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STM32F405 cannot connect using SWD

Question asked by martin on Mar 28, 2014
Latest reply on Apr 1, 2014 by nouha
I'm having the same problem as the one [DEAD LINK /public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/Flat.aspx?RootFolder=/public/STe2ecommunities/mcu/Lists/cortex_mx_stm32/STM32F4%20-%20No%20target%20connected&FolderCTID=0x01200200770978C69A1141439FE559EB459D7580009C4E14902C3CDE46A77F0FFD06506F5B&currentviews=1486]discussed here, i.e., I cannot connect to the stm32F405 using SWD on a board I built. The strange thing is that it was working correctly and then suddenly stopped. Thinking that I may have somehow damaged the stm32, I replaced it but the  symptoms are still the same. I'm using the stlink on a discovery F4 (fw version V2.J19.S0), it works correctly for the stm32 on the discovery board.

What I tried / verified

- The ST-LINK jumpers on the discovery F4 are off
- All power supply pins are at 3.3V
- VCAP1 and VCAP2 are at 1.2V
- VBAT is connected to 3.3V
- BOOT0 is connected to GND, BOOT1 is floating
- NRST goes high after powering on the device
- The SWCLK and SWDIO signals reach the pins (tested with oscilloscope)

Ideas for problems that could be causing this:
- Power supply rise time may be too slow. It takes about 800us to reach 3.3V when powering on the device. NRST goes high approx. 1.5ms after applying power.
- The current board revision only uses 100nF decoupling capacitors, i.e., the 4.7uF cap on VDD and the 1uF VDDA are missing. This will be fixed in the next revision. BTW: is it fine to use 2.2uF on VDDA instead of 1uF?
Any tips on what else to test are highly appreciated.