Hi All
Our tiny Aussie start-up (two greybeards - well and truly) has spent the past 14 months designing and prototyping a modular T&M instrument driven by
two Raspberry Pi RP 2350's.
It provides a DSO, LSA and other useful stuff. It is expandable with attachable
Application Specific Front End pods.
Open source architecture written in Micropython. Signal capture by the Pico 2, display and control by the Pico 2W (also provides Wifi for smart phone/PC integration).
This hardware separation, compartmentalises software development and user customisation (see below).
Full details at
picotronix.com that will be a Kickstarter project shortly - about 130 USD. A brief overview and some design tips for the RP2350 are shared below.
Hope you find it useful. Please feedback with comments, suggestions, questions, and yes, criticism.
Almost forgot. Note to Dave: If this happens to catch your eye, glad to get you a test unit. Will not do too well underwater!!. May survive delicate hammer tap.
Reckon it has a fair chance of passing your customary rigorous and impartial road test of its stated specs.
Picotronix includes the base
Picotronix Command Module that provides a reasonable range of functions to meet common requirements of makers and engineers.
Working on a couple of education oriented pods to enhance use as teaching tool.
The Command Module uses our analog input circuitry and the native capability of the Pico 2 and Pico2 W to capture and display signals:
- Two Analog Inputs (0-3V or 0-16.6 V (ac or dc) conditioned to 0-3 volts. Programmable triggers and levels on both signals.
We’ve characterized Picotronix for a clean 20MHz bandwidth across the full 0-3Vpp range (Students: remember to check the input range when comparing bandwidth of a scope). - Real Time Sampling (RTS) 100 kHz input signal (at 5 samples per cycle for nice tracing)
- Equivalent Time Sampling (ETS) 10MHz 3dB for periodic signal (see down a bit, for how we can subsample at this rate using RP2350 500kSPS).
- Heavy lifting after this is left to the attachable pods
- Eight LSA protected inputs (plus the two analog inputs, if configured to trigger on logic transitions). Another 4 digital lines that are not protected, are best reserved for use with the 12 bit LSA pod.
- Accurate Frequency Counter (accuracy was essential for subsampling).
- Voltmeter and Logic Probe
- Three expansion ports for Application Specific Front End pods (picoPods) enable the unit to be configured to handle a diversity of user configurable (just plug in what you need) functions.
It has two Analog Ports for modules to condition a range of input signal. The Multifunction Port connects 12 RP2350 GPIO lines to an attached pod, as well as outputting the two conditioned analog that also feed to the inputs to the RP2350 ADC.
The pods for the Analog port include a range of single ended modules and two differential pods. The multifunction pods include a
100MSPS MSO Pod (with other useful things),
12 input LSA pod that includes
level shifting on the inputs that are arranged in groups of 4, each configurable for distinct families, and a
Waveform Generator pod.
The above pods use signals generated by the Command Module or received and conditioned by the module. Pods may be designed to receive signals directly (for example, if requiring an increased bandwidth) that essentially use the Command Module for display and interfacing with an external device. Also, pods unrelated to T&M may be designed as an extension to the RP2350.
The image below was taken by the excited other half of the team after 3d printing the first enclosure for the system - so eager to capture for posterity that he forgot to remove the evening's dinner pumpkin from the table, before snapping the pic!
Some stuff that may be of use in other Pi Applications to save you time from our moments of frustration:
- DMA: We coud not work out why we kept getting lost data when trying to DMA from the Input buffers connected to the state machines, until the penny dropped that the 150MHz system clock does not apply to the DMA. It seems restricted to 25 million words (32 bits) a sec. Might be buried somewhere in the data sheet, but it escaped us.
- Capture window of the RP2350 ADC. The RP2350 ADC is limited to 500kSPS. Our analog front-end uses high performance TI Op Amps that have a slew rate that results in a clean sine wave at 20Mhz at a full 3vpp signal. After some tedious experimentation, we figured that the capture window for the 2350 ADC must be in the range of 10-20ns. This does not appear in data sheets but seems to be a stable range. Feedback on this would be really helpful. In combination with a refined algorithm for timing the high and low states of the input signal without jitter, we can determine the frequency of the signal very accurately and sub sample a periodic wave up to 10MHz with 3dB attenuation. After this signal quality drops and is pretty useless at 17MHz.
Design RationaleBy splitting the design into two major subcomponents each controlled by a Pico 2, we achieved two major objectives.
- The first was to confine signal acquisition to the Pico 2 that could precisely maintain jitter free timing without the typical interrupts required to service display, control, and comms with external devices. These functions are performed by the Pico 2W. Our Picogram Data Protocol standardises data transfers between the Pico2 and Pico 2W, and between the Pico 2W and an external device (e.g., smart phone or PC).
- The second objective was to enable software to be developed more or less independently for each sub-component. We have also developed a set of signal emulators that may be loaded to the Signal processor. This permits software development for the Pico 2W without the need to actually apply signals to the device, enabling testing of various routines to process and display the signal.
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