32 kHz vs 3.58 MHz: The Evolution of Key Frequencies

These two frequencies played major roles in electronics; one is still very viable, while the other is completely obsolete.

Engineers are surrounded by countless standard frequencies, many of which are defined by an industry standard. Some are so well-known and commonplace that they need no clarification other than the frequency number itself; others were “celebrities” in their day but no longer. Still, others are with us even if we don’t see them explicitly or are in a diminished role.

Consider two frequencies known simply as 32 kHz and 3.58 MHz (both nominal values) or, to be more precise, 32.768 kHz and 3.579545 MHz, respectively. The first one still has a role, while the second one is obsolete even though it represented a feat of analog-engineering brilliance that was used for over 50 years until it was made obsolete by newer technologies.

Start with 32 kHz

Let’s look at 32.768 kHz first, which in almost all cases is produced by a crystal and accompanying oscillator circuit. This is the base frequency for real-time clocks (RTCs), which must provide one tick per second. Since 32,768 is 2 raised to the 15^th power (2¹⁵), you can create that one-second tick by simple, repeated binary division. That’s easy; all it takes is a series of flip-flops.

It may seem that in today’s world of ubiquitous microcontrollers of all sizes and clock rates embedded in just-about everything, including the kitchen toaster, there’s little need for an independent RTC. However, people still buy standalone, single-function, battery-operated clocks, or AC-line clocks with battery backup via an RTC (AC line-only clocks can use the 50/60 Hz line for timing, so there’s no need for the RTC circuit. Further, some low-end systems use the 32-kHz as their microcontroller clock, which saves money and board space.

The RTC function is so widely used that vendors offer interface ICs ranging from just a basic oscillator for the crystal to devices that do the division and provide the one-second output pulses. Others add additional useful RTC features.

The Renesas IDT1337AG is a low-power serial device with two programmable time-of-day alarms and a programmable square-wave output, Figure 1. Address and data are transferred serially through an I²C bus. The device provides information on seconds, minutes, hours, day, date, month, and year. The date at the end of the month is automatically adjusted for months with fewer than 31 days, including corrections for the leap year. The clock operates in either a 24-hour or 12-hour format with an AM/PM indicator.

32 kHz and 3.58 MHz frequencies — Figure 1. The IDT1337AG RTC IC is more than just an oscillator and divider for the crystal; it includes many additional time-keeping/counting functions and serial interfaces. (Image: Renesas Electronics Corporation)

Other RTC ICs go even further to support designs that need the RTC to operate regardless of the state of the system processor (which may be active, on standby, or inoperable). To meet these requirements, packaged devices such as those in the Snapchat family from ST Microelectronics offer RTC IC-based assemblies that are much more than just the RTC IC. The oscillator circuit, battery backup, and rail/battery circuit are built into the RTC device, Figure 2 and Figure 3.

The next part looks at a frequency at the core of a much more complicated, extremely sophisticated, and quite clever signal-encoding and decoding scheme in mass-market use.