{"id":5416,"date":"2013-02-24T13:21:18","date_gmt":"2013-02-24T21:21:18","guid":{"rendered":"http:\/\/mightyohm.com\/blog\/?p=5416"},"modified":"2022-12-27T17:37:48","modified_gmt":"2022-12-28T01:37:48","slug":"simple-fix-for-bus-pirate-power-supply-oscillations","status":"publish","type":"post","link":"https:\/\/mightyohm.com\/blog\/2013\/02\/simple-fix-for-bus-pirate-power-supply-oscillations\/","title":{"rendered":"Simple fix for Bus Pirate power supply oscillations"},"content":{"rendered":"

\"Modified<\/a><\/p>\n

I picked up this Sparkfun Bus Pirate<\/a> a couple years ago (thanks to Free Day!) It sat in a box for most of its life until recently I needed a simple way to interface a PC to an I2C<\/a> device at work. \u00a0Out came the Bus Pirate.<\/p>\n

I’m fortunate to have an Agilent MSO-X 3000<\/a> series scope at the office. This scope includes I2C decode capability on both analog and digital channels. I had been having some trouble with corrupt data coming in on the digital channels, so before connecting the Bus Pirate I switched to using analog channels 1 and 2 to decode I2C clock and data instead. <\/p>\n

After sending a few test packets with the Bus Pirate, I noticed something weird – the logic high level had a tiny sawtooth waveform riding on it. Not good.<\/p>\n

I zoomed in and saw something like this:<\/p>\n

\"without<\/a><\/p>\n

That 15kHz sawtooth is almost 150 mVpp!<\/p>\n

After removing my device under test, I quickly narrowed the problem down to noise on the Bus Pirate’s 3.3V rail. I initially suspected that this might be some kind of noise coming from the 5V USB supply, so I added an additional 10uF cap in parallel with the 1uF already on the board (C1). The sawtooth increased drastically in amplitude and went down sharply in frequency. That’s when I realized that this wasn’t noise, it was a power supply oscillation<\/strong>.<\/p>\n

The Bus Pirate uses a Micrel MIC5025-3.3YM5<\/a>\u00a0regulator (VR3 in this schematic<\/a>) to create the 3.3V rail from USB’s 5V. In the datasheet, there is a familiar warning: “Ultra-low-ESR capacitors can cause a low amplitude oscillation on the output and\/or underdamped transient response.” The Bus Pirate uses a ceramic cap for C1, I’m not sure of the type but it could easily be NPO\/C0G and quite low ESR. Adding another 10uF ceramic cap lowered the ESR (and SRF) of the parallel combination, bad news for supply stability.<\/p>\n

Realizing that low ESR of ceramic caps was likely part of the problem, I took a different approach. I went around the lab and found the crappiest cap I could find, a 1uF\/50V electrolytic, and put that across C1 instead.<\/p>\n

After adding the new cap, this is what the 3.3V rail looks like:<\/p>\n

\"with<\/p>\n

No more oscillation. This is with the original C1 still in place (see the photo at the top).<\/p>\n

I don’t think Sparkfun has changed the design of their Bus Pirate clone in the years since I purchased mine, so if you’ve got one of these on your bench, throw a scope on the 3.3V DUT supply (after enabling it) and see what you find. You might be surprised.<\/p>\n

Unless I am mistaken, a very similar regulator & bypass cap is used on all<\/strong> Dangerous Prototypes v3 and v4 Bus Pirates<\/a> (including clones), so this is something to watch out for on any Bus Pirate on the wild.<\/p>\n","protected":false},"excerpt":{"rendered":"

