Wednesday, 29 August 2018

BAW!! #5 Lets try a new IC (Part 2 Rushing to finish)

Sometimes I find its less about pretty and more about speed. As mentioned in the previous post the BMA400 is a brand new device not even available but having a board ready to receive it can be a big advantage of bring a product to market.


Certainly not the prettiest board I have designed and i was have some difficultly get the Silkscreen Graphics how I wanted it. 

But just over two days it was finished. I normally take the time to sit with a board just to make sure i haven't made a mistake. But there is one great big one on the silk screen.

The design files can be found at:

Feel free to download, clone and remix how you want :) I'd like to know what you get up too.



Sunday, 26 August 2018

#BAW!! Weeks 5 Lets try a new IC (Part 1 finding the data)

Still running quite far behind the Bubble watch from week 4 is still an in progress project I'll keep everyone updated as I get more work done on it.

Sometimes a change is as good as a rest so its now time to try a new approach. What about a new IC at this point i'm going to pick something new from Mousers website ( mouser is a component disributer I like their new product search )

First item i have found on the page is a new accelerometer from Bosch Sensortech according to the page it was shown of at the most recent CES and receiving awards for innovation. This is many due to its ultra low operating of around 8uA (super low) thats about 20 time lower than their other Accelerometers at around 150uA operating current.

Its always funny with brand new IC products the offer very little information to even Bosch-Sensortec's own page has very limited technical detail. 

These are the only two documents available at this moment in time:  

A nice give away is the schematic on the shuttle board document which shows how the device is decoupled, and give an idea of how its supposed to be connected as typical application.


Another thing to do is look at the previous generation of the device. At the moment Bosch manufacture the BMA280 which is the previous accelerometer device from them.

I took a look at the datasheets of other Bosch devices and discovered the register map for the device are for the most part identical. This will be a good reference from bringing up this device. 

Time to start making a symbol for the part. I have the Pinout.


I draw it into the KiCAD symbol editor I also find it useful to make sure the do not connect pins are drawn in they can be useful for reference later and making sure the footprint is correct. KiCAD can suggest the correct footprint based on the number of pins so it useful to have everything in front of you.

Time to work on a footprint for the part. I am using the datasheet for the BMA456 being one of the more recent accelerometers i can use the application notes to help draw in what I need for this board.    

schematic is now done leaving enough landings for pull up resistors and jumper links to select the different communication modes. The BMA400 supports both I2C and SPI protocols and there is also an I2C address select. 

  




Saturday, 11 August 2018

BAW!! #4 Bubble Watch Part 1 (Falling Behind)

It another week back into KiCAD but this week want to make a board using Retro Bubble Displays from HP.



These little displays are no longer made unfortunately but I have a few knocking around and really love them. They are  HP QDSP-6064 LED displays and where use in calculators around the 70's and into the early 80's. These displays really pretty and have wanted to do a project with them for long time.

So my idea is to try and do a watch, but better i want to get a modern MCU on there and have a debugging watch. something that is can use to send UART or I2C communications in a hurry.

There is one example of a bubble watch based on an the Texas Instruments MSP430, by solderbug.

I'd like something more capable than an MSP430 with an integrated RTC so i don't have to add more components than necessary. I'm looking at using on of the ST  Microelectronics STM32F030C8. I packs a lots of power with a Cortex M0 32-Bit, with lots of peripherals. I comes i'm at around the £1.50 mark which is pretty good for amount of functionality. My only concern is its size   in a 7x7 mm QPF package its going to take quite a lot of  board space.

So i don't have much room to play with if this was going to be a single sided board, I also have to give room for a power supply. I want to get away form lithium-polymer cells (LiPo). If i was to use one then I have to look at how do I protect the battery from over discharging and how do i fit in a charge circuit while not costing the earth.

I have decided to go with a more humble pair of LR44 coin cells and using a boost converter to bring the voltage up. I think i will have to do some clever sleeping with the MCU to get the best out of this, but as the boost converter will run down to 1.2V input I sound get the most out of this format. Also going to have the battery voltage monitored by the MCU so it can issue a warning to change the battery, and as it is two cells in parallel it will be possible to change 1 cell at a time.


