View AN663_4635862.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

rev. 0.1 2/12 copyright ? 2012 by silicon laboratories AN663 AN663 p recision 32? mcu f amily c locking o ptions 1. introduction the 32-bit precision32? mcu family features a highly configurable clock structure. the advanced high- performance bus (ahb) and adva nced peripheral bus (apb) have many options available for clocking. in addition, firmware can disable most pe ripheral clocks to save power. the clock to the module must be enabled before firmware can modify the registers. the first initialization step in all peripheral initialization routines should enable the clock to the module. the corresponding precision32 family reference manual contains detailed descriptive and usage information on each of the clock sources mentioned in this document. 2. clock sources the ahb drives the core and memory on arm ? cortex?-m3 devices. the apb drives the pe ripherals and peripheral registers. the ahb on the precision32 devices has several options for clock sources: low power oscillator, low frequency oscillator, rtc oscillator, external o scillator, pll oscillator, and usb oscillator. the apb clock is either equal to ahb or a divide by 2 version of it, so it is always synchronized with the ahb clock. table 1 describes the oscillators th at can serve as clock sources for the ahb bus. figure 1 shows an example clock control block diagram. table 1. ahb clock source summary oscillator name short name module frequency use case low power oscillator lposcn lposcn 20 mhz or 2.5 mhz high frequency and lower power consumption required low frequency oscillator lfoscn rtcn 16.4 khz low power, slow frequency required; no room for exter- nal rtc watch crystal rtc oscillator rtcnosc rtcn 32.768 kh z low power, slow, accurate frequency required; room for external rtc watch crystal external oscillator extoscn extoscn crystal: 10 khz to 25 mhz cmos: 10 khz to 50 mhz rc or c: up to 1.6 mhz very accurate frequency required; room for external components pll oscillator pllnosc plln 23 mhz to maximum device frequency flexible, fast, and accurate frequency required usb oscillator usbnosc usbn 48 mhz very accurate fast fre- quency required; no flexibil- ity needed
AN663 2 rev. 0.1 figure 1. example precision32 clock control block diagram clock control apb clock ahb clock divider apb clock divider peripheral 0 peripheral 1 peripheral 2 peripheral 3 peripheral 4 ahb clock ram memory area 2 flash memory area 3 memory area 4 lfosc0 lposc0 rtc0 oscillator external oscillator usb0 oscillator pll0 oscillator
AN663 rev. 0.1 3 2.1. low power oscillator (lposcn) the low power oscillator is the default oscillator after reset. the default, factory-ca librated frequency of this oscillator is 20 mhz, but a divide by 8 versi on is also available as an ahb clock source. this oscillator starts very quickly an d automatically turns on and off as needed. this oscilla tor consumes less power than other oscillators, but may not be as accurate over vo ltage and temperature. there are no configuration bits for th is oscillator. figure 2 shows the blo ck diagram for the lo w power oscillator. figure 2. low power oscillator block diagram 2.2. low frequency oscillator (lfoscn) the low frequency oscillator is part of the rtc module and can be selected as an rtc timer clock source in addition to the ahb clock source. the factory-calibra ted output frequen cy of this oscillator is 16.4 khz. this oscillator consumes less power th an other oscillators, but will not be as accurate as the external rtc watch crystal. figure 3 shows the block diagram fo r the low frequency and rtc oscillators. lposcn module lposcn clock low power oscillator automatic start/ stop control divided lposcn clock
AN663 4 rev. 0.1 2.3. rtc oscillator (rtcnosc) the rtc oscillator is part of the rtc module and is derived from the 32.768 khz watch crystal or cmos oscillator. this oscillator can be a source for the ahb or selected as the rtc timer clock source. the load capacitance for the watch crystal is internal and adjustable, so no external components other than the crystal are required. the crystal should be connected dire ctly across the rtc1 and rtc2 pins on the device, and these pins should be configured for analog input mode. the rtc oscillator is more ac curate than the low frequency oscillator, allowing for pr ecision timekeeping with the 32-bit rtc timer. figure 3 shows the block di agram for the lo w frequency and rtc oscillators. figure 3. low frequency and rtc oscillator block diagram rtcn module rtc1 rtc2 rtc external oscillator control low frequency oscillator rtc timer rtcn_out external crystal rtc1 32.768 khz rtc2 external cmos clock load capacitance additional rtc module components frequency adjust rtcnosc clock lfoscn clock rtc1 rtcnosc_out
