Are you an amateur bit by the Time-Nut bug but can’t afford a commercial time interval counter with sub nanosecond resolution and a GPIB interface? Did you find a universal counter on eBay with good time interval resolution but became frustrated figuring out the GPIB interface and writing a command script to get the counter data into your PC so you could analyze it? Are you an experienced Time-Nut tired of tying up multiple expensive commercial counters over extended periods comparing multiple frequency standards to GPS just to establish traceability? The Simple PICTIC is a low-cost, high-resolution, 10-digit interpolating time interval counter with 9600-baud RS-232 serial port designed for GPS monitoring applications to free up your expensive commercial counters for other uses.
The Simple PICTIC sends 10-digit BCD data to a PC using a simple ASCII terminal program to display the data, log the results for analysis, and configure the counter using @@Xy Motorola style serial commands. An 8-IC package thru-hole evaluation board for amateur construction is provided incorporating the core functions, time to voltage interpolators, and automatic calibration system used in the PICTIC module. It has no display or power supply, is 3.8” x 2.5” and easily incorporated into other projects, uses time to voltage interpolation to drastically reduce the hardware required for ease of assembly, and can be built by an amateur for about $50. The Simple PICTIC doesn’t include accumulation, averaging, or statistics modes, input signal conditioning, polarity or trigger point selection, or start channel detection. It requires TTL compatible inputs triggering on the rising edge and a small delay between the start and stop events and between samples for proper operation.
The user can vary the resolution of the Simple PICTIC during construction to suit their needs by installing an optional prescaler, selection of the XO speed, the sampling capacitor value, the interpolator charge current, and the buffer gain and offset resistors. A simple half-can 5v TTL XO is used as the on-board timebase with provisions for connecting and selecting an external TTL timebase. A Simple PICTIC using a 10 MHz XO timebase, no prescaler, and a unity gain buffer provides an interpolator gain of 400 for a resolution of 250ps suitable for GPS monitoring. Increasing the timebase to the maximum 66 MHz rate and using a 2x gain buffer provides a maximum resolution of 18.75ps. For comparison purposes the SR620 counter uses ECL logic, an 80 MHz timebase, 6x gain buffer, and a 12-bit ADC to provide an interpolator gain of 3000 and a resolution of 4.16ps.
The Simple PICTIC documentation, PIC assembly code, and ExpressPCB files are being offered free of charge to members of the Time-Nuts list allowing incorporation of the interpolator and support code into other amateur projects requiring a high-resolution TIC. Sets of 3 Simple PICTIC circuit boards can be ordered directly from ExpressPCB using their free software http://www.expresspcb.com/ExpressPCBHtm/Download.htm and their MiniBoard service for $51 plus shipping. The Mouser part numbers are included and at 12/08 single piece prices the user-assembled cost is about $50 per board to evaluate the PICTIC design for your own applications.
Source code as a zipped archive: pictic_v001.zip
Commented source code as a pdf file: simple_pictic_code.pdf
Documentation (operation, parts list, schematic, board layout) as a pdf file: simple_pictic.pdf
Files necessary to order boards from ExpressPCB: simple_pictic_rev_1_0.zip
The PICTIC II is a later design incorporating a diode switched interpolator with separate start & stop interrupt processing for improved accuracy. Download the following file to construct the latest version of the PICTIC II Interpolating Time Interval Counter.
PICTIC II Files (PIC Code, PCB layout, Schematic and parts list): pictic_ii_rev_003.zip
PICTIC II Schematic in PDF format: pictic_ii_schematic.pdf
PICTIC II Code with Baud rate and Osc adjust commands: pictic_ii_code_rev_103.zip
The 74AC175PC used in the previous versions is no longer in production and no suitable substitute is available. The 74AC175SC SOIC-16 version is still available and the PICTIC II PCB layout file below incorporates the SOIC version as a possible replacement. The schematic and code is the same as the PICTIC II above.
PICTIC II PCB layout for 74AC175SC SOIC: pictic_ii_soic.zip
Another alternative is presented below using two 74AC74 synchronizers instead of the 74AC175. The file includes the PCB layout and schematic files and uses the same code as the PICTIC II above.
PICTIC II PCB layout and Schematic files for the 74AC74 version: pictic_ii_rev_002.zip
Please feel free to contact me at the Email address below if you have questions or need customized PICTIC code for a specific application.
Richard H McCorkle