MCF5216 - Quick SPI Chip Select question

[dreilly]

I have a quick question for you all about chip selects for the ColdFire's QSPI block. (We are going to be using a 5216.)

We are in the process of designing a prototype board with four SPI peripherals (taking up all the available chip select lines), but we also want the ability to add peripherals at a later time. We want to avoid having to add hardware to decode the Chip Select lines, but we do have some spare GPIO available.

Is it valid to drive an SPI access with no Chip Select lines active? We would have to hook up the Chip Select line on our additional part to a GPIO line, of course, and drive it active outside of the QSPI block itself. But if an access like this is valid, we can hook up the parts we are using to the four chip select lines, and worry about expansion later (as long as all required signals are accesable on the board, of course).

Thanks in advance for your help!

--
Alban Edit: Please always mention part number in subject line.


Message Edited by Alban on 2007-07-09 04:52 PM

[Kremer]

  Yes, since the commands you added to command RAM have the QSPI_CS field equal to 0x0F (no chip select lines active).

 Also no problem in using GPIO´s to chip select other devices on the board.

 Also take a look on CSIV bit on QWR to avoid selecting other devices connected to those not driven chip select commands.

 You can do almost everything with this QSPI, but you cannot, for example, drive a device with one of the selected CS lines (and other 3 goes to inactive level defined by CSIV), and, after finishing the commands to this device, let it´s referenced CS line asserted where other 3 keeps like they were. The CS line of this device will go to the same level of those 3 others, defined by the CSIV, since the module isn´t executing any command. Looking on that way, using GPIO to chip select devices seems to give more flexibility.

 Regards

[dreilly]

Thanks for the information! This is exactly what I was looking for....