Good point. I will make use of the enable lines to avoid such glitches. So, I will actually be using 4 GPIOs instead of 3.
In order to select individual slaves, I'll do the following: 1. decoder enable pin will be driven low (all SS outputs high) 2. slave is selected by 3-bit decoder select pattern 3. decoder enable pin will be driven high after all decoder select bits are set Thanks, Robert On Monday, December 4, 2017 at 4:06:08 PM UTC-8, Graham wrote: > > You will also probably need some kind of "enable" line. > You can't guarantee that all three lines are going to transition between > states at exactly the same time, so it is possible that you can get a > transient invalid select while they are changing. > This may or may not be long enough to confuse what you are controlling. > Most 3 to 8 line decoders have some kind of auxiliary enable input. > > --- Graham > > == > > On Monday, December 4, 2017 at 5:02:43 PM UTC-6, Robert M wrote: >> >> Hi all, >> >> I plan to interface with 8 SPI slave devices using the BBBlue along with >> the RoboticsCape libraries. >> >> I plan to use a 3-to-8 line decoder (CD74AC138E) so that I will only need >> three GPIO pins to access the slave select of each device. >> >> Has anyone done anything similar to this before? I presume some form of >> bit-banging will be necessary, though I'm not yet certain what code I need >> to modify to make this possible. >> >> It looks like modifying rc_spi.c >> <https://github.com/StrawsonDesign/Robotics_Cape_Installer/blob/master/libraries/serial_ports/rc_spi.c> >> >> might be my best bet. In particular, it looks like i would need to modify >> the switch statements in *rc_manual_select_spi_slave()* and >> *rc_manual_deselect_spi_slave()*, as well as the gpio_ss array >> initialization in *rc_spi_init()*. >> >> The gpio_ss array currently uses BLUE_SPI_PIN_6_SS1 and >> BLUE_SPI_PIN_6_SS2 GPIOs for slave selects. Will I also be able to use the >> pins on the GPIO headers to control slave selections? >> >> >> Thanks, >> Robert >> >> -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to beagleboard+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/21259c01-9554-4601-ad58-0c62fe09ad47%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
