  We introduce the SAPP (Stellar Abundances and atmospheric Parameters
Pipeline), the prototype of the code that will be used to determine parameters
of stars observed within the core program of the PLATO space mission. The
pipeline is based on the Bayesian inference and provides effective temperature,
surface gravity, metallicity, chemical abundances, and luminosity. The code in
its more general version can have a much wider range of applications. It can
also provide masses, ages, and radii of stars and can be used for stars of
stellar types not targeted by the PLATO core program, such as red giants. We
validate the code on a set of 27 benchmark stars that includes 19 FGK-type
dwarfs, 6 GK-type sub-giants, and 2 red giants. Our results suggest that
combining various observables is the optimal approach, as it allows to break
degeneracies between different parameters and yields more accurate values of
stellar parameters and more realistic uncertainties. For the PLATO core sample,
we obtain a typical uncertainty of 27 ($\rm{syst.}$) $\pm$ 37 ($\rm{stat.}$) K
for T$_{\rm{eff}}$, 0.00 $\pm$ 0.01 dex for log$g$, 0.02 $\pm$ 0.02 dex for
metallicity [Fe/H], -0.01 $\pm$ 0.03 R$_\odot$ for radii, -0.01 $\pm$ 0.05
M$_\odot$ for stellar masses, and -0.14 $\pm$ 0.63 Gyrs for ages. We also show
that the best results are obtained by combining the $\nu_{max}$ scaling
relation and stellar spectra. This resolves the notorious problem of
degeneracies, which is particularly important for F-type stars.
