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Populating External Crystal with TMS320F28035

kerem kahraman
Prodigy 240 points
Other Parts Discussed in Thread: TMS320F28035

Can one populate the external crystal and keep using the internal crystal?


I have written my code on depending on the internal crystal (10 MHz) with the Piccolo ControlCARD. Now I am building my own board and for the future use we may want to use the external crystal. So I don't want to change my code but would like to have the additional external crystal populated.  


When I look at the schematics of the ControlCARD for R1.1 and R1.2, there is a slight difference. For R1.1 both R4 and X1 are not populated so TMS320F28035's 51st and 52nd pins are floating (X1-X2 pins) where as for R1.2 crystal (X1) , C30 and C31 are populated and R4 is not populated.


I used the sample code for HVACI_Sensorless_CLA_F2803x and the system clock speed is based in internal oscillator(10MHz)

// Switch to Internal Oscillator 1 and turn off all other clock
// sources to minimize power consumption
EALLOW;
SysCtrlRegs.CLKCTL.bit.INTOSC1OFF = 0;
SysCtrlRegs.CLKCTL.bit.OSCCLKSRCSEL=0; // Clk Src = INTOSC1
SysCtrlRegs.CLKCTL.bit.XCLKINOFF=1; // Turn off XCLKIN
SysCtrlRegs.CLKCTL.bit.XTALOSCOFF=1; // Turn off XTALOSC
SysCtrlRegs.CLKCTL.bit.INTOSC2OFF=1; // Turn off INTOSC2
EDIS;


So does this mean that I can design my board like Rev1.2 (populate X1, C30-31 and not populate R4) and I still can use my internal  oscillator settings?

  • 0
    TI__Guru 60865 points

    Hi Kerem,

    You can definitely design your board like this.  Furthermore, the internal oscillator is the default clock on power-up and it can be used regardless of what is on X1/X2.  This means you could actually just place the footprint of the crystal and associated components on the board, but not populate them (if you want). If you decide you want to use the crystal later (generally this would be to get a better absolute clock accuracy; the internal oscillators will not be able to achieve better than 1% over temperature) then solder down a crystal and related components and make a small software change to select the crystal after the device boots.