I have a self-designed PCB using an AtMega6450A which will not program. I am using the AVRISP programming module and Atmel Studio 7.
I have several of these PCBs, some program OK, a few don't. The problem scenario is as follows. Starting with a virgin 6450A soldered to the board, I can successfully read the voltage (5.1 volts) and the device signature. Next, I program various fuses (EESave, CLK/8, ClkOut, etc.), but I am still running on the internal oscillator. All of this works fine. The fuses verify, the signature reads flawlessly as many times as I care to query it. Next, I engage the external crystal (7.3728 MHz), which is also part of the PCB assembly, by selecting the appropriate oscillator option fuse. This is Ext. Crystal Osc 3-8 MHz with the longest start-up time option (65msec). On 2 out of 3 of my PCBs this works fine and I can proceed to change the interface speed and program the Flash without an issue. However, on 1/3 of my PCBs the ISP communication with the chip stops right here.
The device signature always reads back as FFFFFF and Studio 7 posts the error window reading: "Unable to enter programming mode. The read device ID does not match the selected device or any other supported devices."
I know about the problem where the oscillator is mis-selected and the IC becomes unresponsive to Studio. This is NOT that problem. The crystal is oscillating very nicely on my O-scope. Plus, I programed the Ext Clock Out fuse (having experienced various clock issues in the past) and the oscillator clock appears on the designated Clk Out pin ( Pin 9) - even though Studio can't read the signature. So, I know I have a viable clock.
The boards were soldered by a very competent craftsman, and I have partially assembled them to make sure the 6450A is working before committing too many other components to the PCB. So, the 6450 is virtually unconnected to any external hardware except the ISP connector ( a 2x3 header) and the crystal and associated caps, 0.01 bypass caps at each Vcc & Gnd pin pair, and two 2.2 uF tantalums near to the 0.01 ceramic bypass caps. AVcc (pin 100) is connected to 5 volts, Aref (pin 98) is bypassed to ground with a 0.01 ceramic cap. I have buzzed out the ISP/SPI connections and looked at these signals on the O-scope. None seem to be dead or stilted.
I suspect there is something unique with the ISP interface on the 6450A. Perhaps a control/GPIO pin I am not connecting, perhaps a fuse setting that is not appropriate, maybe a spate of bad 6450's coming thru the distribution channels ( the came from DigiKey). There are several other questions posted on this topic. I have studied them, but it seems they were resolved as simple hardware problems (bad connections and the like).
Have any Freaks experienced this, or similar, syndrome with the 6450A, or other AtMega models, and how did you resolve the problem? Also, any suggestions will be greatly appreciated.