We have developed our own PCB with an i.MX28, using type MCIMX287CVM4B. There are 128MB DDR2 RAM are installed, which are supplied by an external dc-dc converter, as well as NAND Flash, ethernet phy, 4.3" TFT display and other periphery, also supplied external. So, the internal dc-dc converters are used for cpu interfaces, JTAG and boot mode selection only. As OS we are running Windows CE 6.0.
When we put the first 24 boards into operation, we observed a strange behaviour of the internal DC-DC converter on 4 of these boards:
Case 1: The device is booting properly, but when Windows CE 6.0 switches the display mode to show the desktop, a reboot is performed, caused by internal dc-dc converter that stops switching for ~ 1ms. In consequence, VDDD drops from 1.55V to 1.15V, VDDA drops from 1.8V to 1.6V. After this pause the converter starts switching again, but all voltages rising too high (VDDD (cyan)=2.3V, VDDA (pink)=2.7V, VDDIO (green)=5.1V), while VDD4P2 (yellow) keeps 4.2V continously. It takes nearly 1 ms until all voltage rails have returned to its nominal values.
In picture 2 the pink graph shows the switching node at the external inductor, pin A16, instead of VDDA.
Case 2: The device is not booting at all, VDDD swings from 1.0V to 2.4V, VDDA swings from 1.8V to 2.9V and VDDIO swings from 3.3V to 5.3V.
In both cases we replaced the cpu with new parts and all boards were running properly. So, obvious there was no problem with our board but with some CPU's.
Did anyone of you observed the same behaviour? What could have caused these defects? A failure rate of 17% is too high to enter series production of our board with good feeling.
Although we have put another 15 boards into operation in the meantime without failure, we are still wondering about these 4 defects.
Message was edited by: Grant Whitacre Maik, your images did not come through so I reproduced your discussion entry with images intact. Regards, Grant