MCU 8051 clock cycles and machine cycles

[Jipper]

Hi, I am using the 8051 MCU in Multisim and am attempting to demonstrate the relationship between machine cycles and the 8051's clock signal. The problem I am having is that with the 8051 MCU internal oscillator set to 12MHz (done in the MCU Multisim model itself) it is not possible to trigger a logic analyser with the 8051's 12MHz clock and as a result the time associated with a machine cycle (ie. 12 clock cycles or 1us) is not accurate (when measured with the logic analyser). Maybe there is something simple that I am missing here, but is it possible to disable the internal oscillator in the 8051 model so that the device can be driven by an external oscillator circuit, or alternatively, is there any way in which the internal 12MHz oscillator signal can be accessed in order to allow it to be used to trigger the logic analyser and for comparison with instruction execution. In effect, I wish to be able to show that 1 machine cycle corresponds to 12 cycles of the 8051 clock signal. Any help, greatly appreciated. Thanks 

[Sharanya_R]

Hello Jipper,

You will not be able to take the 8051's internal clock and use it to clock a logic analyser; you can try using an external clock source.  The problem may be occuring becuase in real time the 8051 does not have an internal clock.  If you try an external clock, the internal clock may override it. 

I suggest trying to generate a sperate clock from the 8051 at the same frequency, using it to clock the logic analyzer.

If you have any additional questions, please dont hesitate to get back in touch.