AMD technical article 007
Spartan 7 FPGAs
How a low-cost 28 nm FPGA turns a configuration bitstream into custom logic, memory, arithmetic and I/O for embedded control and signal processing.01
What it is
Spartan 7 is AMD’s small 28 nm field-programmable gate array family for embedded control, industrial interfaces and moderate signal processing. Unlike a CPU, it does not execute a fixed instruction set as its primary function. A configuration bitstream defines many small logic elements and the wires between them, creating a custom parallel circuit after power-up.
The family spans roughly 6,000 to 102,400 logic cells. Every device combines configurable logic blocks, distributed and block memory, DSP48E1 arithmetic slices, programmable I/O and a dual 12-bit analogue-to-digital converter. It has no hard application processor and no multi-gigabit serial transceivers; a MicroBlaze CPU or protocol controller can instead be built from programmable resources.
ProcessTSMC 28 nm HPL1.0 V or 0.95 V core
Logic range6K–102.4K cellsSix devices
Hard resourcesBRAM + DSP + XADCMemory, arithmetic, analogue
External I/OUp to 400 pinsUp to 1.25 Gb/s LVDS
02
Why use an FPGA
A microcontroller processes instructions through a fixed datapath. An FPGA lets the designer create the datapath itself. Multiple counters, interfaces, filters and state machines can therefore run at the same time with deterministic clock-level timing.
| Requirement | FPGA advantage | Cost or constraint |
|---|---|---|
| Precise I/O timing | Signals are captured and generated by dedicated parallel logic. | Timing closure must be proved for the implemented design. |
| Several protocols | Independent hardware blocks can operate concurrently. | Each block consumes finite logic, memory, routing and pins. |
| Custom arithmetic | Bit width and pipeline depth match the algorithm. | Design and verification are more involved than writing ordinary software. |
| Field updates | A new bitstream can replace the hardware function. | Configuration storage and secure update handling are required. |
03
Programmable architecture
04
What happens from power-on
- Load configuration.The FPGA reads a bitstream through a selected mode such as serial flash, JTAG or a processor-controlled interface.
- Authenticate and decrypt.When enabled, the configuration engine applies AES-256 decryption and SHA-256 authentication checks.
- Configure resources.Bits define lookup-table truth tables, register behaviour, routing switches, I/O standards, clocks and block settings.
- Release start-up.After configuration checks and clock sequencing, global reset is released and the implemented circuit begins operating.
- Process in parallel.Independent logic pipelines handle inputs on every clock without a central instruction scheduler.
- Reconfigure when required.A replacement bitstream changes the circuit; supported designs may use partial reconfiguration for selected regions.
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Device range
| Device | Logic cells | DSP48E1 | Block RAM | Maximum I/O |
|---|---|---|---|---|
| XC7S6 | 6,000 | 10 | 180 Kb | 100 |
| XC7S15 | 12,800 | 20 | 360 Kb | 100 |
| XC7S25 | 23,360 | 80 | 1,620 Kb | 150 |
| XC7S50 | 52,160 | 120 | 2,700 Kb | 250 |
| XC7S75 | 76,800 | 140 | 3,240 Kb | 400 |
| XC7S100 | 102,400 | 160 | 4,320 Kb | 400 |
Package limits apply. The number of bonded I/O pins depends on the selected package, not only the silicon device. Migrating within a package family still requires pin, bank-voltage and timing checks.
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Principal capabilities
| Resource | Published capability | Use |
|---|---|---|
| Logic fabric | Six-input LUTs, flip-flops, carry chains and distributed RAM. | Boolean logic, state, counters and small memories. |
| Block RAM | 18 Kb and paired 36 Kb configurations; up to 4,320 Kb. | FIFOs, frame buffers, coefficient stores and processor memory. |
| DSP48E1 | 25 × 18 multiplier, adder/accumulator and pipeline registers; up to 160 slices. | Filters, motor control, image processing and fixed-point arithmetic. |
| I/O | Up to 400 pins; 1.25 Gb/s LVDS; 3.3 V support on selected banks. | Sensor, converter, memory and board-level interfaces. |
| External memory | Soft DDR3/DDR2/LPDDR2 controllers; documented DDR3 rates up to 800 Mb/s. | Capacity beyond on-chip BRAM, implemented with fabric and I/O resources. |
| Analogue | Dual 12-bit, 1 MSPS XADC with on-chip sensor channels. | Power, temperature and external analogue monitoring. |
| Security | AES-256 bitstream decryption, SHA-256 authentication, eFUSE and Device DNA. | Protects configuration confidentiality and device identity when correctly provisioned. |
| Embedded processor | MicroBlaze soft core, over 200 DMIPs in documented configurations. | Runs control software while custom logic handles deterministic datapaths. |
07
Design and verification flow
| Stage | What the tools do | Evidence required |
|---|---|---|
| Describe | Capture RTL in VHDL/SystemVerilog, assemble IP or use higher-level synthesis where appropriate. | Simulation and interface-level functional tests. |
| Synthesise | Convert behaviour into LUTs, registers, BRAM and DSP resources. | Resource report and warnings reviewed. |
| Place and route | Choose physical sites and programmable connections. | Timing constraints met across all defined clock domains. |
| Generate bitstream | Encode the routed design and selected security options. | Configuration mode, voltage and boot-image checks. |
| Validate hardware | Exercise the implemented circuit on the target board. | Signal integrity, clock-domain crossing, reset and fault tests. |
| Maintain | Version source, constraints, IP, toolchain and bitstream. | Reproducible build and controlled update path. |
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Terms
- FPGA
- An integrated circuit whose logic and interconnect are configured after manufacture.
- LUT
- A small memory implementing a Boolean truth table.
- CLB
- A configurable logic block containing LUTs, registers and carry logic.
- BRAM
- Dedicated on-chip block memory.
- DSP slice
- A hardened arithmetic block for multiplication and accumulation.
- RTL
- A hardware description of registers and data transfers.
- Bitstream
- The binary configuration image loaded into the FPGA.
- Timing closure
- Proof that routed signal paths meet clock constraints.
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Limits of the evidence
- Logic-cell counts are vendor-normalised capacity figures, not physical LUT counts.
- Maximum I/O, memory rate and LVDS throughput depend on device, package, speed grade and board design.
- Spartan 7 contains no hard serial transceivers; high-speed serial protocols may require external components or a different FPGA family.
- Peak DSP rates assume suitable pipelining, placement, arithmetic width and clock speed.
- Security features are effective only with correct key storage, boot policy and update procedures.
- Product longevity through 2040 is AMD’s published plan, not a guarantee of every package or speed grade.
- The embedded video and playlist are third-party educational material, not specification sources.
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Sources
- AMD Spartan 7 product pageFamily scope, applications and longevity.
- Spartan 7 product briefDevice range, resources, I/O and system features.
- DS189: Spartan 7 data sheetElectrical, switching and timing specifications.
- DS180: 7 Series overviewLogic, BRAM, DSP, I/O and package tables.
- UG470: 7 Series configurationBoot modes, bitstream loading, security and start-up.
- Xilinx 7 Series FPGA Deep Dive (2022)BYU Computing Bootcamp video supplied for this article.
- Xilinx playlistbonfireprocessor playlist supplied for this article.