Only in some cases will the 8051's internal pullup resistor be suitable as a bias source for lone connection right into the base of an NPN transistor. You must understand the design requirements, transistor gain, collector current load of the transistor and other factors to determine if the internal pullup will adequately bias the transistor.

You must also look specifically at the specs of the particular derivative part in use (not all parts are exactly the same) and properly consider the VDD voltage applied to the MCU.

Keep in mind that the two clock wide fast pullup mechanism on the MCU pins may also be creating a transient surge of current into the transistor base that could be harmful to some transistors. You cannot freely dump however many mAmps that you wish into a transistor base without properly comprehending the capabilities of the device.

Also be aware that some MCU derivatives in the 8051/8052 family may also be configured for push-pull type outputs. You are looking for some serious possible damage to either the MCU or the transistor if there is a direct connection between a port pin that is in push pull mode and a transistor base.

Michael Karas