Every HART FSK modem needs a stable time base to generate the 1.2 kHz and 2.2 kHz tones at the right frequencies. The HART physical layer is unforgiving about this — drift the tone frequencies too far and the carrier-detect and demodulation margins shrink. How that time base is supplied is a real design decision, and on a modem like the Microcyber HT5700 there are three ways to do it: an external crystal, an external CMOS clock, or the chip’s internal RC oscillator. Each trades cost, board space, current, and frequency accuracy differently.

This article walks through the three options on the HT5700 and where each one fits.

Why the clock frequency matters

The HT5700 transmits and receives by switching between a 1200 Hz mark and a 2200 Hz space. Those audio-band tones are derived from a much higher reference clock, so the accuracy of the reference clock sets the accuracy of the tones. On the HT5700 the frequency error of the tones is specified at ±0.5% over −40 to +85 °C and ±1% over the full −40 to +125 °C range when the internal oscillator is used; with a crystal or external clock, the tone accuracy follows the accuracy of that source.

The clock generation is selected through three pins — XTAL_EN, CLK_CFG0, and CLK_CFG1 — and the same block can optionally drive a buffered clock back out on the CLKOUT pin.

Option 1: External crystal

A 3.6864 MHz crystal across the XTAL1 and XTAL2 pins gives the most accurate and most stable reference. Microcyber’s reference design uses the ABLS-3.6864MHZ-L4Q-T, whose datasheet recommends two 18 pF load capacitors. To keep current low and stray capacitance down, the crystal, its capacitors, and ground should sit as close to the modem as possible.

The cost is current and parts. The crystal oscillator adds typically 33 µA (60 µA maximum) with 16 pF loading, and it occupies the board space of the crystal plus two capacitors. There is also a start-up delay — the crystal oscillator power-up time is typically 2.1 ms with 16 pF loads.

Use it when tone-frequency accuracy is the priority and the extra part and start-up time are acceptable.

Option 2: External CMOS clock

If the host system already has a clean clock available, it can be fed straight into the modem: drive the XTAL1 pin from the external clock source and leave XTAL2 open. The expected source frequency on the HT5700 is 3.6864 MHz (within a 3.6496–3.7232 MHz window).

This removes the dedicated crystal from the modem entirely — the accuracy then comes from whatever generates the system clock. One constraint: when a CMOS clock is used, no buffered output is available at CLKOUT.

Use it when the host already provides a suitable clock and you want to avoid a modem-specific crystal. This is the same idea as driving a HART modem from an MCU clock output — covered in detail for the HT1200M in our HT1200M clock guide; the difference is that the HT5700 takes the clock on XTAL1 rather than through a dedicated input pin scheme.

Option 3: Internal RC oscillator

The HT5700 integrates a 0.5%-precision RC oscillator running at 1.2288 MHz. To use it, tie XTAL1 to ground and leave XTAL2 open. This removes the external crystal and its capacitors completely, which is the option to reach for when board space or BOM cost is the dominant concern.

The trade-offs are accuracy and current. Tone-frequency error widens to ±1% at the temperature extremes, and the internal oscillator draws typically 87 µA (110 µA maximum) — more than the crystal oscillator. Its advantage on the timing side is a fast start: internal-oscillator power-up time is typically 25 µs, against roughly 2.1 ms for the crystal.

Use it when minimizing components and board area matters more than holding the tightest tone-frequency accuracy across the full temperature range.

CLKOUT: reusing the modem’s clock

Whichever source is selected (except a CMOS clock), the HT5700 can output a buffered clock on CLKOUT for the rest of the board to use:

• With the crystal oscillator: 3.6864 MHz, 1.8432 MHz, or 1.2288 MHz
• With the internal RC oscillator: 1.2288 MHz only

Driving CLKOUT adds current, since it has to drive whatever load sits on the pin — that load should be kept under 30 pF for clean edges and low consumption.

Quick comparison

Choosing for your design

The decision usually comes down to three questions: Do you have a clean system clock already? If yes, the CMOS-clock option removes a part for free. Is board space or BOM cost the binding constraint? If yes, the internal RC oscillator removes the crystal at the cost of some accuracy and current. Do you need the tightest tone accuracy across the full temperature range? If yes, the external crystal is the safe choice.

The HT5700 supports all three on the same silicon, so the clocking scheme can be chosen per product without changing the modem. For the related approach to clocking the HT1200M HART modem from an MCU, see our HT1200M clock guide. For the HT5700 pin descriptions, clock-configuration table, and connection diagrams, see the HT5700 user manual or contact Microcyber at info@microcyber.cn.