hi,

Though baud rate of this is set at 57600bps and sampling frequency should be 57600bps*(2data/33bits) = 3491Hz, it looks like possible input is 800Hz at most from the ocilloscope. This is quite a strange judging from a sampling theory. So, I know there're some point that makes transmission speed much lower than expected.

sure, there are:

1) while(SCONV); // wait for ADC completion
2) while(!TI);
// then some doing
// then load SBUF

So, some time is "ate" with wait loop till conversion has been done by ADC. Look at manual to see this value.

In addition, some time is lost because you wait till transmission has been done, then prepare new data and send it finally. To avoid it, prepare data to be ready for sending before wait TI flag. Something like:

	tmp = ((ADCDATAH&0x0F) << 4) | ((ADCDATAL&0xF0) >> 4);
	while(!TI);
	TI = 0;
	TB8 = 1;
	SBUF = temp;

Regards,
Oleg