Part Number: MSP430FR2633
We are building a battery-powered CapTIvate device that must run in the field for about a decade so power consumption is critical. Out hardware team has designed a prototype that provides a 192kHz external clock with the intent of not utilizing the DCO/FLL. I.e. we will drive MCLK/ACLK directly from the 192kHz input at XT1.
As you know, the clocks are quite versatile, which adds some complication, especially given that we are trying to do something that is probably not particularly common.
Anyway, it looks like this can be done via XT1BYPASS:
And the driver library provides an interface for entering this mode CS_bypassXT1():
From the vanilla BSP generated by the Captivate DC, I’ve made the following changes:
1) Added a call to CS_bypassXT1().
2) Commented out the call to CS_turnOnXT1LFWithTimeout(). (This call was failing with the 192kHz crytal- I assume due to it being out of range for this part?)
3) Selected XT1CLK as source for both MCLK and ACLK (Not sure I can use 192kHz crystal as an input to ACLK in bypass mode given that this part does not support high-speed crystals?).
4) Commented out the calls to initialize FLLREF and SMCLK (not sure this is right but according to the diagram below, SMCLK can only be sourced from DCO or a high frequency crystal, which is not supported by this part but I’m not sure if this is a constraint when using bypass mode and a 192kHz crystal.).
5) Adjusted by baud rate of my debug terminal for the 192kHz clock in order to generate 9600 baud:
Issue
Clock initialization is below. I’m seeing a couple of issues. 1) I’m printing a “hello world” to a terminal and this was working at the higher clock speed/baud rate. Not getting it now. 2) The call to CAPT_appHandler() in my main loop is not returning.
Questions.
1) By way of documentation, the data sheet mentions “LFXTBYPASS”, which appears to be referring to “XT1BYPASS” described by the user guide.
2) Can both SMCLK and ACLK be sourced from an external crystal running at 192kHz if in bypass mode?
3) Is this feasible within the timing needs for CapTIvate?
4) What am I misunderstanding?
5) Is there an example of using bypass mode? I didn’t see one in Resource Explorer.
Thanks very much for your time.
PS - I thought that maybe 192kHz was too fast for ACLK, even in bypass mode, so I used 8 as a divider and adjusted by UART baud rate for 9600 given a 24kHz clock and that also failed to print.