netpp node [Spartan6-LX9]
I am glad to announce a new user evaluation platform module called ‘netpp node’. Its motto is ‘IoT on FPGA done right’:
- Integrated high speed UDP stack
- dagobert network SoC with various configureable interfaces (SPI, UART, PWM, I2C) and pin multiplexing options.
- SDK: GCC, GDB hardware debugger
- Up to 6 analog I/O channels with ‘analog population option’
- Piggy backs onto a custom I/O PCB using two 2×16 pin headers
- In-Field upgrade option (reprogramming of user images with fallback scenario)
The default firmware runs a fully functional netpp stack for remote control and measurement.
Its main applications:
- Reliable computing (safety relevant applications with tamper-safe main loop watchdogs)
- Guaranteed real-time response in network for scaleable applications (100s of units). Performance outlines:
- Up to 1200 netpp continuous property requests per second verified
- Push-on-Demand streaming of arbitrary dataports (high speed ADC) at maximum network bandwidth (10 MByte/s)
- Simple DSP applications and smart analog measurement (low power, filtering and differential options)
- Evaluation of next generation ZPU architecture for embedded GNU style developers
First prototypes are finished and are running 24/7 in the test bench at this moment.
Things are going very smooth so far, just a minor capacitor change will be required for the v0.1 series.
- netpp node fact sheet [ PDF: main-netpp-node ]
For analog I/O, U3 on the board is by default populated with a MSP430G2553, functioning as a smart ADC that is controlled from the Dagobert SoC via i2c. All relevant ADC configuration registers are directly accessible via netpp. For instance, we access the low level registers through a process browser panel as shown above to play with the parameters. The process view panel automatically updates the volatile properties from the netpp peer device. The ADC10 variant of the netpp node provides up to six analog channels internally sampled at up to 200ksps. When in synchronous acquisition configuration (SPI master), only five channels can be used.
Differential 16 bit sigma-delta ADC
The alternate population option with a MSP430F2013 provides a Sigma-Delta 16 bit ADC with differential inputs and programmable gain amplifier. This variant provides three different input channel configurations using the provided analog input pins on this board. Moreover, the internal temperature is available in a separate channel.
‘Push on demand’ data streaming
By default, the analog sensors are polled, i.e. a measurement value is delivered upon request by the master. For synchronous sampling however, a ‘push’ strategy might be desired, where a netpp node delivers a value stream to a data logger or database. This can be netpp (where the netpp node acts as a master), however for high speed data transfers (‘network scope’), a low overhead UDP stream is more desirable. The dagobert SoC features a data port option with programmable slots to stream I/O channels as well as analog values using a standard real time protocol with 90 kHz time stamps.
Monitoring netpp packet performance
Packet behaviour in a real network is measured using the Wireshark protocol analyzer.
The figure below shows some example netpp transaction log that the netpp node handles at a very low CPU overhead based on direct register accesses.The red bars is the effective number of query responses using somewhat ineffective ping-pong requests. The performance can be increased by accumulating data into larger buffer properties.
For i2c or SPI transactions however, the packet rate is expected way lower.
For high speed performance like MJPEG video streaming, a separate UDP/RTP queue can be set up within the firmware to reach maximum throughput. However, there is no handshaking using this method.
The image below shows a repeated property query from within Python. The pauses are introduced by external disturbance (stress test) that causes a packet drop – and the netpp engine to timeout and re-synchronize.
Improved RX/TX queue
With an improved packet FIFO on FPGA, I was able to crank up the number of netpp requests per second, as shown in the Wireshark trace below. This test makes sure that several netpp clients can poll the netpp node at high frequencies without disturbing each other. The blue trace is a repeated poll of the full property tree, the red bars are the timed queries from a process viewer daemon. With no other disturbance, we get the occasional drops (e.g. at 45s, 101.5s) due to the queue running full
The default boot loader firmware supports self-programming over the cable. That means, the netpp_node can be supplied remotely with a new firmware image via a simple upgrade procedure over netpp. If the uploaded image is faulty, the system will fall back to the default boot loader. However, if the new design itself has errors, the system will be unable to recover unless the reset button is pressed.
As the full model of this design is available for simulation, we can verify the system effectively against stress situations. In particular, network safety is of outmost importance. The test procedure check list of the dagobert SoC:
- ARP and ping flooding
- netpp packet performance test
- Broken packet handling
- Lost interrupt scenario (packet queue desynchronization)
- Jumbo packet flooding