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How to us prescale of Timer on lm4f120 launchpad?

Zheng Zhao
Prodigy 60 points

Other Parts Discussed in Thread: TM4C1233H6PM

Hi~~thanks~

Now i need a waveform with 20ms period and 0.5ms~2ms duty cycle.

But the Sysclock of lm4f120 is 80MHZ(I need the highest speed to process the PID).so timer0~timer5(16/32 bit timer) 's max period is :12.5ns*0xffff(65535)=0.819ms,that can't reach 20ms..

I also see the prescale function of timer, but how to use it to achieve clock division??confusing.....

  • 0
    TI__Intellectual 1380 points

    Hi Zheng,

    So you have three options here, but first some math

    the clock is 80Mhz, so there is a tick every 1/80,000,000 = 0.000,000,012,5 seconds. assuming you want to use a 16bit timer then the max value it could hold would be 2^16 = 65536 so the maximum length of a standard 16bit timer would be 0.000,000,0125 * 65536 = 0.0008192 seconds. you are asking for something that at a minimum can count 2ms = 0.002s and at a max 20ms = 0x020s. Thus I see three options

    Option 1: use a larger timer.
    use the 32bit timer, for this you can use the full width 32 bit timer, or the 64bit timer in split mode.

    Option 2: use counters
    keep global values that keep track of the number of times a timer has executed. say we set the timer to go off ever 0.0001 seconds, then you would have a global variable that would incriment every time the timer executes. So when the global value reaches 20 then .002s would have passed. 200 for .02s . use a polling function to keep checking the values and update the GPIO pin accordingly.

    Option 3: Use Prescaler
    There are two ways to think about how the prescaler works. Either it makes each tick take more time, or it gives the counter more digits of precision. I reccomend looking at http://www.ti.com/lit/ds/symlink/tm4c1233h6pm.pdf Table 11-5 on page 682.
    also you can look at the code implemented by Jordan Wills at

    https://github.com/EuphonistiHack/marble-maze/blob/master/TivaWare_c_Series-1.0/examples/boards/ek-tm4c123gxl-boostxl-senshub/maze_receiver/maze_receiver.c
    line 293 and 300 for the prescaler in  use.

    And if you still cant figure it out look at the TimerPrescaleSet() function in Timer.c in driverlib.

    Best of Luck!

    -Austin