[avr-chat] Mega16L with SPI Flash communication problem.

[Zehao Chang]

I'm trying to read/write between a Mega16L and ST's M25P40 extenal  
Flash memory. Both are running at 3.3V.

When I do a write instruction, the serial output of the flash stays  
low, which is fine. But when I do a read instruction, the serial  
output pulses high and then exhibits capacitive decay, and does not  
look like a digital signal.  Here's what the output of the flash chip  
look like when there is no load (not connected to microprocessor). It  
seems to go high for the duration of the chip select signal and then  
decays exponentially:
http://www.zehao.com/temp/spiflash_miso.jpg

Here's the corresponding chip select signal (0 means chip is selected):
http://www.zehao.com/temp/spiflash_select.jpg

If I connect the output pin to the AVR, it becomes a flat ~2V signal.  
I've tried 3 different chips and they all do the same thing. Can  
anyone give me an advice or hazard a guess as to why this is happening?