I picked up this Sparkfun Bus Pirate a couple years ago (thanks to Free Day!) It sat in a box for most of its life until recently I needed a simple way to interface a PC to an I2C device at work. \u00a0Out came the Bus Pirate. I’m fortunate to have an Agilent MSO-X 3000 … Continue reading Simple fix for Bus Pirate power supply oscillations<\/span> →<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[5],"tags":[461,463,462,168,464],"class_list":["post-5416","post","type-post","status-publish","format-standard","hentry","category-electronics","tag-bus-pirate","tag-capacitor","tag-oscillation","tag-sparkfun","tag-stability"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_shortlink":"https:\/\/wp.me\/pioCd-1pm","jetpack_sharing_enabled":true,"jetpack_likes_enabled":true,"jetpack-related-posts":[{"id":5807,"url":"https:\/\/mightyohm.com\/blog\/2015\/02\/cool-tools-adafruit-edition\/","url_meta":{"origin":5416,"position":0},"title":"Cool Tools: Adafruit Edition","author":"Jeff","date":"February 9, 2015","format":false,"excerpt":"SMT\/SMD Resistor and Capacitor Books This\u00a00805 SMD resistor and capacitor book\u00a0is just like the ones sold on eBay and just as useful. 0603's are here. 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Tantalum cap to the rescue!","author":"Jeff","date":"March 1, 2013","format":false,"excerpt":"This week I finally received my Bus Pirate v4\u00a0from Seeed Studio. The first thing I did after opening the box was to probe the 3.3V rail with the BP internal power supply enabled. Here's what I saw: That looks familiar! \u00a0Except this time it's an oscillation at 5.9kHz and 400mVpp!\u2026","rel":"","context":"In "Electronics"","block_context":{"text":"Electronics","link":"https:\/\/mightyohm.com\/blog\/category\/electronics\/"},"img":{"alt_text":"Bus Pirate v4 3.3V rail oscillation","src":"https:\/\/i0.wp.com\/mightyohm.com\/files\/flickr\/8519611367_0e791f659c_o.png?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1547,"url":"https:\/\/mightyohm.com\/blog\/2009\/03\/arduino-mega-hack-a-day\/","url_meta":{"origin":5416,"position":2},"title":"Arduino MEGA Spy Shots on Hack a Day","author":"Jeff","date":"March 17, 2009","format":false,"excerpt":"Hackday posted the first image of the new Arduino MEGA platform, shown above.\u00a0 According to Eliot, the MEGA features \"12 PWM connections, 36 Digital I\/O, and 16 analog inputs... 4 hardware UARTs and an I2C bus as well.\" Arduino MEGA - Hack a Day.","rel":"","context":"In "Microcontrollers"","block_context":{"text":"Microcontrollers","link":"https:\/\/mightyohm.com\/blog\/category\/microcontrollers\/"},"img":{"alt_text":"","src":"https:\/\/hackadaycom.files.wordpress.com\/2009\/03\/arduino_mega.jpg?w=350&h=200&crop=1","width":350,"height":200},"classes":[]},{"id":1318,"url":"https:\/\/mightyohm.com\/blog\/2009\/02\/sparkfun-stencil-and-solder-paste-class-notes\/","url_meta":{"origin":5416,"position":3},"title":"Sparkfun Stencil and Solder Paste Class Notes","author":"Jeff","date":"February 12, 2009","format":false,"excerpt":"Yesterday, Sparkfun had a solder paste stencil class at their HQ in Boulder, CO. @esawdust was there and took lots of great photos and notes about the class: Sparkfun Stencil and Solder Paste Class | PCB, Solder Paste | sawdust.see-do.org Wish I could have been there, it looks like everyone\u2026","rel":"","context":"In "Books and Resources"","block_context":{"text":"Books and Resources","link":"https:\/\/mightyohm.com\/blog\/category\/books-and-resources\/"},"img":{"alt_text":"Sparkfun Electronics Solder Paste Stencil Demo","src":"https:\/\/i0.wp.com\/www.mightyohm.com\/blog\/wp-content\/uploads\/2009\/02\/sfesolderpastestencil08.jpg?resize=350%2C200","width":350,"height":200},"classes":[]},{"id":1858,"url":"https:\/\/mightyohm.com\/blog\/2009\/05\/my-new-oscilloscope-the-agilent-dso1014a\/","url_meta":{"origin":5416,"position":4},"title":"My new oscilloscope – the Agilent DSO1014A","author":"Jeff","date":"May 26, 2009","format":false,"excerpt":"Over the Memorial Day weekend I had a chance to spend a little bit of time with my new Agilent DSO1014A oscilloscope. 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