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the SS pins on the microcontroller are just standard IO pins so you can enable internal pull-ups providing your SPI devices are active low.(most SPI devices are active low but there are exceptions.)
Hmmmm but when I have an ISP programmer actively programming my AVR, I can't have internal pull-ups enabled, right?
The programmer does not use any SS pins. Only the MISO, MOSI, SCK, RST, GND and Vcc pins.So I guess you should be fine. Anyway, the pull ups are enabled only when the pin is set as an input pin. If the pin is in special function mode, the pin is set by that function as needed.
You can daisy-chain only the simple SPI I/O devices. Things like EEPROM won't work and need individual chip selects. And external resistors to disable the chip selects when the device is powered, but still in reset.
ISP connects to reset, MOSI, MISO, and CLK. The SPI devices should ignore MOSI/MISO/CLK when disabled.
Quote from: GearMotion on February 25, 2010, 06:41:27 AMYou can daisy-chain only the simple SPI I/O devices. Things like EEPROM won't work and need individual chip selects. And external resistors to disable the chip selects when the device is powered, but still in reset.you can use internal pull-ups on the chip selects (SS) too.
Quote from: GearMotion on February 25, 2010, 06:41:27 AMISP connects to reset, MOSI, MISO, and CLK. The SPI devices should ignore MOSI/MISO/CLK when disabled. again,this is only true on *some* AVRs.for example, i'm working on the ATmega2561 at the moment which does share CLK but uses completely different pins for MOSI and MISO.(i keep repeating this because i wasted a day once trying to workout why i could not program an AVR using the "normal" SPI pins...)
Again: You need to make sure that the SPI devices are disabled during ISP. Don't rely on the internal pull up during reset.
And external resistors to disable the chip selects when the device is powered, but still in reset.
Anyone see a problem with using 3kohm for the SPI CS pullup pin, or is that potentially too strong?