SLAU847D October 2022 – May 2024 MSPM0L1105 , MSPM0L1106 , MSPM0L1227 , MSPM0L1228 , MSPM0L1228-Q1 , MSPM0L1303 , MSPM0L1304 , MSPM0L1304-Q1 , MSPM0L1305 , MSPM0L1305-Q1 , MSPM0L1306 , MSPM0L1306-Q1 , MSPM0L1343 , MSPM0L1344 , MSPM0L1345 , MSPM0L1346 , MSPM0L2227 , MSPM0L2228 , MSPM0L2228-Q1
This section describes the procedure for selecting the external ROSC resistor (including computing the achievable SYSOSC accuracy) as well as the procedure for enabling the mode through the device SYSCTL registers.
SYSOSC Overall Frequency Accuracy with TI Recommended ROSC Resistor
The device-specific data sheet includes specifications for overall SYSOSC frequency accuracy when the TI-recommended ±0.1% tolerance, ±25-ppm/°C, TCR resistor is used for ROSC. The TI-recommended resistor enables a variety of applications, including UART communication. If the overall accuracy values in the device-specific data sheet with the TI-recommended resistor specifications meets the application and cost requirements, then there is no need to compute the accuracy for an alternate resistor or alternate temperature ranges.
However, if a higher accuracy or reduced cost resistor is needed, the designer can use a more precise or less precise resistor for ROSC to balance cost with frequency accuracy for a given application scenario.
SYSOSC Overall Frequency Accuracy with Designer Selected Resistor
Assuming an ideal (zero error, zero temperature drift) reference resistor, the FCL mode improves the SYSOSC circuit accuracy to the level specified in the fSYSOSC:"SYSOSC frequency accuracy when frequency correction loop (FCL) is enabled and an ideal ROSC resistor is assumed" section of the SYSOSC specifications table in the device-specific data sheet. The overall SYSOSC frequency accuracy in FCL mode which is achievable for a given external resistor and temperature range is determined by combining the following error sources to determine the total frequency error:
The following table gives an example of how to compute the accuracy for different conditions, given the following typical application scenario:
Step | Description | MIN | TYP | MAX | UNIT |
---|---|---|---|---|---|
1 | Define the operating temp range of the application. | -40 | 25 | 85 | ℃ |
2 | Select the target fSYSOSC frequency (4 or 32MHz) | 32.00 | MHz | ||
3 | Look up the corresponding fSYSOSC ideal resistor accuracy from the device-specific data sheet based on the temperature range in Step 1. | -0.80 | 0.93 | % | |
4 | Compute the min and max fSYSOSC with an ideal resistor based on the selected frequency in Step 2 and the accuracy range in Step 3. | 31.744 | 32.298 | MHz | |
5 | Select an ROSC resistor with acceptable tolerance (the TI-recommended ±0.1% tolerance used in this example) | 99.9 | 100 | 100.1 | kΩ |
6 | Note the temperature coefficient of resistance (TCR) of the ROSC resistor. | ±25 | ppm/℃ | ||
7 | Compute the max temperature variation from 25°C (nominal) based on the temperature range defined in Step 1. | | 25 - (-40) | = 65 | ℃ | ||
8 | Compute the max ROSC drift over the max temperature variation (from Step 7) and the TCR (from Step 6). | -0.16 | 0.16 | % | |
9 | Compute the min and max ROSC resistance including tolerance (from Step 5) and temperature drift (from Step 8). | 99.74 | 100.26 | kΩ | |
10 | Compute the overall ROSC resistor accuracy (in percent), using the overall ROSC resistance range from Step 9 vs. the nominal resistor value. | -0.26 | 0.26 | % | |
11 | Compute the variation in the min and max fSYSOSC by degrading the min/max fSYSOSC computed in Step 4 by the ROSC accuracy computed in Step 10. | 31.66 | 32.38 | MHz | |
12 | If desired, convert the raw min/max fSYSOSC computed in Step 11 into a %-based accuracy format relative to the target fSYSOSC from Step 2. | -1.06 | 1.20 | % |
To increase the SYSOC accuracy with FCL, follow this procedure: