Re: A/D Conversion Clock Select bits confussion ..
You certainly coverted all the bases! Good job.
Instead of bothering to graph all the data and try and make sense of it, Let's start with this:
The 25K20 has a TAD spec of 0.7us min.
At 64MHz, that means all ADCS settings except for FOSC/64 and FRC violate minimum times (Table 19-1).
Using the FOSC/64 setting gives a TAD of 1us
Computing Tc from equation 19-1 for each of the three source resistances gives:
1K = 0.27us
10K = 1.2us
39K = 4.2us
Computing TACQ = Tamp + Tc + Tcoff, given Tamp = 5us and Tcoff = 0 (25degC) produces:
1K = 5.27us
10K = 6.2us
39K = 9.2us
So, since TAD = 1us, TACQ must be a min of ~6-9 TAD for the three different resistances.
Removing all of the measurements that don't fit the above you get:
Pretty consistent. It shows that waiting longer than the minimum doesn't get you anything, but it also doesn't change anything either.Code:***************************************************** 1k resistor in series with input (TACQ=5.27us) ***************************************************** Input voltage 25.7mV FRC 6 TAD Result: 7 FOSC/64 6 TAD Result: 7 FRC 8 TAD Result: 7 FOSC/64 8 TAD Result: 7 FRC 12 TAD Result: 7 FOSC/64 12 TAD Result: 7 FRC 16 TAD Result: 7 FOSC/64 16 TAD Result: 7 FRC 20 TAD Result: 7 FOSC/64 20 TAD Result: 7 Input Voltage: 1.650V FRC 6 TAD Result: 511 FOSC/64 6 TAD Result: 511 FRC 8 TAD Result: 511 FOSC/64 8 TAD Result: 511 FRC 12 TAD Result: 512 FOSC/64 12 TAD Result: 511 FRC 16 TAD Result: 511 FOSC/64 16 TAD Result: 511 FRC 20 TAD Result: 511 FOSC/64 20 TAD Result: 511 Input Voltage: 3.286V FRC 6 TAD Result: 1020 FOSC/64 6 TAD Result: 1019 FRC 8 TAD Result: 1019 FOSC/64 8 TAD Result: 1019 FRC 12 TAD Result: 1019 FOSC/64 12 TAD Result: 1019 FRC 16 TAD Result: 1019 FOSC/64 16 TAD Result: 1019 FRC 20 TAD Result: 1019 FOSC/64 20 TAD Result: 1019 ***************************************************** 10k resistor in series with input (TACQ=6.2us) ***************************************************** Input voltage 25.7mV FRC 6 TAD Result: 7 FOSC/64 6 TAD Result: 7 FRC 8 TAD Result: 7 FOSC/64 8 TAD Result: 7 FRC 12 TAD Result: 7 FOSC/64 12 TAD Result: 7 FRC 16 TAD Result: 7 FOSC/64 16 TAD Result: 7 FRC 20 TAD Result: 7 FOSC/64 20 TAD Result: 7 Input voltage 1.650V FRC 6 TAD Result: 511 FOSC/64 6 TAD Result: 511 FRC 8 TAD Result: 511 FOSC/64 8 TAD Result: 511 FRC 12 TAD Result: 511 FOSC/64 12 TAD Result: 511 FRC 16 TAD Result: 511 FOSC/64 16 TAD Result: 511 FRC 20 TAD Result: 511 FOSC/64 20 TAD Result: 511 Input voltage 3.286V FRC 6 TAD Result: 1020 FOSC/64 6 TAD Result: 1019 FRC 8 TAD Result: 1019 FOSC/64 8 TAD Result: 1019 FRC 12 TAD Result: 1020 FOSC/64 12 TAD Result: 1019 FRC 16 TAD Result: 1019 FOSC/64 16 TAD Result: 1019 FRC 20 TAD Result: 1020 FOSC/64 20 TAD Result: 1019 ***************************************************** 39k resistor in series with input (TACQ=9.2us) ***************************************************** Input voltage: 25.7mV FRC 6 TAD Result: 7 FOSC/64 6 TAD Result: 7 FRC 8 TAD Result: 7 FOSC/64 8 TAD Result: 7 FRC 12 TAD Result: 7 FOSC/64 12 TAD Result: 7 FRC 16 TAD Result: 7 FOSC/64 16 TAD Result: 7 FRC 20 TAD Result: 7 FOSC/64 20 TAD Result: 7 Input voltage: 1.650V FRC 6 TAD Result: 511 FOSC/64 6 TAD Result: 511 FRC 8 TAD Result: 512 FOSC/64 8 TAD Result: 511 FRC 12 TAD Result: 511 FOSC/64 12 TAD Result: 511 FRC 16 TAD Result: 512 FOSC/64 16 TAD Result: 511 FRC 20 TAD Result: 511 FOSC/64 20 TAD Result: 511 Input voltage: 3.286V FRC 6 TAD Result: 1019 FOSC/64 6 TAD Result: 1019 FRC 8 TAD Result: 1019 FOSC/64 8 TAD Result: 1019 FRC 12 TAD Result: 1019 FOSC/64 12 TAD Result: 1019 FRC 16 TAD Result: 1019 FOSC/64 16 TAD Result: 1019 FRC 20 TAD Result: 1019 FOSC/64 20 TAD Result: 1019
The interesting thing is as you look through the raw data you can see the effect of not using the proper TAD setting for the clock freq you're using, or not waiting/setting TACQ to at least the minimum time.
Bookmarks