Electrical – SPI max distance on flexible circuit

flexiblepcbpcb-designspi

I want to run a number of sensors up the length of a flex circuit, they all communicate via SPI and I will run chip selects individually to each device.

I need the flex circuit width to be minimal so its likely I will have tracks in close proximity running the entire length of the flex circuit.

Im concerned with how this will affect the SPI comms with increasing length.

EDIT: The SPI will run at a maximum speed of 1MHz, data will be active on both MOSI and MISO, One mircoprocessor will sit at one end of the flex circuit as master, comunnicating with the chain of sensors.

I'm not looking for an exact figure just ball park, so that i can decide o na length to test with first.
This answer SPI max distance gives the max length for cables, but i think that this is not applicable to that of a flex circuit.

How long can I go with the SPI on a flex circuit?

Best Answer

A bi-directional SPI bus (master -> slave -> master) is the most problematic to deal with. Consider the much simpler master to slave comms - both data and clock remain in time-sync on their route to the slave.

However, when the slave transmits back to the master it does so by syncing to the delayed clock it receives from the master. That delay is due to the propagation time through the channel (cable, PCB etc..). Then there is the extra delay of the data coming back i.e. two lots of propagation delay and the master might end up trying to receive data that has shifted a significant part of one clock cycle.

With a 1 MHz clock, the period is 1000 ns and if the received data from the slave has shifted more than about a quarter of this time (250 ns) then expect problems.

The velocity of propagation of free space is 300,000,000 m/s or, put differently, 3.333 ns per metre. Cable may double this figure and I wouldn't expect this to be any worse for a PCB so assuming a worst case of 7 ns per metre, you could run 250/7 metres and not hit delay problems. That's nearly a 36 metre round trip or 18 metres of cable.

I'm not saying that signal levels will be totally adequate after 18 metres but that 18 metres should not produce a show-stopping round-trip delay.

There are also reflections to consider and the rule of thumb is that they become significant if the cable length approaches one-tenth of the wavelength of the highest relevant frequency. What frequency you might ask? Well the clock is 1 MHz and it contains harmonics that are usually regarded as important up the the seventh so, the frequency in question is 7 MHz. This has a wavelength in free space of about 43 metres and if run in a "slow" cable this might be 20 metres.

For this reason (and the one-tenth rule of thumb) I wouldn't want to go further than 2 metres without paying some attention to line terminators. They can be added to SPI lines without too much difficulty but, if you don't want them then I'd say 2 metres is your limit.

Related Solutions

Electronic – Arduino SPI limit

The number of SPI slaves is not limited. In fact, you can add digital muxes (multiplexors) and control more slave devices than you have digital pins on the Arduino.

However, the SPI was design for communication over short distances within a box. So, the physical size of the bus can become a problem (bus capacitance, EMI). If you have to make a long-distance ruggedized SPI, there are application notes on the subject: Extending the SPI bus for long-distance communication.

Electronic – SPI multiple slave termination

SPI allows the so called "Daisy-Chain" and multi-slave topology. You seems to need multi-slave. Note that altough you will need a separate signal for the CS pin of every chip, you can share the MOSI/MISO/CLK pins between your nodes.

For your parameters, I think that a proper routed board will suffice since you are not working at high clock/signal speeds.

I never heard of SPI pins being terminated. You must indeed take care of grounding and avoiding short-circuits.

Note also, that I2C looks like a more appropriate solution for connecting several devices in a BUS (altough with less speed).

You can have your setup like this:

HOST_MOSI -> DEVICE1_MOSI, DEVICE2_MOSI, ..., DEVICEn_MOSI
HOST_MISO -> DEVICE1_MISO, DEVICE2_MISO, ..., DEVICEn_MISO
HOST_CLK -> DEVICE1_CLK, DEVICE2_CLK, ..., DEVICEn_CLK

HOST_PIN1 ->DEVICE1_CS
HOST_PIN2 ->DEVICE2_CS
  ...
HOST_PINn ->DEVICEn_CS

You might want to read link 1.

Best Answer

Related Solutions

Electronic – Arduino SPI limit

Electronic – SPI multiple slave termination

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