Turbo speed limitation
Engine torque is limited in case of high pressure ratios across the compressor to protect the turbo from being damaged by over revving.
The pressure ratio is the absolute pressure from the turbocharger divided by its inlet pressure.
The charge air absolute pressure sensor (688) and the atmospheric pressure sensor (539) are used to calculate the pressure ratio.
High pressure ratios arise when driving with heavy engine loads at altitudes where the atmospheric pressure is low. A high pressure ratios gives high turbine speed.