AN663 rev. 0.1 5 2.4. external os cillator (extoscn) the ahb clock may be derived from the ex ternal oscillator module. the external oscillator su pports four different external oscillators: crystal, cmos, rc, and c. figure 4 depicts a block diagram fo r the external oscillator module. figure 4. external oscillator block diagram extoscn module extoscn clock xtal1 xtal2 external oscillator control external crystal xtal1 xtal2 10 m ? external rc oscillator vio xtal2 external c oscillator xtal2 external cmos oscillator xtal2
AN663 6 rev. 0.1 2.4.1. crystal mode when operating in crystal mode, the external oscillator module supports crystals ranging from 10 khz to 25 mhz. the module requires a 10 m ?? resistor across the crystal pins and two loading capacitors between the crystal pins and ground. both of the crystal pins must be configured for analog input mode. figure 5 shows the external crystal oscillator hardware configuration. figure 5. external crystal oscillator configuration the capacitors provide the load capacitance required by th e crystal for correct oscillati on. these capaci tors are ?in series? as seen by the crystal and ?in parallel? wit h the stray capacitance of the xtal1 and xtal2 pins. note: the recommended load capacitance depends upon the crystal and the manufacturer. refer to the crystal data sheet when completing these calculations. equation 1 describes the equation for determining th e load capacitance for the two capacitors. the c a and c b values are the capacitors connec ted to the crystal leads. the c s value is the total stray capacitance of the pcb, which is typically between 2 pf and 5 pf per pin for a typical layout where the crystal is as close as possible to the pins. equation 1. crystal load capacitors if c a and c b are the same (c), the resulting equation is shown in equation 2. equation 2. simplified crystal load capacitors for example, using equation 2 with a 32.768 khz tuning-fork crystal with a recommended load capacitance of 12.5 pf placed as close to the pins as possible (3 pf pe r pin) results in crystal load capacitors of 13 pf each. crystal oscillator circuits are quite sensit ive to pcb layout. the cryst al should be placed as close as possible to the xtal pins on the device. th e traces should be as short as possible and shielded with ground plane from any other traces that could introduce noise or interference. sim3xxxx device xtal1 xtal2 extoscn module 10 m ? c c c l c a c b ? c a c b + --------------------- c s + = c l c 2 --- - c s + =
AN663 rev. 0.1 7 2.4.2. cmos mode when operating in cmos mode , the external oscilla tor module supports cm os oscillators with output frequencies ranging from 10 khz to 50 mhz. the external cmos cl ock should be connected directly to the xtal2 pin configured in digital input mode. the xtal1 pin should be left floating. the cmos oscillator mode is available with a divide by 2 stage, ensuring the clock has a 50% duty cycle. 2.4.3. rc mode the external oscillator module supports output frequencies up to 1.6 mhz when operating in rc mode. the rc components should be connected to xtal2 configured for analog mode, while xtal1 should be left floating. figure 6 shows the external rc o scillator hardware configuration. figure 6. external rc oscillator configuration the capacitor used in the rc network should have a value no greater than 100 pf, and the resistor should be no smaller than 10 k ? . for very small capacitors, the parasitic capaci tance in the pcb layout may dominate the total capacitance. the oscillation frequency can be determined by equation 3, where f is the frequency in mhz, r is the pull-up resistor value in k ? ., and c is the capacitor value in the xtal2 pin in pf. equation 3. rc oscillation frequency the rc oscillator mode is only available with a divide by 2 stage, which ensures that the clock derived from the external oscillato r has a duty cycle of 50%. th e equation for the extoscn ou tput frequency is shown in equation 4. equation 4. extoscn output frequency in rc mode sim3xxxx device xtal1 xtal2 extoscn module vio c r f 1.23 10 3 ? rc ? -------------------------- - = f out f 2 -- - 1.23 10 3 ? 2r ? c ? -------------------------- - ==
AN663 8 rev. 0.1 2.4.4. c mode when operating in c mode, the external oscillator module supports frequencies up to 1.6 mhz. the capacitor should connect directly to xtal2 configured for analog mode. xtal1 should be left floating. figure 7 shows the external c oscillator ha rdware configuration. figure 7. external c oscillator configuration to determine the required module setting s, select the capacitor to be used and find the frequen cy of oscillation according to equation 5, where f is th e frequency of oscillation in mhz, c is the capacitor value in pf, v dd is the device power supply in volts, and kf is the k factor. equation 5. c oscillation frequency the c oscillator mode is only available with a divide by 2 stage, whic h ensures that the cl ock derived from the external oscillato r has a duty cycle of 50%. th e equation for the extoscn ou tput frequency is shown in equation 6. equation 6. extoscn output frequency in c mode sim3xxxx device xtal1 xtal2 extoscn module c f kf cv dd ? --------------------- = f out f 2 -- - kf 2c ? v dd ? ------------------------------- ==
