AN60590 explains the diode-based temperature measurement using PSoC 3 / PSoC 5.
The temperature is measured based on the principle of diode forward bias current dependence on temperature. PSoC 3 / PSoC 5 has on-chip 8-bit IDACs, and a 20-bit Delta Sigma ADC, which enable accurate, high-resolution temperature measurements. An external diode-connected transistor is used for temperature measurement.
AN60590 is available here http://www.cypress.com/?rID=42993
