-megabit bitstreams.

XCF01SVOG20C Specifications

The XCF01SVOG20C represents the entry-level option in the Platform Flash lineup. Despite its modest 1Mb density, it’s perfectly sized for smaller Spartan-3 and CoolRunner-II designs.

XCF04SVOG20C Specifications

The XCF04SVOG20C offers four times the storage capacity while maintaining the same compact footprint. This makes it suitable for mid-range Spartan-3 and Spartan-3E configurations.

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XCFxxS Pin Configuration and Functions

Understanding the pinout is critical for successful board design. Both the XCF01SVOG20C and XCF04SVOG20C share the same 20-pin TSSOP package with identical pin assignments.

VOG20 Package Pinout

Critical Pin Descriptions

OE/RESET (Pin 6): This bidirectional open-drain pin serves dual purposes. As an input, pulling it low holds the address counter in reset and forces DATA into high-impedance. As an output, the PROM holds this pin low during internal power-on reset. Connect to the FPGA’s INIT_B signal for proper sequencing.

CE (Pin 2): Chip Enable controls whether the PROM actively drives data. When CE goes high, the device enters low-power standby mode. Many designs connect this to the FPGA’s DONE signal—once configuration completes, DONE goes high, disabling the PROM to reduce power consumption.

CF (Pin 8): The Configuration pulse output allows JTAG-initiated configuration without power cycling the FPGA. This open-drain output pulses low when the JTAG CONFIG instruction executes.

CEO (Pin 4): Chip Enable Output enables cascading multiple PROMs for larger bitstreams. When the first PROM finishes outputting its data, CEO goes low to enable the next PROM in the chain.

Configuration Modes and Timing

The XCFxxS series supports two FPGA configuration modes, both using serial data transfer.

Master Serial Mode

In Master Serial mode, the FPGA generates CCLK and the PROM responds with data on each clock edge.

Slave Serial Mode

In Slave Serial mode, an external clock source (typically from the FPGA in master mode) drives the PROM’s CLK input. The timing requirements are similar, but the FPGA controls the configuration pace.

Configuration Sequence

The typical power-up configuration sequence follows these steps:

1. Power supplies ramp up (VCCINT, VCCO, VCCJ)
2. PROM holds OE/RESET low during internal POR
3. PROM releases OE/RESET after POR completes (~200µs typical)
4. FPGA detects OE/RESET release and asserts configuration mode
5. PROM outputs serial data on D0 synchronized to CLK
6. FPGA receives bitstream and completes configuration
7. FPGA asserts DONE, which can drive CE high for standby

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Programming the XCF Series PROMs

Programming the XCF01SVOG20C and XCF04SVOG20C requires JTAG access and appropriate software tools.

Required Hardware

Software Options

Programming Procedure with iMPACT

The standard programming workflow using Xilinx iMPACT:

Step 1: Generate PROM File

• Open iMPACT and select “Create PROM File”
• Choose “Xilinx Serial PROM” for XCFxxS devices
• Select target PROM density (1Mb for XCF01S, 4Mb for XCF04S)
• Add your .bit file and generate .mcs output

Step 2: Connect and Detect

• Connect JTAG cable to target board
• Launch iMPACT and initialize chain
• Verify PROM detection (check device ID)

Step 3: Program Device

• Right-click detected PROM
• Select “Program” and browse to .mcs file
• Enable “Verify” option for production programming
• Execute programming sequence

JTAG Chain Considerations

When the XCF PROM shares a JTAG chain with the target FPGA, device ordering matters:

The JTAG IDCODE for XCFxxS devices follows this pattern:

FPGA Compatibility Matrix

Not every FPGA works with every PROM. Here’s the compatibility breakdown for the smaller XCFxxS devices:

For larger FPGAs like Spartan-3 XC3S1000 and above, you’ll need to cascade multiple PROMs or upgrade to the XCFxxP series.

Cascading Multiple PROMs

When a single PROM lacks sufficient capacity, cascading provides the solution. The CEO (Chip Enable Output) signal chains devices together.

Cascade Wiring

All D0 outputs connect together (directly or through isolation resistors) since only one PROM drives data at any time.

Design Security Features

The XCF series includes data protection mechanisms to prevent unauthorized bitstream extraction:

Once programmed and protected, the PROM will only output data through the normal configuration interface—JTAG read operations return all zeros or blocked responses.