AN663 rev. 0.1 9 2.5. pll oscillator (pllnosc) the plln module available on sim3xxxx devices is a digital pll capable of generating output frequencies between 23 mhz and the maximum device operating frequ ency. the pll has configurable output ranges and frequencies, multiple reference clock inputs, locking on either the falling or ri sing edge of the reference, dco frequency lsb dithering to provide finer average output frequencies, spectrum spreading to reduce generated system noise, low jitter, and fast lock times. the plln module also supports three output modes: fr ee-running dco, frequency-locked, and phase-locked. the pll can lock on a reference frequency and then sw itch to free-running dc o mode to save power. figure 8 displays the plln module block diagram. figure 8. pll module block diagram plln module phase and frequency adjuster cal dither range digitally-controlled oscillator (dco) spectrum spreading output frequency (f dco ) reference frequency (f ref ) outmd m + 1 n + 1 extoscn clock usbn oscillator divided low power oscillator rtcn oscillator
AN663 10 rev. 0.1 2.6. usb oscillator (usbnosc) the usb oscillator is part of the usbn module and oper ates at a factory- calibrated 48 mhz. when connected to the usb bus, the clock recovery mechanism can constantly ad just the oscillator frequenc y relative to the full-speed frame reference, creating a very accurate clock source. this oscillator is with in the usb specification across voltage and temperature, allowing the application syst em to be hardware usb certified without an external oscillator. this oscillator can be used as the ahb clock source ev en if the usb perip heral is not used. figure 9 displays the usbn module block diagram. figure 9. usb module block diagram usbn module d+ d- extoscn internal usbn oscillator clock recovery pllnosc usbnosc transceiver serial interface engine
AN663 rev. 0.1 11 3. clocks and the peripherals the apb clock drives most peripherals and their registers. the exceptions to this are lower power peripherals that run from the rtc oscillator or the low frequency oscillator directly. for peripherals that run from the apb clock, disabling th e clock to the module in the device clock control module (clkctrl) will disable the clock to bo th the module and the module?s regist ers. for peripher als that run from another clock so urce, disabling the clock to the module will disable the clock to the module?s registers only and the module will continue to run. 4. reset behavior on sim3xxxx devices, the low po wer oscillator (lposcn) is the default oscillato r after a reset. in addition, the clocks to most peripherals are disabled to save power. the clock to the module must be enabled before firmware can modify the registers. the first initializati on step in all peripheral in itialization routines should enable the clock to the module.
AN663 12 rev. 0.1 c ontact i nformation silicon laboratories inc. 400 west cesar chavez austin, tx 78701 tel: 1+(512) 416-8500 fax: 1+(512) 416-9669 toll free: 1+(877) 444-3032 please visit the silicon labs technical support web page: https://www.silabs.com/support/pages/contacttechnicalsupport.aspx and register to submit a technical support request. silicon laboratories and silicon labs are trademarks of silicon laboratories inc. other products or brandnames mentioned herein are trademarks or registered trademarks of their respective holders. the information in this document is believed to be accurate in all respects at the time of publication but is subject to change without notice. silicon laboratories assumes no responsibility for errors and omissions, and disclaims responsib ility for any consequences resu lting from the use of information included herein. a dditionally, silicon laboratorie s assumes no responsibility for the functioning of und escribed features or parameters. silicon laboratories reserves the right to make changes without further notice . silicon laboratories makes no wa rranty, rep- resentation or guarantee regarding the suitability of its products for any particular purpose, nor does silicon laboratories as sume any liability arising out of the application or use of any product or circuit, and s pecifically disclaims any an d all liability, including wi thout limitation conse- quential or incidental damages. silicon laborat ories products are not designed, intended, or authorized for use in applications intended to support or sustain life, or for any other application in which the failure of the silicon laboratories product could create a s ituation where per- sonal injury or death may occur. should buyer purchase or us e silicon laboratories products for any such unintended or unauthor ized ap- plication, buyer shall indemnify and hold silicon laboratories harmless against all claims and damages.

▲Up To Search▲

Price & Availability of AN663

	To Download AN663 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .