Adafruit has listed a new clock generator breakout as product 6534: the "Si5351C 8 Channel TCXO Clock Generator, 2.5KHz-200MHz, STEMMA QT." The price is $14.95. When we checked the product page it said "Coming soon" and offered a notify-me button, so the board has been announced and priced but is not yet in stock. This report is based on the product page, the Skyworks datasheet, Adafruit's driver repositories and the Learning System guide for the older board.

Adafruit already sells an Si5351 breakout. The new board differs in two ways. It has eight outputs instead of three, and its reference is a temperature-compensated crystal oscillator (TCXO) instead of a plain crystal. For anyone using these boards as cheap bench references or local oscillators, the second change probably matters more.

The chip: Si5351C, not the usual Si5351A

Skyworks' Si5351A/B/C-B datasheet (Rev 1.3, August 27, 2021) describes the family as I2C-programmable any-frequency CMOS clock generators plus VCXO. They can generate "up to eight non-integer-related frequencies from 2.5 kHz to 200 MHz" with exact frequency synthesis (0 ppm error). The datasheet lists typical output period jitter below 70 ps peak-to-peak, a 25 or 27 MHz crystal input, a core VDD of 2.5 or 3.3 V, and output supplies (VDDO) of 1.8, 2.5 or 3.3 V. It also claims PCIe Gen 1 compatibility and support for an HCSL-compatible output swing.

The number of outputs depends on the package. The datasheet lists eight outputs for the 20-QFN (4 x 4 mm), four for the 16-QFN and three for the 10-MSOP. It also says the Si5351C variant takes a CLKIN reference. That fits a board built around a separate reference oscillator, here a TCXO, instead of a bare crystal on the chip's XA/XB pins.

The reference: a 25 MHz TCXO with published tolerances

The product page names the reference part as 1XTW25000MAA, a 25 MHz TCXO, and copies its tolerance figures from the oscillator's specifications:

  • After two reflow passes: +/-1.5 ppm
  • Over temperature, -30 to +85 C: +/-0.5 ppm
  • Versus supply: +/-0.2 ppm
  • Versus load: +/-0.2 ppm
  • Aging: +/-1.0 ppm per year

The datasheet's "0 ppm error" claim only means the synthesizer adds no error of its own when it divides the reference down. Every output is still exactly as accurate as the reference. Adafruit's guide for the older three-output board lists a plain 25 MHz crystal, and a plain crystal drifts as the room or the enclosure warms up. A TCXO rated at +/-0.5 ppm across -30 to +85 C is a better reference for anything that has to stay on frequency over a session, such as a beacon, a receiver's local oscillator or a test signal for checking a counter. One limit to keep in mind is the +/-1.5 ppm post-reflow figure. It means an individual board can start up to 1.5 ppm off nominal, so precise work still needs calibration against a known reference.

Eight outputs, two frequency ranges, and a discrepancy

There are eight independent outputs, according to Adafruit, and each one can have a different frequency. The product page says CLK0 through CLK5 cover 2.5 kHz to 200 MHz, which matches the datasheet's headline range.

The page disagrees with itself about CLK6 and CLK7. The prose description says these two outputs reach "about 20 kHz to 50 Mhz." The bulleted spec list on the same page says 20 kHz to 150 MHz. The datasheet's headline sentence quoted above covers the family as a whole and does not settle which figure applies to these two outputs. Until Adafruit corrects the page, plan around the lower 50 MHz figure if you need CLK6 or CLK7 above a few tens of megahertz, and check the per-output limits in the Skyworks datasheet before you commit a design to them. The 2.5 kHz to 200 MHz range on CLK0 through CLK5 is consistent across both sources.

Adafruit gives jitter as below 70 ps peak-to-peak typical, the same figure as the datasheet.

Outputs, connectors and power

The outputs swing 3 Vpp by default. Adafruit says you can lower the swing using the VDDA, VDDB, VDDC and VDDD pads on the board. Those pads presumably correspond to the chip's output-supply banks, given the 1.8, 2.5 and 3.3 V VDDO options the datasheet lists.

For connections, the board has a standard breadboard header. It also has footprints for optional edge-launch SMA connectors on alternating outputs, four in total. Adafruit sells the SMA connectors separately. For comparison, the older STEMMA QT three-output board had a single SMA, on output #1, according to the Learning System guide.

An onboard 3.3V LDO and level shifters make the board ready for use with 3V or 5V power and logic, per Adafruit. As the name says, it also has STEMMA QT connectors, so you can plug it into a Qwiic or STEMMA QT chain without soldering. Adafruit gives the board size as 46.0 x 17.7 x 4.8 mm.

Software

Adafruit lists Arduino and CircuitPython/Python libraries. The CircuitPython driver is adafruit/Adafruit_CircuitPython_SI5351, which depends on Adafruit CircuitPython and Bus Device and is documented on docs.circuitpython.org. The Arduino library lives in the adafruit/Adafruit_Si5351_Library repository, whose latest GitHub release is 1.6.0, dated September 22, 2026. We did not read that library's README, so we cannot say whether release 1.6.0 explicitly supports all eight outputs or the Si5351C's CLKIN reference path. Check that before you start writing firmware. The product page says Adafruit has a tutorial and library to get you started. At the time of writing, the Learning System guide we read covered only the earlier three-output board.

What It Means for Makers

The board suits people who need several programmable clocks from one cheap, I2C-controlled part and care about how much those clocks drift. That includes amateur radio builders making VFOs, beacons or quadrature LO chains, people building or calibrating frequency counters and logic analyzers, and embedded engineers who want several unrelated reference clocks on a prototype without fitting a separate oscillator for each one. Educators covering PLLs and fractional dividers get eight outputs to scope from a single STEMMA QT cable.

Compared with Adafruit's earlier Si5351 breakout, this board replaces three outputs and a plain crystal with eight outputs and a TCXO whose temperature tolerance is published, and it offers up to four SMA footprints instead of one. It does not replace a lab synthesizer. The outputs are CMOS square waves with typical jitter below 70 ps peak-to-peak, and the datasheet's frequency limits still apply.

Adafruit lists the board at $14.95 (USD, as listed in late October 2026). The SMA connectors cost extra. When we checked, the product page said "Coming soon" with a notify-me option. The board has been announced and priced, but it is not yet in stock, and Adafruit has not given a ship date on the page.

Sources