[FAQ] AM625: LFOSC usage in the device

Hello Koichiro Tashiro 

Thank you for the query.

You will have to use the clock tree tool to understand the clock routing to peripherals.

Let me check in parallel and come back.

Regards,

Sreenivasa

Any updates for this item?

Hello Koichiro Tashiro 

Thank you for following-up.

We do not expect many customers to use this clock source.
However, some customers may decide to use this as a source for a real-time clock based on the accuracy requirements.

Did you have a chance to explore the Clock Tree Tool?

Regards,

Sreenivasa

Hi Sreenivasa,

Kallikuppa Sreenivasa said:

Did you have a chance to explore the Clock Tree Tool?

Yes, I checked it, but the Clock Tree Tool is not quite useful to check where the LFOSC is used.
First of all there is no "LFOSC" in the tool. I guess it is called "RCOSC" in the tool.
And it seems all Timers can use "RCOSC", but many other modules(McASPx, GPIOx, I2Cx, McSPIx, UARTx, etc.) can use it.
So it is not possible to answer the customer's question from the tool.

Thanks and regards,
Koichiro Tashiro

Hello Koichiro Tashiro 

Thank you.

Please refer additional inputs that could help understand:

None of the three supported reference clock frequencies or operating frequencies of the internal PLLs are able to produce a 32768 Hz clock with an integer divider. So it will not be possible to implement an accurate RTC inside our device using one of our timers without a more accurate clock source.
All of the internal clock options will see significant drift relative to a true 32768 Hz clock.
The customer will need to connect a 32768 Hz crystal to WKUP_LFOSC0 if they want to implement an internal RTC that does not have significant drift between updates from the host.
Using an external RTC device may be a better option for most customers and will definitely be a better option if they need an RTC with battery backup.

The connection of 32K is use case dependent and based on the performance requirement an external LFOSC can be used.

Regards,

Sreenivasa

Hi Sreenivasa,

Thanks for clarification.
I understood LFOSC is needed to use accurate RTC function inside SoC.
Otherwise, LFOSC is an optional.

Thanks and regards,
Koichiro Tashiro

Hello Koichiro Tashiro 

Thank you and Your understanding is correct.

Regards,

Sreenivasa

Hello Koichiro Tashiro 

Here are some guidelines and recommendations for using the LFOSC

Koichiro Tashiro said:

Otherwise, LFOSC is an optional.

7.11.4.1.3 WKUP_LFOSC0 Internal Oscillator Clock Source

When WKUP_LFOSC0 is used, follow the below section for clock implementation.

Refer the below figure for he recommended crystal circuit.

Figure 7-21. WKUP_LFOSC0 Crystal Implementation

Table 7-24. WKUP_LFOSC0 Crystal Electrical Characteristics

7.11.4.1.4 WKUP_LFOSC0 LVCMOS Digital Clock Source

When not used refer below section for connecting the SoC LFOSC0 pins 

7.11.4.1.5 WKUP_LFOSC0 Not Used

Note: it is recommended to provide provision to ground WKUP_LFOSC0_XI input and WKUP_LFOSC0_XO output through a 0R for future enhancements.

Regards,

Sreenivasa 

Hi Sreenivasa,

Thanks for updates.

The customer found below description in TRM section 12.7.4.4.5.1 1-ms Tick Generation.


Other than accurate RTC, 1-ms tick generation is not possible without LFOSC, correct?

Thanks and regards,
Koichiro Tashiro

Hello Koichiro Tashiro 

Please refer below. I guess the 1-ms tick is for the RTC as described below.

The only available source for a 32.768-kHz clock is the LFOSC. The HFOSC is not capable of
generating a true 32.788-kHz frequency.

Regards,

Sreenivasa

Hello Koichiro Tashiro 

Pls let me know if you have any additional query or if i can close the thread.

LFOSC0 clock is optional.  The system configuration/uses cases will dictate if the low freq clock is required, but the processor can operate without it.

Regards,

Sreenivasa

Hi Sreenivasa,

I have one more question. There are other 32k clk sources called "CLK_32K_RC" and "HFOSC0_CLKOUT_32K" in the device.
I think these frequency is not accurate enough, but still usable as 32k source?

Thanks and regards,
Koichiro Tashiro

Hello Koichiro Tashiro 

Thank you.

Koichiro Tashiro said:

have one more question. There are other 32k clk sources called "CLK_32K_RC" and "HFOSC0_CLKOUT_32K" in the device.
I think these frequency is not accurate enough, but still usable as 32k source?

Correct.

The RC could also have wide variation over the temperature.

Regards,

Sreenivasa

Hi Sreenivasa,

Yes, we can close it for now.

Thanks and regards,
Koichiro Tashiro