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TMS320F28026 internal oscillator

I'm developing a programming algorithm for this micro through an external programmer. I'm able to flash the micro through an external oscillator with this configuration:

VDD (pin 43) -> GND with 1,2 uF capacitor)
VSS (pin 44) -> GND
negVREGENZ (pin 34) -> GND
VDDIO (pin 35) -> VPROG (3,3V with 2,2 uF capacitor)
VSSA (pin 12) -> GND
VDDA (pin 11) -> VPROG (3,3V with 2,2 uF capacitor)

XCLKIN (pin 23) -> external oscillator

+ SCIRXDA, SCITXDA, negTRST, negXRS, GPIO37 (pin 22), GPIO34 (pin 19) driven by my programming routine

Now i want to use the internal oscillator, so i left floating XCLKIN pin but when i send the autobaud character i don't receive answer. I noticed that XRS is going down every 600 us so is the watchdog which is resetting the uC for the BOR circuit (i think). But with the external oscillator the configuration worked! and the voltage is within the range specified with the datasheet. Do someone have any suggestion?