The Motorola MC68HC908QY4 is utilized to generate an NTSC video signal applicable in various scenarios. One application is an NTSC Test Pattern Generator, creating basic patterns to assess a TV’s geometric distortion, high voltage regulation, and interlace quality. Another application is a VCR Pacifier, enabling a VCR to record audio-only signals like those from a radio. This device also includes a real-time clock displayed either in video or via the TV’s closed captioned decoder. Additionally, low-bandwidth data from the 68HC908QY4’s A/D inputs can be presented similarly, effectively turning the VCR into a two-channel data logger. As a bonus feature, a Video Line Trigger is included, counting video lines from an external NTSC video source and generating a pulse to trigger an oscilloscope at the selected line(s).
To achieve an integer divider, we must begin at twice the color frequency, which amounts to 7.15909 MHz. Considering the QY4’s bus frequency as clock/4, we require an input clock of 28.63636 MHz (7.15909 * 4). (It’s worth noting that the QY4’s maximum clock is 32 MHz.)
In NTSC video, there are two standards: monochrome and color. The color standard is compatible with monochrome receivers. But its adoption was not without challenges. For the monochrome standard, the specifications entail a horizontal rate of 15.750 KHz, a vertical rate of 60.0 Hz, and two interlaced fields of 262.5 lines, resulting in 525 lines per frame at 30 Hz.
The original NTSC setup demanded a clock speed of 28.63636 MHz and utilized a crystal within the standard CMOS crystal oscillator circuit depicted on the right. (At this frequency, R2 is not necessary.)
However, applications involving Closed Captioning required a 32 MHz clock. By that time, I had already assembled the board and lacked space to incorporate a 32 MHz oscillator module close to the QY4.
Upon substituting the 28.63636 MHz crystal with a 32 MHz one. The circuit failed to oscillate at 32 MHz. Instead, the frequency dropped significantly, resembling more of a 10 MHz rate.