turn on suggestions

Auto-suggest helps you quickly narrow down your search results by suggesting possible matches as you type.

Showing results for

- Silicon Labs Community
- :
- Silicon Labs Community Knowledge Base
- :
- Microcontrollers Knowledge Base
- :
- 32-bit MCU Knowledge Base
- :
- CLFXO_TS Parameter and LFXO Load Capacitance Tunin...

- Silicon Labs Community
- Forum
- Welcome and Announcements
- Silicon Labs Knowledge Base
- Microcontrollers
- 8-bit MCU
- 32-bit MCU
- Wireless
- Bluetooth / Wi-Fi
- Mesh
- Proprietary
- Other Products Category
- Optical/RH/Temp Sensor
- Timing
- Interface
- Other Products
- Hardware and Software Tools
- Simplicity Studio and Software
- Discussions
- General Discussions and Suggestions
- Chinese Forum
- ChineseForum
- Share
- Projects
- Contests
- Tools
- Software Libraries
- Development Kits
- Reference Designs
- Third Party Tools
- Training
- Video-Tutorials
- Lectures
- White Papers
- Blog
- Official Blog of Silicon Labs
- Chinese Blog

- Article History
- Subscribe to RSS Feed
- Mark as New
- Mark as Read
- Bookmark
- Subscribe
- Email to a Friend
- Printer Friendly Page
- Report Inappropriate Content

In the **LFXO Configuration** section of the CMU chapter on EFM32 Series 1 and EFR32 devices, the following equation is presented for calculating the value of the CMU_LFXOCTRL register's TUNING bit field:

TUNING = ((desiredTotalLoadCap × 2 - Min(C_{LFXO_T})) / C_{LFXO_TS})

What is or where do I find the CLFXO_TS parameter? I need this in order to adjust the LFXO's tunable load capacitors to match the specification for the crystal in my design.

For the equation above to get you a register bit field value, the result needs to be dimensionless, i.e. it cannot have any units. Needless to say C_{LFXO_TS} must be a datasheet or other parameter, ideally quantified in pF.

Looking at the table under **Electrical Specifications → Electrical Characteristics → Oscillators → LFXO** in any EFM32 Series 1 or EFR32 datasheet, C_{LFXO_TS} is not listed. However, right under the C_{LFXO_T} parameter, which is in the equation above, there is a specification for the on-chip tuning capacitor step size, in pF, with the symbol SS_{LFXO}.

This parameter makes sense in the equation above because if you divide the numerator by the step size, you get a value without units. Likewise, min(C_{LFXO_T}) is specified in the datasheet as 8 pF, so you know that if 16 pF is desired for the total loading capacitance of a given crystal, then TUNING = 0 gets the minimum C_{LFXO_T} value of 8 pF.

Finally, it follows that if C_{LFXO_T} is intended to be read as "LFXO tuning capacitance" then reading C_{LFXO_TS} as "LFXO tuning capacitance step size" isn't much of a stretch. Another way to clear up this confusion is simply to rewrite the equation above as:

TUNING = ((desiredTotalLoadCap × 2 - Min(C_{LFXO_T})) / SS_{LFXO})