8051assemblymicrocontroller
I'm trying to design an 8051 microcontroller circuit which uses timers/counters, and the circuit I'm using for inspiration uses a clock generated using the XTAL1 and XTAL2 pins of the IC, shown in the picture.
What I don't understand is why this clock is necessary for using the counter function of the IC, and where the output of the clock goes. I tried using a logic probe to test where the clock was outputted but couldn't figure it out.
No you don't need an ADC in between the ZCD and the 8051.
You must have your simulation setup incorrectly, as it should work as shown. the only think I can think of is that SPICE does not like floating parts of the circuit, so the voltage source on the other side of the transformer may need a high resistance connection to the secondary, in order to keep SPICE happy. This is at least the case for LTSpice, I don't know about Multisim.
Anyway, here is the circuit simulated in LTSpice, it seems to work fine. Note the high value resistance between primary and secondary (R1)
Simulation
EDIT
It seems as though you have not given us all the detail - you mention an LED not lighting up though (unless I'm missing something) there is no LED shown on the schematic. Also, though I am no 8051 assembly expert, if the code you have posted is the entire firmware, then I'm not sure how you expect it to do anything useful. There is no SETB or MOV instruction which could be used to set an output pin.
I can't answer this specific question, but I have been trying to work through some timer problems of my own. Searching for that application note has an interesting 2nd hit on the ST forums..
One user volunteers some sad truths about the documentation:
The timers' description in ST's materials is an incredible mess. Instead of one properly written chapter, dealing with all the features and pointing out those omitted in the lesser-capable timers, they inflated the UMs with several copies of the timers' description, plagued with factual errors, omissions, apparent copy/paste and forgot-to-delete-this errors - plus the basic text is still of very poor quality.
For the 'F4xx, which I use, I tried to write down some of the most obvious errors in a "doc errors" writeup, http://www.efton.sk/STM32/STM32F4xx_doc_errors.txt ; but as far as the timers' chapters go, I'd recommend ST to discard the existing and rewrite it from scrap.
For your amusement, for example, have a glimpse at Table 49 on page 363 in RM0313...
JW
Hopefully you find some answers in his errata... I have a lot of reading to do :/
Best Answer
The MCU processor requires a clock to do (almost) anything. Aside from perhaps the RST input which is probably partially asynchronous, everything happens after some number of clock edges.
None of the code will run without a clock, so, for example, the timer module cannot be set up without a clock and some code.
The 8051 is a rather old microcontroller and requires an external clock (either a pulse train or there is an internal oscillator which can use a bare crystal). Some relatively newer processors may have an internal RC clock, as an option or perhaps as the only clock source.