Your situation looks rather simple, because you have only two sensors and furthermore the signal of one sensor (temp sensor) is not changing very quickly. So, you can put the highest priority on catching the edges of your RPM sensor.

A good idea is to give the (properly conditioned) RPM signal to one of the /INT inputs. Read the chapter "Timer/Counters" of this link

http://www.nxp.com/acrobat_d...WARE_1.pdf

to see, how the /INT inputs work together with the internal timer/counters to tremendeously ease your RPM measurement.

Theoretically, two ways of determining the RPM are possible: Measuring the time period between two (or more) edges or counting edges during a known period of time. Which one is preferred mainly depends on actual height of RPM:

If the frequency is rather low, then measuring the time period between succeeding edges is advantageous. But if the frequency is rather high, then counting edges per known period of time is better.

You could do the measurement of temperature at a point of time, when no edge is to be expected, for instance immediately after detecting such an edge. Another way, especially when consequently using the interrupt abilites of 8052, is to measure the temperature when refreshing the display, which could be three times a second or similar.

When using interrupts, it's a good idea to keep the interrupt service routine as short as possible.

Kai