lm3s5632

Naveen Bhat said:

I have one doubt till today

Just one?    Staff/I - others here - have (many!)

Thank you for providing that link to your (retired/past) MCU.     BTW - firm/I have (just over) 100 pcs. of  that (past/deceased) MCU!     Under 'proper' conditions - it (may) be possible to, 'Pry them away - from our Cold (yet not dead) Hands!'     Those less 'timely & strategic' in their planning - likely 'missed' the (tiny) window - to acquire sufficient stock for (highly predictable - 'future') needs!

To your questions:

You may employ only the external crystal oscillator (via pins Osc0 & Osc1) for 'normal' MCU operation.    You (must)  'add'  the proper components to  'XOsc0 & XOsc1' - if and only if - you seek to 'Hibernate and/or employ the MCU's RTC.'

The MCU manual IS your guide - it makes 'special note:'  

• "When the ADC module is in operation, the system clock must be at least 16 MHz.    When the USB module is in operation, MOSC must be provided with a clock source, and the system clock must be at least 30 MHz.
• The main oscillator must be connected to one of the following crystal values in order to correctly generate the USB clock: 4, 5, 6, 8, 10, 12, or 16 MHz.    Only these crystals provide the necessary USB PLL VCO frequency to conform with the USB timing specifications."

Thus - it appears 'best' for you to employ an external crystal (or xtal osc.) [introduced to Osc0/Osc1] and program System Clock to 16MHz - or above.

The MCU's (many) Timers - all run from the System Clock.   (Timer section of the manual - neatly diagrams.)    There is NO special clock input for the Timers (as you asked) and NO special clock for USB operation.   (other than the restriction upon xtal values - well detailed, above.)

Has your (one) doubt - now - been adequately resolved?      (such appears - to 'well be' the case...)

Hello Naveen,

To add to cb1's comments, the LM3S5632 MCU is in NRND status, i.e. Not Recommend for New Designs, so please look to use our TM4C devices instead. The TM4C123x family likely will provide what you need.

Naveen Bhat said:

I have seen in (censored) pic controller uses normal oscillator and timer oscillator separately

Such a question - rather clearly - should be addressed to (that) vendor - is that not so?

In your behalf - I can 'hazard a guess' (albeit - w/some logic) - yet the source proves best.

It may be that providing a completely different, Timer Clock source provides (some) advantage during:

• those cases in which the 'rapid and eased' adjustment of Timer Clock's Frequency is mandated
• when that Timer Clock source is 'adjustable' - and may be (near) exactly 'dialed-in' - to match an MCU generated frequency
• such lower frequency clock source will most always consume less power from the MCU
• it may prove that 'sharing' the MCU's System Clock (as is done here) imposes (some) 'restriction or degradation' - upon (certain) of the MCU's operation

Now - while such, 'External Timer Clock Source' (may) exist via another's MCU - such proves NOT the case - w/your MCU.     Thus - are you not asking for some 'unreachable dream?'

I gleaned this info from past service as Tech Mgr. - at a similar 'semi-giant.'     (notably - that past firm - and this firm - shared the SAME building!)    (We're told that the secret tunnels - were (almost) completed - prior to my past firm - moving across the street!)    And do note - my (slight) awareness of such 'tunnels' - in no way - denotes my participation in - nor encouragement of - their creation...