TSC channels vs. steps & registers

Walter Cromer

I'm reading through the TRM for the ARM335x as I am using a BeagleBone Black for a new product line and need to use the PRUs and the TSC_ADC.

I understand that the ADC is an 8 channel device. The TRM includes 16 step configuration registers. I don't understand why there is a difference. It's a newbie question for sure but I'd appreciate some enlightenment and if the answer is obvious, please go gently. I am an old dog, refreshing on old tricks, learning new ones, and enjoying most of the treats that come along with them.

Thanks,

WC

over 5 years ago

0 Brad Griffis over 5 years ago

TI__Guru*** 125430 points

The steps are for specifying which channels should be sampled, in what order, and with how much delay between samples.

0 Walter Cromer over 5 years ago in reply to Brad Griffis

Intellectual 305 points

I'm being a dunce here probably. I see in the TRM (SPRUH73Q–October 2011–Revised December 2019, starting on page 1842) a table with registers listed such as STEPCONFIG1 and STEPDELAY1 up to STEPCONFIG16 and STEPDELAY16 with every register between 1 and 16 listed. They all have different offsets. So, with only 8 channels of A/D, I was expecting there to just be registers STEPCONFIG1 -> STEPCONFIG8. How do these map to the actual A/D channels? Is STEPCONFIG1 channel 0? If so, then what does STEPCONFIG8 map to and STEPCONFIG9, and so on?

Thanks for your help!

0 Brad Griffis over 5 years ago in reply to Walter Cromer

TI__Guru*** 125430 points

It is configurable. Please see bits 22:19 in the STEPCONFIGx register. You pick the channel. If you want to sequence through the channels in order you would configure

STEPCONFIG1[22:19] = 0

STEPCONFIG2[22:19] = 1

STEPCONFIG3[22:19] = 2

STEPCONFIG4[22:19] = 3

STEPCONFIG5[22:19] = 4

STEPCONFIG6[22:19] = 5

STEPCONFIG7[22:19] = 6

STEPCONFIG8[22:19] = 7

The intent is that you have flexibility to specify any order you want. For example in the Linux touchscreen driver we have 5 steps that sample x and 5 steps that sample y. In the corresponding ISR we discard the min/max values for each coordinate and then average the remainder. In that example we used 10 steps for just 2 of the channels. There doesn't have to be a one to one correspondence.

Best regards,
Brad

+1 Walter Cromer over 5 years ago in reply to Brad Griffis

Intellectual 305 points

Ok, I'm getting it I think, but slowly. Let me see if I do by offering an example of my own.

So, I see now that STEPENABLE lets me enable the steps that I want to be executed. (I missed that concept.)

So if I had an application that had a sensor A that needs to be read every 10ms and sensor B that only needs to be read every 10 seconds, I could wire channel 1 to sensor A and assign it to step 1 and wire channel 2 to sensor B and assign it to step 2.

Then at startup, when I want to read both sensors, I enable steps 1 and 2 but not 3-16. This saves time on the read as 3-16 aren't needed so I don't read them. Then after the initial read when I don't need to read sensor B until a minute passes, I can change STEPENABLE so only step 1 is enabled and execute a read every 10 ms. Only sensor A would be read. Then at 10 seconds, I change STEPENABLE so steps 1 & 2 are enabled and when read is triggered, sensors A and B are read.

BTW - I don't have a real-world application like this but I guess someday I could.