This paper presents a validation study of a smartphone for detection of heel strike and foot flat during gait, in comparison with a validated in-shoe plantar pressure system. The aim of the study is to produce a smartphone gait analysis system that is able to estimate gait parameters in a non-controlled environment such as the home. The smartphone system using the built-in tri-axial accelerometer of the phone, and provides a reliable estimation of the number of steps and the stride-to-stride interval (ISI). Comparison with the results produced by an F-Scan mobile system showed an excellent relationship (R2=0.97). When Detrended Fluctuation Analysis was applied to the ISI calculated for each system, no significant differences were observed for a paired t-test. These findings open the way for other gait features such as gait velocity, walking distance and step length to be calculated using smartphones. Such a technique could be used to detect the loss of complexity in signals due to advanced age or disease in order to assess frailty and risk of falls in the elderly in ecological conditions.