An all-digital frequency-locked loop (ADFLL) based frequency synthesizer with a built-in FSK modulator for medical implants communication systems (MICS) band applications is presented. The ADFLL uses a high resolution single-bit digital ΣΔ frequency discriminator in the feedback path and a ΣΔ phase accumulator in the reference path, achieving fractional resolution. The ADFLL uses a digital IIR-based loop filter followed by a digital-intensive ΣΔ current-steering DAC and a first-order-hold filter. The ADFLL achieves 9.5Hz frequency resolution, spanning the ISM 400MHz-410MHz band. The worst-case near-integer spur of -55dBc and a phase noise of -83dBc/Hz at 300kHz offset is measured. The ADFLL is fabricated on a 0.18um CMOS process, occupying 0.14mm2 die area, with a quiescent current consumption of 700uA.