Altera_Forum
Honored Contributor
Honored ContributorCyclone IV nCONFIG pin is not stable
Hello everybody,
I have a strange hardware problem and hope somebody can help me. I made two Cyclone IV testing boards. Both have the same problem: After power on, everything seems ok. The nCONFIG pin is stable at about 2.9 V. My VCCIO = 3.0 V for all banks. After some time, about several seconds to several minutes, the nCONFIG pin voltage collapses to a much lower level. At first it was around 0.7 V. Moreover, this voltage is not stable. It jumped between about 0.7 V and 2.5 V randomly and quickly. After a lot of searching and trying we think that it should lie in the bad reflow soldering process, meaning that the FPGA chip is not well soldered at some pins, or there might be a weak and not continuous short circuit (contacting R = about several KOhm) between nCONFIG pin and the nearby GND pins. Therefore we sent the two boards back to the soldering compnay and they heated the boards again with a longer time and the highest acceptable temperature for the FPGA. After the re-heating, it gets better: the nCONFIG voltage jumps now between about 2.0 V and 2.8 V, sometimes as low as 1.5 V, but not yet reaches the digital low level. So the board can work, meaning we can configure the FPGA with JTAG and program the EPCS chip. Also before the re-heating, the FPGA could be configured via JTAG and EPCS before the nCONFIG voltage collapsed. Now it can be configed at any time. But I am afraid after some time, the nCONFIG voltage might drop and below the logic low threshold. After a detailed analysis again, I think the nCONFIG pin has a low resistance to the GND. Maybe it is a dirt contacting the pins and causes a contact resistance now around 20 k. The reasons that I come to this thought are the following phenomena besides what I described above: 1. The voltage only goes low. 2. The voltage is always good when the chip is cold. If the chip is getting warm (the temperature is still below 30 °C), it takes much less time, sometimes only 5 - 10 seconds, that the nCONFIG voltage collapses. Maybe the higher temperature causes some expansion and makes the "short circuit" resistance to be contacted. What is strange is that both boards have almost the same problem. If it is because of a dirt underneath the chip, it should be highly impossible that such random located dirts could cause the same problem in two boards. I also checked my circuit to see if my circuit has problems, but cannot find. In order to isolate the possible problems, for one board at first, I only had the FPGA and the most basic components (power supplies, clock circuit, JTAG, EPCS chips) on the board to ensure the basic operation of the FPGA. Several other irrelavent chips (SRAM, ADC, etc.) are also present. The nCONFIG pin has ONLY a 10k Ohm pull-up R connected to the VCCIO. The problem is still there! In the Altera user's manual, it says that nCONFIG keeps low before all power supplies are good. I measured all three power supplies. They are all good: VCCINT = VCCD_PLL = 1.2 V with about 20 mV peak-to-peak ripples VCCA = 2.5 V with about 10 mV peak-to-peak ripples VCCIO = 3.08 V with about 30 mV peak-to-peak ripples The FPGA type is EP4CE115F23C8N, 1.2V core. The clock is 50 MHz generated with an silicon clock oscillator. For the configuration, I use JTAG and AS. MSEL[3:0] = '0011'. Does anybody know this problem? Can anybody help me? What could this problem lie in? I appreciate for any hints! Michael
