  The processes leading to the birth of high-mass stars are poorly understood.
We characterise here a sample of 430 massive clumps from the ATLASGAL survey,
which are representative of different evolutionary stages. To establish a
census of molecular tracers of their evolution we performed an unbiased
spectral line survey covering the 3-mm atmospheric window between 84-117 GHz
with the IRAM 30m. A smaller sample of 128 clumps has been observed in the SiO
(5-4) transition with the APEX telescope to complement the SiO (2-1) line and
probe the excitation conditions of the emitting gas, which is the main focus of
the current study. We report a high detection rate of >75% of the SiO (2-1)
line and a >90% detection rate from the dedicated follow-ups in the (5-4)
transition. The SiO (2-1) line with broad line profiles and high detection
rates, is a powerful probe of star formation activity, while the ubiquitous
detection of SiO in all evolutionary stages suggests a continuous star
formation process in massive clumps. We find a large fraction of infrared-quiet
clumps to exhibit SiO emission, the majority of them only showing a
low-velocity component (FWHM~5-6 km/s) centred at the rest velocity of the
clump. In the current picture, where this is attributed to low-velocity shocks
from cloud-cloud collisions, this can be used to pinpoint the youngest, thus,
likely prestellar massive structures. Based on the line ratio of the (5-4) to
the (2-1) line, our study reveals a trend of changing excitation conditions
that lead to brighter emission in the (5-4) line towards more evolved sources.
Our analysis delivers a more robust estimate of SiO column density and
abundance than previous studies and questions the decrease of jet activity in
massive clumps as a function of age.
