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AD9835 FSYNC voltage level

I'm designing a signal generator using a AD9835. I'm interfacing it via a PIC16F88, using these pins for the SPI bus: SDO=pinB2, SCK=pinB4, SS=pinB5. I connect those pins to the correspondent pins in the AD9835: pinB2->SDATA, pinB4->SCLK, pinB5->FSYNC.

I tried with the hardware and the software version of the SPI protocol (this PIC has SPI dedicated hardware), so the PIC's pins that I used can be changed.

I'm also using a 20MHz crystal for the PIC's clock and a 50MHz crystal oscillator for the AD9835.

The SDATA and the SCLK lines are working correctly, but I found that the FSYNC high voltage is only 1V, where it should be 5V as in the others. I tried disconnecting the pinB5 from the circuit and reading the values directly from it and in this case it works correctly, 5V.

Even knowing that a SPI bus doesn't need pull-up resistors, I tried nonetheless, but the result was the same, 1V.

What could be possibly happening in with the AD9835 that it forces the high voltage to 1V only?

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