CC1352P7: RAT not accurate without external 32kHz crystal

Shuyang Zhong

Part Number: CC1352P7

Hi team,

A customer is using a custom board to develop their proprietary application, their board does not have a 32kHz external crystal, and they found the RAT has an error of several milliseconds after the device wakes up from standby. To my understanding, it is because the RAT synchronizes with RTC after wake up, however the RTC is not accurate because it uses RCOSC. This has been verified by the customer with soldering on a external crystal, which fixes the problem.

The question is, is there a way to achieve the accuracy without a 32kHz external crystal? The power consumption is not important in this application, so the customer thinks of an idea that keeping the RF core in idle and do not enter standby, is it possible?

Another question is that the customer found the timer module of the M4 core is still accurate after waking up, but the RAT was not. What's the difference between the source of these clocks that can lead to the behavior?

Thanks.

Shuyang

over 2 years ago

0 Siri over 2 years ago

TI__Guru* 79421 points

The RAT accuracy depends on the system HF clock while the RF core is active and on the LF clock while the RF core is powered down.

From the data sheet (section 7.18.3.5) you will see that the Calibrated RTC variation can on some devices be as much as +-1000 ppm (typical +-600). Are you seeing larger variation than that?

Are you using the TI drivers so that calibration is enabled?

If current consumption is not an issue, it is fully possible to disallow standby.

Siri

0 Shuyang Zhong over 2 years ago in reply to Siri

TI__Expert 3895 points

Hi Siri,

The customer needs more accurate clock than 600 ppm so they decided to go with disabling standby.

Now the question is how to force the RAT to use the HF clock as clock source? Does simply set power policy to WFI can do that? If the policy is WFI, will RAT always use HF clock?

BR,

Shuyang

+1 Sid over 2 years ago in reply to Shuyang Zhong

TI__Mastermind 20332 points

Hi Shuyang,

As you mentioned, the sync of RAT timer happens with the RTC on wake up. If power consumption is not an issue, not allowing the radio to power down is an option. This should keep the RAT timer functioning with the HF clock source.

using the RF_control() API is an option to do this.

uint32_t powerManagement = 0;
RF_control(rfHandle, RF_CTRL_SET_POWER_MGMT, &powerManagement);

The documentation for this can be found here.
https://dev.ti.com/tirex/content/simplelink_cc13x2_26x2_sdk_5_20_00_52/docs/rflib/html/group__rf__driver__cc13x2__cc26x2.html#ga5b7345d53c66e3168216234046359823

Regards,

Siddanth

Sub-1 GHz

Sub-1 GHz forum

CC1352P7: RAT not accurate without external 32kHz crystal