Oscillate GPIO pins as fast as possible for a DAC

Matthew Normyle

Hi,

I'm utilizing the MSP432 Launchpad, and have built a 6-bit resistor ladder DAC. I've connected that DAC to P4.0 - P4.5, and am trying to find a way to oscillate those pins as quickly as possible through a sin wave lookup table.

However, when I set the DCO to 48MHz and simply run a for loop flipping a bit (P4.0 for example) the output is barely over 1MHz. Shouldn't this be significantly faster running at 48MHz?

In a broader sense, what I'm trying to do is iterate through a constant array lookup table and output a particular value to the 6 bits of DAC. Running a loop at 48Mhz, I am able to produce a 28khz sine wave... which I think means I am updating the values of the GPIO pins at around 1.8MHz.

What am I missing here? Is there some specific clock setting for the pins I have not set?

over 9 years ago

0 Clemens Ladisch over 9 years ago

Guru 287110 points

To find out where the cycles go, look at the machine code generated by the compiler for the loop.
By writing the loop in assembler manually, you might improve this, but not by much.

The fastest way to flip those bits would be to avoid the CPU, and to use a timer and the DMA controller to copy the table entries to P4OUT (this would affect all eight bits).

0 Matthew Normyle over 9 years ago in reply to Clemens Ladisch

Prodigy 100 points

Hi, thanks for the reply. Is there any particular good code example for using the timer and DMA for this task?

0 Dennis Eichmann over 9 years ago in reply to Matthew Normyle

Guru 74080 points

Unfortunately example code from TI for the DMA does not seem to be available for any MSP, at least I haven't seen any so far. But maybe you're lucky and someone did something similar already. Otherwise...- > user's guide :\

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Oscillate GPIO pins as fast as possible for a DAC