XE8806A SEMTECH [Semtech Corporation], XE8806A Datasheet - Page 49

• Manufacturer Part Number: XE8806A
• Description: Ultra Low-Power Low-Voltage
• Manufacturer: SEMTECH [Semtech Corporation]

XE8806A/XE8807A
7.11 32 kHz frequency selector
A decoder is used to select from the high prescaler, the frequency tap that is the closest to 32 kHz to operate the
low prescaler when the Xtal is not running. In this case, the RC oscillator frequency of ±35% will also be valid for
the low prescaler frequency outputs.
The next table shows how the RC trimming values in the RegSysRcTrim1 and RegSysRcTrim2 registers select
the 32 kHz frequency. The least significant bits of the RcFreqFine word are not used.
In order to ensure the correct frequency selection for the low prescaler when having an external clock, a proper
value must be set in the RC trim registers. The code can be selected from the table below as a function of the
frequency ratio between half the frequency of the external clock and 32kHz. If the frequency is not set correctly, all
timings derived from the low prescaler will be shifted accordingly (e.g. watchdog frequencies) and some
peripherals may no longer function correctly if the deviation from 32kHz is too large (e.g. the voltage level
detector).
© Semtech 2006
www.semtech.com
7-10