SPI slave randomly missing bits in response

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I’ve connected an MCP3008 to a raspberry pi. First on a solderless breadboard, where everything worked correctly. Then on a prototyping board, where the only difference is that the chip is externally powered. The problem is now that the response from the slave is occasionally missing bits (the slave fails to pull the signal up for every bit that it should). Here is a series of readings taken at a 0.01 s interval:

[3.4, 3.39, 3.29, 0.0, 3.4, 3.4, 3.4, 2.55, 0.0, 2.98, 3.39, 0.0, 3.39, 3.3, 3.19, 3.4, 2.98, 3.4, 3.4, 0.58]

The readings are normalized for Vref = Vin = 3.4.

I’ve tried:

• Redoing the soldering,
• replacing the MCP3008,
• adjusting the SPI frequency (currently at 1.35 MHz),
• checking the connections and voltages using a multimeter
• using the piscope to examine the serial communication (tricky, because if I set the frequency low enough to get a good reading, the chip rarely returns any values at all)
• grounding unused inputs

The PCB has copper wire from the header to the chip for the SPI interface, about 5cm long. As far as I can see, that should be fine for SPI. SpiDev is set to use mode 0.

Any other ideas as to what might be going wrong would be hugely appreciated.

Update:

Connecting the Raspberry Pi ground to the external ground solved the issue, so it looks like the lack of common reference voltage caused the mcu to sometimes not pull high above the necessary threshold.adcraspberry-pispishareedit  follow  flagedited 1 hour agoasked 10 hours agoboileau10533 bronze badges New contributor

• 1Time for a real scope … – Tom L. 10 hours ago
• 2Decoupling caps? – DKNguyen 10 hours ago
• 1I have caps on all the inputs, same as on the breadboard. – boileau 10 hours ago
• 1@TomL. Yeah, I think I’ll finally have to bite the bullet and pick up an oscilloscope. But in this case it will only rule out bad wiring, I think. – boileau 10 hours ago
• 1@boileau It could show a noise issue, connection instability, or brownout. You said in your post that you’re missing bits, but your 3.4V readings show that you’re missing entire bytes. Which is it? – DKNguyen 9 hours ago 
• 1do you have components available to go back to solderless breadboard? … if you do have the components, then go back to a working version and switch to external power supply – jsotola 8 hours ago 
• 1@DKNguyen if an entire byte is missing, then bits are necessarily missing (8 to be precise). – boileau 1 hour ago

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1 Answer

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Question

The OP is testing MCP3008 with SPI speed 1.35MHz, and found results inaccurate. How come?

Answer

Update 2020aug07hkt1219

Now I am searching my old lab log to hopefull find something useful to suggest the OP to troubleshoot. I read my old schematic and found two things the OP might consider.

(1) Use separate analog grounds and digital grounds, as shown in the schematic below.

(2) Use a digital voltage reference with small series resistance, to maximize sample and hold capacitor current, so to prevent the not enough time filling up at high frequencies.

(3) Use one differential input channel, instead of single ended, to avoid noise problems. The three other not used differential inputs should be grounded, also to prevent noise, (ESD, back EMF) surge/spike problems.

(4) The schematic also reminds me that MCP3008’s Chip Select (CS) pin is also used to “Shut Down”. The Op might not be aware that there shutting down might need some time to wake up, otherwise the next conversion might not be accurate. This is just brainstorming, and I have not looked into this minor details for these particular hip, but I do have annoying experiences of not being aware that some chips’s initialization and waking up take a long time, in the order of milliseconds.

(5) One more thing is that the OP is powering MCP3208 with 5V. In this case Rpi 3V3 logic SPI signals should NOT directly drive 5V device. I almost always do not use directly Rpi’s 3V3 GPIO/UART/I2C/SPI signals. I always shift up Rpi’3 3V3 signals to 5V (using TSX/TBX0102/4/6/8)

I forgot if there is any other precautions things I need to observe. So I searched my reading logs and found the following:

Update 2020aug07hkt1149

I made a careless mistake in reading the datasheet, resulting a wrong conclusion. Please ignore this wrong answer. I will try to improve my answer later. My apologies.

Part A – Datasheet spec summary

A.1 – max data rate = 20ksps (Errata – should read 200ksps).

A.2 – conversion time = 3 SPI byte transactions, or 24 cycles.

Part B – Circuit analysis

B.1 – Example Case 1 : SPI Frequency = 1MHz, => conversion time = 24us.

B.2 – Max sps = 1/24us ~= 40kHz.

B.3 – Conclusion: 1MHz means 40ksps is too high for MCP3008 to handle.

Errata – Conclusion is wrong. Max sps of 200ksps should imply max SPI frequency around 4MHz

Part C – Troubleshooting suggestions

C.1 – Suggest to lower SPI frequency from OP’s 1.35MHz to 100kHz (Note 1) and test again.

Notes

N.1 – Spec says min frequency > 10kHz, or sample and hold cap leaks.

N.2 – Volt source Rs < 1k, or sample and hold cap input current too small to fill up in time.

N.3 – Using SPI frequency might have a same trouble as N.2 above: sample and hold cap does not have enough time to fill up.

N.4 – My always dodgy calculation is not proofread.

References

(1) MCP3004/3008 2.7V 4-Channel/8-Channel 10-Bit A/D Converters with SPI Serial Interface – MicroChip 2000feb

(2) MCP3008 for Rpi Tutorial – M Sklar, AdaFruit, 2019jul

(3) MCP3008 ADC readings not accurate Problem – tlfong01, rpi.stackexchange 2029may 22

Appendices

Appendix A – MCP3008 Operation

Appendix B – The OP’s missing bits in his MCP3008 ADC Conversion Results

I was curious which bits were missing. So I converted the decimals to binary to try to find a clue.

dec 3.40 = bin 11.01100110011001100110
dec 3.39 = bin 11.01100011110101110001
dec 3.30 = bin 11.01001100110011001101
dec 3.29 = bin 11.01001010001111010111
dec 3.19 = bin 11.00110000101000111101
dec 2.98 = bin 10.11111010111000010100
dec 2.55 = bin 10.10001100110011001101
dec 0.00 = bin 0.000000000000000000000

My thought for 15 seconds and jumped to the conclusion that the missing bits should be random, so I give up diving deeper.

End of answershareeditdeleteflagedited 17 secs agoanswered 5 hours agotlfong0142733 silver badges66 bronze badges

• 1Wow, thanks for the answer. It inspired me to connect the raspberry pi ground to the external ground. Doing that solved the issue! – boileau 3 hours ago
• @boileau, How nice to hear that you have found the solution. By the way, I appreciate very much the way you present the problem concisely but still with all the necessary helpful details. It has been a pleasure learning problem solving with you. Have a great project. Cheers. – tlfong01 11 mins ago   

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