Unobtrusive Debugging for Actel AX and RTAX-S FPGAs

1. An Unobtrusive Debugging Methodology for Actel AX and RTAX-S FPGAs Jonathan Alexander Applications Consulting Manager Actel Corporation MAPLD 2004

2. Logic Design Challenges • Designs often don’t work the way they were intended to the first time • Non-Device Issues • Signal Integrity • VIH/VIL • Ground/Vcc Bounce • Cross Talk • Termination • Edge rates • Power supply noise • Assembly • Solder shorts • Component orientation • Component alignment • PCB Design or PCB Manufacturing • Spacing rules • Shorts/Opens • Device Issues • Timing Problems • External setup/hold • Clock skew • Cross-clock domain paths • Software/Timing model bug • Device speed (faster or slower than expected) • Device Problems • Damage due to electrical overstress (EOS) • Defect • Packaging

3. Debugging Challenges • Non-Device Issues • Signals can be directly probed on a PCB • Power supplies can be probed • Resistance can be measured • Components can be replaced • JTAG tests for continuity • Must have test points and test headers for access to signals • Device Issues – ASIC • Custom test vectors offer high coverage for the specific design implemented • Test points and test blocks built into device • Limited access to internal nodes • Long and expensive re-spins

4. Debugging Challenges (Cont’d) • Device Issues – Reprogrammable FPGAs • Device can be reprogrammed to access internal node activity • Debuggers are available for pre-determined node access • Manufacturers have very high test coverage and can retest devices • Re-place and route required to view different nodes • Timing issues very difficult to detect due to requirement of a new place and route • Device Issues – Axcelerator FPGAs • Manufacturer has very high test coverage for production screen • Built-in probe circuitry gives access to virtually every net in the design without additional programming or redesign • Design-specific test vectors are needed for manufacturer failure analysis

5. Axcelerator Design and Debug Flow

6. Probe Setup • Silicon Explorer 2 • Serial port connection between PC and Probe header on board • 100MHz asynchronous sampling • Multilevel triggering • Four internal probe channels • 18 total logic analyzer channels (4 may be used for internal probing) • Requires 5V or 3.3V power. Power can be taken from the PCB (~1 Amp required) or from supplied power converter. • 18 X 64K sample buffer

7. Axcelerator Probing Setup

8. Axcelerator Probe Circuitry • JTAG Test Access Port (TAP) used for control interface • Silicon Explorer connects to JTAG TAP to designate XY coordinate of cell to observe. • Cell output is transmitted to one of four available probe output pins. • Dynamic Internal Node Access • Nodes can be selected and changed while device is in full system operation • Selecting a node has no impact on design performance Registers Control Registers

9. Axcelerator Cell Probe Selection Example

10. Silicon Explorer Logic Analyzer 4 1 3 2

11. Silicon Explorer Logic Analyzer 1. Probe Control • This section shows what signal each probe is assigned to • This section will also shows what the Checksum of the device is, allowing the user to verify that the device has been programmed with the correct design 2. Node Listing • This section shows all the nets/nodes that can be probed 3. Waveform Viewer • This is the window where all waveforms captured by the Silicon Explorer II are displayed. 4. Menu • This is where all the controls are located • It allows manipulation of the waveforms

12. Probe Performance • Maximum observable signal speed • Worst case RTAX-S simulations show 100MHz signals can be observed without distortion • Typical case RTAX-S simulations show that up to 150MHz signals can be observed without distortion

13. Probe Guidelines • The Silicon Explorer gives access to internal nodes through XY coordinates • Built in logic analyzer can be used to view signals at 100MHz sample rate • Oscilloscope or other logic analyzer can connect to probe outputs to view signals with higher resolution • Measuring delays • The probe circuit is not designed to accurately reflect internal delays. Only logic states and timing approximations should be considered • Errors due to timing can be observed such as hold and setup violations on a flip flop.

14. Probe Guidelines • Design Tips • Reserve the probe pins in Actel’s Designer software. This will prevent the probe pins from being used as IOs • Avoid assigning probe pins as inputs or bi-directionals. If the pins are needed for IO, use them only as non-critical outputs • Avoid assigning JTAG pins as inputs or bi-directionals. If the pins are needed for IO, use them only as non-critical outputs • Do not program the security fuse in the FPGA. This will disable the probe circuitry in the device. • The probe circuitry allows four simultaneous internal signals to be monitored with a maximum of two signals per tile. • 70 Ohm series resistors are recommended on every probe connection

15. Conclusion • Real-time observation of internal nodes allows you to Find: • Timing violations • Logic errors • Large glitches • Un-obtrusive probe circuitry means: • No need to re-place and route design • No additional delay added to design when probing • No FPGA logic gates needed