The MCU has a an RTC that can be used to wake up from standby mode. I fact there is no reason that when the display is bing looked at that the device has to be in any run mode at all. 

This means the device and display could be drawing less than 10uA. :) (I'm this mode the device could run up to 11,000h hours).

So far at writing this I have the base power board routed and ready to go.

I unfortunately I am only just finished with routing the base/power board. I'm pretty happy with it, the LR44 cell holders are taking the majority of the board area but I have managed to find a little room for the boost converter.


My problems are to do with the display board. The number of through hole pins are making it difficult to put the board into a format that I'm happy with.

  
  I was hoping i could somehow fit the MCU directly under the display, and as yet have not room for any IO breakout to allow add on modules. 

I think my approach is to put SMT landings for the bubble displays and modify some DIP 12 sockets to be able to solder them onto the pads. I thank i'm going to take the same approach with the crystal and SWD header. 
After a very quick search it seems SMT DIP sockets are freely available. 

This is how far the project is a the writing of this blog I'm going to push ahead to try and complete it had hope that I can not fall too far behind with boards to follow. 

I think my problem is this was quiet a complex project to try and complete in a week. and with this design change its set myself back a little. What this project is teaching me is sometime a redesign is what is needed and I have to just get on with it.

I hope to follow up soon with this design finished. Please let me know what you think, any comments are always welcome thank you to everyone who has given me feedback. 

Wednesday, 1 August 2018

BAW!! #3 Into KiCAD and RabidCHARLIE

Board A Week, Week 3 Into KiCAD and RabidCHARLIE

When designing PCB's there several software packages to choose from, these can range from free Open Source options all the way up to full fledged highly integrated software suits. My normal choice for PCB design is Autodesk Eagle, the freeware version I use at home is a great tool but limited by number of layers and board size. More info in the different versions of eagle can be found here.

When starting Board A Week I said one of the things I wanted to try was to use other design tools on of the other design tools I wanted to really give some time to is KiCAD.

KiCAD is an integrated design suit with schematic editor, layout editor and 3D viewer, I have wanted to give KiCAD a try for many years but previous attempts have been unsuccessful. The unfortunate true when you have worked with a certain piece of software for years its becomes quiet difficult to learn a new way of doing things, and consequent have returned to eagle where I'm familiar.

So lets jump into it :


I started of by you throwing down a few parts into the schematic editor and trying to understand how the schematic editor relates to the layout editor using the classic 16 x 2 Character display and an ATMega32u4 MCU. I was thinking along the lines of making some kind of industrial looking watch.

I got to a point that I realised this board would be far too complex to complete before the week was up, especially as I was still getting used to how the KiCAD operates.

I have left my first design as I left it in the GitHUB Repo I don't believe i'm deleting failed attempts as they help me to learn from my mistakes.

So in the end I restarted the board and idea to something useful and simpler a board with Charlieplexed LEDs. Charlieplexing  is a method of multiplexing LEDs by using there diode properties (only allowing current to flow in one direction. This little board could be good for testing IO speed on different devices.


KiCAD  has a very particular workflow that must be adhered to when going between working with the schematic and working with the board design.

  1. Work/Create/Modify Schematic
  2. Assign Foot prints to parts
  3. Export Netlist (important this is done after assigning footprints)
  4. Open Board layout 
  5. Import Netlist (make sure the import options are set correctly whether you want to over write footprints or remove parts that have been removed from the schematic etc.)
  6. If something is wrong repeat from step 2      



The routing tools are much the same airwires are draw between connected pads and you have to connect them with wires. In KiCAD the airwires are always up to date as well, in eagle unfortunately you have to keep hitting the restnest button to get an up-to date render of the airwires which can get annoying when trying to layout the parts in a convenient way.   



The silk and artwork import tools are amazing they import as a library item making them easy to move around.

  

The Gerber export was really straight forward and had valid Gerbers at the click of a button. Then wrapped them up in a zip file and put them into the Eurocircuts board viewer. Perfect!!!

All in all really happy with using KiCAD its powerful and precise and i think there is a lot more to learn with it. Next weeks project involves making a custom part so some new skills to look at with the software.