Timer Prescaler

Stephen Bolanos

Other Parts Discussed in Thread: EK-TM4C1294XL, TM4C1294NCPDT

Hi all,

Apologies, I am revisiting an old issue of mine - measuring duty cycle. We are considering the use of the TimerPrescaleSet () function to extend the range of the Timer used to capture edge events. However, I have a few questions that I can't quite find the answer to in the datasheet or the driver library.

The datasheet mentions that the value of Timer n and the Prescaler are latched into GPTMTnR and GPTMTnPS respectively. Accessing the driverlib/timer.c file, the function TimerValueGet() obtains a different value and my program seems to continue working. Why is that so?
Is there a function that obtains the snapshot value of the prescaler? Looking again in the driverlib/timer.c file, TimerPrescaleGet() obtains the the GPTMTnPR, which again, is not the value that I would like to capture. However, since TimerValueGet() works fine, would TimerPrescaleGet() also work fine?

In short, am I required to access the registers directly and perform bit manipulation in order to obtain the correct values during each event should I use the prescaler.

For reference:

I am using EK-TM4C1294XL development board with a TM4C1294NCPDT MCU.
CCS 6.1.0 as a development environment

Thank you,

Stephen

over 10 years ago

0 Amit Ashara over 10 years ago

TI__Guru**** 244440 points

Hello Stephen

TimerValueGet does return the value of GPTMTnR register. However there is no API for getting the value of the GPTMTnPS. This API would need to be done by the application. In this your application needs to access the register directly.

Regards
Amit

0 Stephen Bolanos over 10 years ago in reply to Amit Ashara

Intellectual 260 points

Is it a must that I obtain the value in GPTMTnPS and do bit manipulation in Edge Time mode to have a "24-bit" timer? Or is this added automatically somewhere?

Thank you,
Stephen

0 Amit Ashara over 10 years ago in reply to Stephen Bolanos

TI__Guru**** 244440 points

Hello Stephen,

In the 16 bit mode the read of the GPTMnR will result in the 23:16 as Prescalar and 15:0 as the 16 bit value

Regards
Amit

0 Stephen Bolanos over 10 years ago in reply to Amit Ashara

Intellectual 260 points

Hi Amit,

Thank you very much for your help, that cleared a lot of things up!

For those outside looking in, this information can be found in page 1012 in the datasheet (something I should have seen beforehand).

Thank you,

Stephen

0 Stephen Bolanos over 10 years ago in reply to Amit Ashara

Intellectual 260 points

Hi Amit,

It's been a bit and I've been playing around with this value, and it occurred to me that wouldn't it make more sense for the pre-scalar value be the LSB of the register? Otherwise, wouldn't I need to shift bits 23:16 to the right (or left) 16 bits to obtain a "true" 24-bit counter?

For example, I set the prescale the value for 255. I grab the value in GPTMTnR and it results in - 1111.1111.0000.000.0000.0000 - this is read as 16711680, however, the real number is 255. Is this the case, or is this already accounted for?

Another question, in the datasheet, it is indicated that - Bits 31:24 always read as 0. What is the case should the value exceed 2^24? Does this then rollover into the unused bits?

Thank you,

Stephen

0 Amit Ashara over 10 years ago in reply to Stephen Bolanos

TI__Guru**** 244440 points

Hello Stepehen,

Yes, in the 16 bit mode it is a roll over condition.

Regards
Amit

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Timer Prescaler