Minimum spec oscilloscope for rpi spi

Question

Hey, I'm fairly new to the electronics scene and was wondering what the minimum specs for an oscilloscope would be if i needed to look at raspberry pi spi transactions? thanks.

Problemchild · Answer

Yeah the problem is that been new he doesn't know what he needs and no doubt will get conflicting advice. I suggested the cheap logic analysers since they are far from perfect they do represent good effective value and not a money to kick out doing most things OK. That way he can learn exactly what he wants and still have plenty of toy budget for the next thing

Robert Peter Oakes · Answer

As SPI is only upto about 125MBits on the PI but typically way slower, then a 100Mhz scope should work, but if your coding it then you can select a slower SPI speed and therefor use a lower bandwidth scope. It depends what your trying to look at, if it is the actual waveforms then you will need 100Mhz min. If it is actually to protocol decode it and verify the operation then a logic analyzer may be more effective. the reason I say this is because at 100Mhz, if your trying to look at the waveform you will need probably nearer a 500Mhz scope in order to see the rising and falling edge speeds more accuratly, a 100Mhz scope looking at a 100Mhz square wave will show an attenuated sine wave due to being at the limits of the scopes bandwidth and the edges of a 100Mhz square wave is way faster than 100Mhz, even a 500Mhz scope will show some rounding of the edges Do you know the speed of the SPI but in your instance ?? another option if it is a SPI bus your looking at is a Logic Analyzer like a Saleae or similar like a bus pirate

michaelkellett · Answer

It depends on how fast you will crank the SPI clock - I'm not an RPi expert but a quick web search suggests it can do at least 32MHz clock but a lot of the time you wouldn't go this fast. To see logic states on the SPI you need a scope with 4x the clock bandwidth and to do signal integrity at least 10x the bandwidth an at least 20x the sample rate. But a basic scope isn't a lot of help with spi - what you really need is a logic analyzer - which you could buy stand alone (cheapish) or as part of a mixed signal scope - lots more money. Some of the nicer scopes offer SPI decoding (usually as a paid for extra) and with a 4 channel scope this can be pretty good. You can also get SPI only analyzers and this can be very cheap (compared with scopes) but is much more limited. I can't advise anything specific without knowing your budget and a bit more about what your need to do - mostly people expect a scope to do lots of things so if you can define some of the other things you would like it to do that would help. MK

Problemchild · Answer

I would suggest that since in this instance you are looking to track digital signals you may want to look at something like the Saleae logic 8/16 port Analysers which you can get clones of for a few pounds. I have several and they work very well they should be good for a few megs therefore OK for most SPI traffic if not at the ultimate speed. (Yes you can always get faster ones). Scope wise you could do much worse than the RIGOL1054Z which can also do serial logic decode at around 280 ish £

Robert Peter Oakes · Answer

There is a difference between Data Rate and Bit Rate, you can crank the bit rate upto the 125Mhz and it probably does exit the PI at that speed but only for that byte of data, the time it takes the code to loop around and grab the next piece of data will slow down the overall rate considerably resulting in considerable degration in overall datarate resulting in the figures arround 10 or 20Mhz, this actual result may depend on the activity the pi is doing during the transfer

gdstew · Answer

What Michael says is correct, especially for digital signals to get a reasonably accurate look at rise/fall times and other critical timing. I have an old (but nice) Tektronix TDS 1012 100MHz, 1 Gs/s two channel w/external trigger digital scope that I bought for exactly that reason. The main drawbacks to the scope is a small buffer (2.5K samples), and it is only two channels. 4 channel and 200 MHz (2 Gs/s) models also exist. The TDS 1012 was around $1200 USD when I bought it and I still consider it a good investment although I do wish now that I had sprung for the 4 channel 200 MHz model.

shabaz · Answer

Hi John,

From what I can tell, the processor peripheral goes to a very high speed (125MHz according to the BCM peripherals doc that was published), but the Linux driver may limit it (no idea) and then there is the general rule that any signal above 30MHz should always send warning alarms if it is apparent that they are constructed without care over routing those signals. I assumed no care was taken, because it routes to a typical DIL header, and no transmission line. (In a similar vein, any plastic breadboard design would be worriesome if designed for 30MHz+). With that guesstimate in mind, I nevertheless tested at 32MHz SPI clock (square wave, so needs more bandwidth than 32MHz for a nice shape) when I connected it to the XMOS startKIT board quite a while back. I don't have signal traces though : ( And perhaps there could be bit errors and unreliability. It might not be a very nice waveform. Probably higher values could be tried with experimentation (and attempts at minimising reflections), but going above 32MHz clock may not be very sensible without a lot of testing.

For the SPI communication bursts over a short length ribbon cable between the RPI and XMOS board, 32MHz 'worked' (i.e. it might not be reliable at all over a longer period, or some changing conditions, and a lower value may be more prudent - but it was so long ago, I only have screenshots of the logic analyzer capture from that time, I don't seem to have scope captures from then.