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Video trigger generator for oscilloscope

This project generates a trigger output for an oscilloscope. It can select which line we want of the video signal (NTSC only).

jacques-pelletierJacques Pelletier
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I made a video trigger generator several years ago, with some old school electronics. That version is completed and working. It uses an LM1881 and hardware comparator, which is programmed by a recuperated microcontroller + additional RAM and EPROM. The user interface uses ordinary push buttons and a 3-digit 7-segment display.

The current version works, using an OLED display and only one chip: a Microchip ATmega328PB.

This chip replaces all the hardware logic of the previous version by the on-chip timers and analog comparator. It's a very good exercise for using timers and the analog comparator in a bit unusual way. Code and schematics available below.

The user interface works fine. Since it uses the chip's hardware, the trigger output has no jitter.
I plan to replace the push buttons by capacitive sensors with the Q switch

The old school circuit uses a LM1881 to do the sync separation. The sync pulses are counted by a 4040 and compared with a 74688. I compared my circuit with a commercial trigger generator, and it works the same.

The new version uses the ATmega328PB's internal analog comparator to separate the sync pulses. The internal counters are used to check the field (odd/even), count the lines and compare the selected line.

The software part takes care of the user interface. It debounces the push button switches and drives the OLED display. I used a modified font for this project. More on this later.

ATmega328PB-BOB.ods

Pinout spreadsheet

spreadsheet - 389.47 kB - 02/09/2021 at 06:33

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ATmega328PB-BOB.pdf

Pinout pdf file

Adobe Portable Document Format - 123.50 kB - 02/09/2021 at 06:33

Preview
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video-trig.sch

Schematics for the circuit in Kicad

x-kicad-schematic - 21.15 kB - 01/23/2021 at 06:02

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video_trig-v1.1.tar.bz2

Tweaked the numbers to fit with my scope which has video trigger. The only minor nick-nack is the 4th line on the even field is close to the vertical sync, so this one specifically doesn't work. Otherwise, it seems to be all fine.

x-bzip - 15.11 kB - 01/03/2021 at 02:15

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video_trig-v1.0.tar.bz2

This is the fully working version. The code should be documented well enough to reproduce the design. It generates a stable jitter-free trigger output (rising edge) for any scope.

x-bzip - 15.08 kB - 01/02/2021 at 22:18

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View all 8 files

  • Time to go on other projects

    Jacques Pelletier • 01/03/2021 at 02:23 • 0 comments

    Last update for this project for now. It works well and served as a proof of concept. This was a good exercise to use the on-chip peripheral for generating a trigger output for an oscilloscope. It uses only a Microchip ATmega328PB, a few swiches and components.

    I won't be needing this for myself anymore since I have a scope with this feature built-in.

  • Project development completed

    Jacques Pelletier • 01/02/2021 at 22:26 • 0 comments

    I finally made this project work after much head scratching.

    The result is a stable trigger output for any scope. This version uses common push button switches.

    The design could use capacitive switches instead with the Q switch library. To be tried next.

View all 2 project logs

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