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Mar 16, 2010
The STM requires a communication protocol to exchange commands and data with the operator interface on the host PC, and will be known hereafter as ‘STMbus’. Request and Response Commands and data are encoded into formatted message frames and sent via TCP/IP over a dedicated ethernet link. Communication between the STM and host PC consists of pairs of messages, a request message and a response message. Either party may initiate a request, but every request must have an acknowledgement response from the other. Message Frame Format I have decided to base the Message Frame on ASCII-mode MODBUS (with a few... Continue reading
Posted Dec 25, 2009 at Andrew Starr's STM Project Journal
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The PCB has arrived and Sierra have done their usual high-quality job. I'll be loading the PCB manually, except for the switchmode IC U3. This is a DFN package, and can only be soldered with tools that I don't have. Fortunately a friendly contract-manfacturing company that my employer uses will do it as a favour - yay! I'll be posting more pics of the loading of this PCB soon. Continue reading
Posted Oct 27, 2009 at Andrew Starr's STM Project Journal
Ok, got a simple network service running on the micro board. All it does is listen on port 8080 for incoming connections, and when there is one, echos back text strings that the client sends it. So now I need to add some code to the Java applet to open a socket, send some strings, and check they're echoed back. Then I can start thinking about a format for the STM comms protocol. Continue reading
Posted Oct 25, 2009 at Andrew Starr's STM Project Journal
Here is a PDF of the schematic and PCB of the piezo amp. I'm getting a little ahead of myself with the electronic design - time to turn my attention back to the software while I wait for the HV power supply PCB to arrive. I have a basic 'hello world' Java applet already, now I need to add functionality for opening a network socket back to the microcontroller board. Continue reading
Posted Oct 15, 2009 at Andrew Starr's STM Project Journal
The heart of the amplifier is a LTC1650 16-bit DAC driven in unipolar mode with a precision 3.0V reference. The voltage output is fed to a pair of low-noise op amps (OPA452) in a differential configuration with a gain of 11. The amp should have a full-scale output of 66V. The waveform data is sent to the DAC via an optoisolated SPI interface. The optoisolation, combined with the isolated supplies, should ensure that there are no ground loop issues, or excess noise pickup from the microcontroller board. Datasheet for LTC1650 Download PDF of PIEZO AMP Rev A Schematic I'll post... Continue reading
Posted Oct 13, 2009 at Andrew Starr's STM Project Journal
I've finished laying out the PCB. PDFs of the PCB and Schematic can be downloaded here. I get my PCBs made by Sierra Circuits (www.protoexpress.com), using their No touch service. Their Auto File Verification system catches most silly mistakes, there's no setup charge and the quality is always excellent. While the PCB is being made (2-3 weeks), I'll be working on the design of the Piezo driver amp. Continue reading
Posted Oct 7, 2009 at Andrew Starr's STM Project Journal
L1,2,3 changed to Wurth Elektronik 744773122 (22uH, 1A) D1,2 changed to Motorola 1N5819 (40V, 1A) 12V DC supplied by Tracopower TEN5-2412WI isolated DC-DC converter module LED to indicate when power has been applied to the DC-DC converter module These changes have been made to reduce the layout size of the switchmode and to eliminate the need for a separate PCB for the isolated 12V supplies in the prototype rig. These changes have no (simulated) effect on the performance of the supply. I'll post an updated schematic along with the PCB, once completed. Continue reading
Posted Sep 30, 2009 at Andrew Starr's STM Project Journal
This is the complete schematic for the piezo amp HV power supply. It is supplied with 12V fom an isolated DC-DC converter module (not shown). The 12V is regulated by a LM317 down to 9V to avoid stressing the LT3471. The LT3471 generates +/-37V which is in turn regulated via linear regulators (LM317 and LM337) down to +/-35V. Download PIEZO AMP HV PSU Rev A Continue reading
Posted Sep 25, 2009 at Andrew Starr's STM Project Journal
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The STM needs a supply of high positive and negative voltage to power the amps that drive the piezo tube. I have chosen a switchmode design to avoid the need for mains power (with its obvious hazards) and large transformers (magnetic fields). The particular switchmode IC I have selected switches at 1.2MHz, so switching transients are at high frequencies that can be easily filtered. The design is physically compact and high current loops are very small. I have incorporated many similar supplies in designs with sensitive analogue circuitry and have invariably found their performance to be excellent. The power supply... Continue reading
Posted Sep 22, 2009 at Andrew Starr's STM Project Journal
I now have Web and FTP services working concurrently under the uC/OS-II kernel. They are using the serial flash as a file store (FAT16). I also have an 'LED blinker' task running at the same time to function as a 'heartbeat' so that I know everything is fine with the kernel while I'm testing Web and FTP. The RCM5700 has successfully served a 'hello world' Java applet to my browser - the next step is to have the applet make a socket connection back to a listening task on the RCM5700. Continue reading
Posted Sep 20, 2009 at Andrew Starr's STM Project Journal
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As mentioned previously, the STM will need a file store from which to serve user interface web pages to a requesting client. As the Dynamic C development system does not currently support partitioning the program flash for the use of the FAT file system library, I have had to add an external flash chip. An examination of the RCM5700 schematic, and of the module itself, revealed that Rabbit have made provision for adding a serial flash chip to the module, but have simply not loaded it for the RCM5700. Presumably, a future Minicore module based on the 5000 series will... Continue reading
Posted Sep 20, 2009 at Andrew Starr's STM Project Journal
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Here's a block diagram of the STM, showing the major functional parts of the system. Continue reading
Posted Sep 17, 2009 at Andrew Starr's STM Project Journal
The following is a summary of the major design decisions I have made so far. Each section will be covered in detail in future posts as the project progresses. Mechanical construction Scanner I've chosen to use a 'tube' piezo with quadrant electrodes; this seems a mechanically compact and simple solution (compared to tripods, bimorphs etc.). It also seems to be by far the most popular choice in other STM designs I've seen. Coarse approach The problem of bringing the sample into tunnelling range of the tip without adding excessive vibration or mass to the system seems ideally suited to a... Continue reading
Posted Sep 17, 2009 at Andrew Starr's STM Project Journal
This site documents my efforts to design and construct a Scanning Tunnelling Microscope. It is intended to be a resource for others who have a similar interest. The design goals of the project are: an instrument capable of: Atomic resolution at room temperature and in ambient air (with a suitable sample). Surface modification and lithography experiments. All waveform generation and feedback control to take place in the digital domain. Microscope hardware to be of a compact and easy-to-operate design. Microscope operation to be highly automated. Current Task (as at 26/12/2009): Writing a comms framework for the firmware and loading the... Continue reading
Posted Sep 14, 2009 at Andrew Starr's STM Project Journal