This paper describes a sub-1-V 2.4-GHz ZigBee transmitter (TX) with scalable output power (P) and system efficiency. It features a function-reuse class-F topology unifying the digital-controlled oscillator (DCO) and power amplifier (PA), designated as DCO-PA. Unlike the existing currentreuse topologies that rely on transistor stacking, here the power consumption of the DCO and PA-driver is absorbed into the DCO-PA without losing the voltage headroom, while allowing a low supply voltage. The DCO-PA also benefits from a six-port transformer with customized coupling coefficients and turn ratios to jointly perform the functions of resonant tank and output matching network, saving the chip area. A fractional-N all-digital phase-locked loop (ADPLL) realizes a two-point data modulation. Its phase-interpolated time-to-digital converter prevents time-consuming calibration. The entire TX fabricated in 65-nm CMOS occupies a 0.39-mm active area. The standalone DCO-PA shows a peak efficiency of 26.2% at a 6-dBm P, and a back-off efficiency of 17.7% at a -4.3 dBm P under a scalable supply voltage (0.3-0.7 V). The system efficiency, including the ADPLL, is 22.6% (14.5%) at 6-dBm (0-dBm) P. The HS-OQPSK modulated output complies with the ZigBee spectral mask with an adequate margin and the error vector magnitude is 2.29%.