STM32F407 - should Vcap1/Vcap2  have supply voltage if unprogrammed? - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: September 29, 2014, 09:52:35 pm
content: I've got an STM32F407 (LQFP100) I'm trying to program on a custom board.I can't get any reading from the CPU at all. It's not getting warm, and Vcap1/Vcap2 measure 0.0V each. No shorts to ground/Vcc on them.Pinout appears correct - it matches the Discovery board.On my Discovery board, they measure 1.25V each. But maybe they are only available after programming to take it out of some kind of low power mode? Can anyone confirm?

author: dannyf
author_rank: Super Contributor
timestamp: September 29, 2014, 11:01:04 pm
content: What does the datasheet say about what should be attached to them?They are called V***cap*** for a reason, I suppose.

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2014, 07:44:18 am
content: I've attached 2.2uF ceramic capacitors to them as recommended in the datasheet, this also matches the discovery board.

author: Kjelt
author_rank: Super Contributor
timestamp: September 30, 2014, 01:26:18 pm
content: What voltage rating are the caps, are they X7, and do they fullfill the ESR requirements of the datasheet ?The Vcap c's are used by the internal regulator, if there is no voltage on them I suppose the internal regulator is off, no idea what is causing that, maybe the controller does not startup?

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2014, 07:23:57 pm
content: X7R, discovery board uses similar ones but 1206, it says zero ohm ESR is OKEven if they were wrong, I should see oscillation but I see nothing.I'm left thinking I killed the chip, maybe during soldering. I've only got one more, so will try that...These chips are pricey £7.50 each and ST does not offer free samples to students

author: Kjelt
author_rank: Super Contributor
timestamp: September 30, 2014, 08:31:25 pm
content: Hmmm would look a bit further first if i were you.Check all other voltages, reset pin correct? and osc pins for clock and also check ic orientation, sure pin1 is pin1?

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2014, 09:35:45 pm
content: Reset voltage doesn't matter, high or low, no change. Programmer drives it correctly anyway. Pinout correct on all pins that should matter, such as programming SWD CLK/DATA, NRST and BOOT0. Yes it's the correct way.No shorts on Vcap1/2 either - about 7k to ground. Discovery board is 4k. Seems within reason though, doubt this is highly controlled parameter.

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2014, 09:51:45 pm
content: QuoteMany ready-made boards aren't that much more expensive than that.QuoteUnwise of them.QuoteIf so, chuck it up on bad luck and move on,Seriously, debugging is about finding bugs where you least expected.And in this endeavor, you have made it quite difficult for others to help you.

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2014, 10:16:18 pm
content: Probably pretty wise of them. I'm just going to be a loss on their books for the time being. Obviously I'm not going to be involved in purchasing large quantities of ST chips for 5+ years, but in the future it might influence my decision to design them in. We'll see.What additional information would you like? Here's a schematic of my MCU section, if it helps. (rename to *.ps and open in Adobe or Evince reader.)The PCBs were £21 for 10, so far spent £125 at Farnell on this project... ST sent email after I put in Farnell order telling me they would not ship the items. (I wish they'd rejected me immediately if they had to. I waited 3 days then put the Farnell order in thinking they'd shipped them. That cost me additional shipping charges.)Yeah, debugging is usually the best part of engineering - but it's just frustrating when you come to this weird black box and can't figure out if it should be doing what it is. At least I have a near reference - the STM32F4 discovery board - to work from.

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2014, 10:35:57 pm
content: QuoteThink about a complete stranger coming to you for help. He said: "my circuit is correct; my PCB is correct; everything I have done is correct; just that my circuit doesn't work. Help me please."What do you think you would do?You would say "go pound sand!": if everything he did is correct, it is just bad luck and there is nothing anyone could do to help.In reality, there is obviously something wrong. But if he isn't going to help you help him, well, time to move on.

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2014, 10:43:02 pm
content: I guess I see your point, I could have been more specific. What I am asking for is non obvious points.Such as, what is the rise time on your 3.3V supply? I had a board that would not work under blah uS/V with an STM32F4... (I've checked this too, though)My original question was relating to Vcap1/Vcap2 voltages and if anyone has had experience with these being low before programming.I've since put a new processor on, but it's late now, so I'll try it tomorrow. Work 9-5 now, but don't get back until 6.30pm, so development time is limited.

author: tom66
author_rank: Super Contributor
timestamp: September 30, 2014, 11:00:46 pm
content: New chip on. Got warm when Vcap2 had short across it. Removed power and short, applied power again. Not warm, no Vcap voltage.I think it's fried, simple soldering error. Ouch. (The original chip never got warm at all, there were no soldering errors.)This is why I stopped soldering late at night.I'm reluctant to buy another chip though, until I am sure they are the fault.Think I'm migrating back to Tiva MCUs... I don't like these STM32s much!

author: dannyf
author_rank: Super Contributor
timestamp: September 30, 2014, 11:54:37 pm
content: QuoteIt means soldering isn't for you.Before powering you, you should inspect for shorts / open circuits. After that, measure for shorts - between Vdd/GND.After that, put a current limiting resistor on the power line to power up the circuit first - that way, if there is a short, it wouldn't damage the chip. The size of the current limiting resistor will depending on the load / current consumption of your chip. Your chip will likely consume 10 - 20ma normally. With 3.3v Vdd, I would put a 110ohm or so resistor there -> your chip may not run with it in but it allows you to see how much current is being drawn. If that number is much higher than expected, stop and inspect.Having said that, you may check out those discovery or nucleo boards. The TIVA boards are really nice too - very feature rich.

author: Kjelt
author_rank: Super Contributor
timestamp: October 01, 2014, 07:51:40 am
content: ouch bad luck. To your defense a 407 is not the first easy step to take, even board layout could become an issue with high frequencies although probably not the cause here. I personally find the STM32F407 discovery a very cheap board for €15.- a piece, i do not understand that they can make them for that amount of money but if it is too big for your use than that is a shame. Soldering them can be a PITA you always have to measure if none of the pins bridged or even make decent contact to the board. Use a microscope and multimeter. I can not read your file, renamed to .ps but my adobe reader does not support it.

author: dannyf
author_rank: Super Contributor
timestamp: October 01, 2014, 10:14:46 pm
content: QuoteLikely that they are losing money on those things. They probably think of it as an investment into their brands and their future customers.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 02, 2014, 05:25:11 am
content: Quote from: tom66 on September 30, 2014, 11:00:46 pmI think the VCaps are decoupling for internal LDOs, so shorting them to ground might have severe consequences. The STM32F40x seem to be pretty hardy though. I've put 5V directly onto VDD a couple of times and still had the chips work afterwards.Tiva? Blech. Don't like 'em.