Using SPI pin for general I/O

Martin King · Joined: 03 Mar 2005 Posts: 12

I recently had an issue with Portb.4 on a Mega16. I have a motor running between two soft limits (reed switches) every so often the motor would "glitch" as if one of the limits was being hit and occasionally it would miss one of the soft limits (always the same one) completely and hit the hard limit shutting the system down. Turned out that the affected soft limit was connected to Portb.4 which is one of the SPI lines, moving everything to Portc solved the problem, but I could do with using PortB for general I/O so I guess I have to disable the use of port B for SPI somehow?

Martin.

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

SPI is only enabled if you use the CONFIG SPI = HARD command in Bascom, else it is disabled by default on startup. As long as SPI is not enabled, you should be able to use the port for general I/O.

Be aware some development boards insert resistors into the SPI lines since they are also used for ISP programming. Again, as long as you disconnect the ISP interface, you can use those lines as general I/O.

Martin King · Joined: 03 Mar 2005 Posts: 12

>Be aware some development boards insert resistors into the SPI lines since
>they are also used for ISP programming.

I did wonder about that but B.4 is the SS line and isn't used for programming. It could simply be that that pin is borked, when I get a chance I'll swap it out for a new chip, unfortunately it's now running an automated test which would be a real pain to abort and re-start.

Cheers.
Martin.