  The increasing number of spectra gathered by spectroscopic sky surveys and
transiting exoplanet follow-up has pushed the community to develop automated
tools for atmospheric stellar parameters determination. Here we present a novel
approach that allows the measurement of temperature ($T_{\rm eff}$),
metallicity ($[{\rm Fe}/{\rm H}]$) and gravity ($\log g$) within a few seconds
and in a completely automated fashion. Rather than performing comparisons with
spectral libraries, our technique is based on the determination of several
cross-correlation functions (CCFs) obtained by including spectral features with
different sensitivity to the photospheric parameters. We use literature stellar
parameters of high signal-to-noise ($\textrm{SNR}$), high-resolution HARPS
spectra of FGK Main Sequence stars to calibrate $T_{\rm eff}$, $[{\rm Fe}/{\rm
H}]$ and $\log g$ as a function of CCFs parameters. Our technique is validated
using low $\textrm{SNR}$ spectra obtained with the same instrument. For FGK
stars we achieve a precision of $\sigma_{T_{\rm eff}} = 50$ K, $\sigma_{\log g}
= 0.09~ \textrm{dex}$ and $\sigma_{\textrm{Fe}/\textrm{H}]} =0.035~
\textrm{dex}$ at $\textrm{SNR}=50 $, while the precision for observation with
$\textrm{SNR} \gtrsim 100$ and the overall accuracy are constrained by the
literature values used to calibrate the CCFs. Our approach can be easily
extended to other instruments with similar spectral range and resolution, or to
other spectral range and stars other than FGK dwarfs if a large sample of
reference stars is available for the calibration. Additionally, we provide the
mathematical formulation to convert synthetic equivalent widths to CCF
parameters as an alternative to direct calibration. We have made our tool
publicly available.
