[sdiy] Many SPI slave devices on the same PIC master BUS..

Brian Willoughby brianw at audiobanshee.com
Tue Mar 2 02:42:59 CET 2021


p.s. I almost always design for SMD, and I jump right to a fabbed PCB for the first prototype. However, with an unanswered question like "number of devices supported on a single SPI bus," I'd be tempted to design with optional jumpers and a buffer/driver chip so that both approaches could be prototyped with the same board, simply by varying the assembly. Each signal path could have a couple of SMD jumpers to allow for direct or buffered options. You might not be starting out with a PCB right away, so you may not face this particular challenge.


> I don't think there should be a strict limit. Any fan-out issues could be handled by buffer/drivers on MOSI and SCK, although MISO would be difficult unless the drivers were tri-stated by the same Chip Select used for each chip.
> 
> I seem to recall designing with as many as 3 SPI slave devices on a PIC, maybe even 5.
> 
> I think you'll want to 'scope all three bus signals to look at the edges, and to decide whether a boost from a driver is needed.
> 
> Which PIC part number are you using? The data sheet should have a chapter on the pin electronics specific to SPI.
> 
> Brian
> 
> 
> On Mar 1, 2021, at 16:55, Jean-Pierre Desrochers wrote:
>> Did anybody used in the past as much as 6 x SPI slaves devices
>> driven by the same micro master bus (MISO, MOSI, SCK)?
>> Obviously one at the time and each of them having their own separate Chip select pulse ? 
>> I cannot see anywhere if there is a limit for SPI slaves sharing
>> the same bus (loading, fan out..)
> 



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