Power Supply Design Guidelines

Proper power sequencing prevents configuration failures and potential device damage.

Voltage Requirements

Current Consumption

Power Sequencing

The recommended power-up sequence:

1. VCCINT and VCCO can ramp simultaneously
2. VCCJ should reach valid level before JTAG operations
3. All supplies stable before OE/RESET releases
4. Total POR time: ~200µs after supplies valid

Important: The I/O pins are 5V-tolerant only when the device is properly powered. Applying 5V signals to an unpowered PROM can cause damage.

Troubleshooting Common Issues

Configuration Fails After Programming

Symptoms: PROM programs successfully, but FPGA won’t configure

Causes and Solutions:

• Verify bitstream matches target FPGA
• Check VCCO level matches FPGA configuration bank voltage
• Confirm CLK signal integrity (no excessive ringing)
• Verify OE/RESET connection and timing

JTAG Chain Not Detected

Symptoms: iMPACT cannot find PROM in chain

Causes and Solutions:

• Verify VCCJ is within specification
• Check TCK, TMS, TDI, TDO connections
• Confirm no shorts on JTAG signals
• Try slower JTAG clock frequency

Intermittent Configuration

Symptoms: Configuration works sometimes, fails randomly

Causes and Solutions:

• Check power supply ripple and noise
• Verify adequate decoupling (0.1µF minimum per supply pin)
• Examine signal integrity on D0 and CLK
• Consider temperature effects if near limits

Useful Resources and Documentation

Official Xilinx/AMD Documentation

Download Links

• AMD Documentation Portal: docs.amd.com
• DS123 Datasheet: Search “DS123” on AMD documentation site
• ISE Design Suite: Required for iMPACT (legacy download)
• xc3sprog: sourceforge.net/projects/xc3sprog

Distributor Resources

Frequently Asked Questions

Can I use the XCF01SVOG20C with Spartan-6 FPGAs?

The XCFxxS series was designed primarily for Spartan-3/3E and earlier devices. While you might get it working with Spartan-6 in some configurations, Xilinx recommends using SPI flash (like the S25FL series) for Spartan-6 designs. The Spartan-6 configuration interface expects different timing and protocols that the XCF series doesn’t fully support.

What’s the difference between VOG20 and VO20 packages?

Both are 20-pin TSSOP packages with identical pinouts. The “G” suffix indicates lead-free (RoHS compliant) solder ball composition. The VOG20 uses matte tin (Sn) finish, while the older VO20 used tin-lead solder. For new designs, always specify VOG20 to ensure RoHS compliance.

How do I cascade XCF01SVOG20C with XCF04SVOG20C?

You can cascade different density PROMs, but place the smaller device first in the chain. Connect PROM 0 CEO to PROM 1 CE, and wire both D0 outputs together. When generating the PROM file in iMPACT, specify both devices and their densities—the tool will split the bitstream appropriately. Remember that total capacity equals the sum of individual PROM sizes.

Why does my PROM show as “secured” and won’t erase?

If the PROM was programmed with security bits enabled, you’ll need to perform a full chip erase before reprogramming. In iMPACT, try the “Erase” operation first, which should work even on secured devices. If that fails, some devices have a “Bulk Erase” or “Override” command that clears all protection. Check UG161 for the specific unlock procedure for your device revision.

Is the XCF04SVOG20C still in production?

As of recent AMD documentation, the XCFxxS series has reached end-of-life status. While stock may still be available through distributors, new designs should consider alternatives like SPI flash memories with appropriate configuration logic. For legacy designs requiring drop-in replacements, check with authorized distributors for remaining inventory and potential last-time-buy options.

Conclusion

The XCF01SVOG20C and XCF04SVOG20C Platform Flash PROMs represent a proven solution for Xilinx FPGA configuration storage in countless legacy and production designs. While newer FPGAs have moved to SPI flash and other configuration methods, understanding these devices remains essential for maintaining existing systems and supporting legacy hardware. Whether you’re troubleshooting a configuration problem or designing a replacement board for an older system, the fundamentals of XCF series operation—proper power sequencing, JTAG programming, and cascade configuration—will serve you well.

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Complete guide to Xilinx XCF series Platform Flash PROMs including XCF01SVOG20C and XCF04SVOG20C specifications, pinouts, programming with iMPACT, FPGA compatibility, and troubleshooting tips for configuration memory designs.