MSP430FR2353: Design questions

1) I suspect the answer is Yes, but I really should leave this to the EEs.

2) No. Once you turn on DACEN/OAEN, the output goes right to the pin [Ref Data Sheet (SLASEC4C) Fig 6-3 + observed behavior]. The PSEL=0b11 setting only serves to turn off the Schmitt trigger, so it doesn't get in the way.

3) Yes. Set ADCRES=1.

b) I'm not completely sure where they got 0.52us from in DS Table 5-21, since my calculator says 4/6MHz=0.66us, but the difference is negligible. Assuming SHT=0, I get (4+12+1)/6MHz=2.8usec which is about what you got.

c) You probably shouldn't get bogged down by Tsettle=0.1us, since that's about half an ADC clock, i.e. you probably can't measure it. As a practical matter, there is no settling time.

d) See also (c). You do need to account for the time it takes to switch the channel in the ADC, which you probably can't do in less than about 15 CPU clocks.

More generally, you need to account for:

A) The (CPU) time to start the ADC,  figure out that it's finished, and  fetch the result.

B) Any extra (S/H) time you need if your input isn't very low impedance.

Hi,

Thank you for replay Bruce.

1) I think the SAC OA can be used as buffer for SAC DAC ( SLAA833A page 11) E2E team please confirm

2a) I think i can use OAEN bit 8 from SACxOA register , when '0' output on pin high impedance, when '1' output on pin is from OA

Can you tell what is the delay when switching OAEN bit 8 from 0->1 and 1->0 ?

2b) Or i can connect GND on OAx+ pin and then switch PSEL port between '00' and '01'

Can you tell what is the delay when switching PSEL bit 0-1 from 00->01 and 01->00?

3a)The questions is how many values can we measure in 10us ?

3b) They got the 0.52us from the following formula tsample = ln(2^(n+1)) × τ where τ = (Ri +Rs)*(Ci+Cex)

Rs = External source resistance

Ri = Internal MUX-on input resistance

Ci = Input capacitance

Cex = External capacitance

Why does your calculator say 4/6MHz for tsample , where did you get the formula?

3c/d) how do you know that it needs 15 CPU clk-s to switch from one input to other ? is this specified in DS?

So for 24MHz -> 625ns?

4) what is the time needed for CPU to start the ADC?

Will the CPU give interrupt when the conversion is finished?

Br,

David.

2a/b) Data sheet (SLASEC4C) Table 5-25 indicates an OA settling time of 1-4.5 sec (depending on OAPM).

3b) That's 4 ADCCLKs (SHT=0) at 6MHz (max ADC clock). You could actually make this shorter by generating your own sampling pulse (SHP=0) but the logistics might be tricky. 

3c) I wrote the code in my head and estimated the clocks for each instruction. Maybe you can do better -- write the code and count the clocks using User Guide (SLAU445I) Chapter 4. Or, better yet, code it and measure it.

4) UG Fig 21-6 shows that (with SHP=1) it can take up to 1 ADC clock from trigger to S/H. An interrupt will be triggered at the end if you set ADCIE0.

More generally, I suggest you buy a Launchpad (MSP-EXP430FR2355) and try/measure all these things before you "bet your project" on them. I don't think you've described more than maybe 100 lines of code.

Hi Srinivas,

on question regarding internal voltage reference.

1)

If we select 1.5V internal reference for ADC can we then select 2.5V internal reference in the same time for DAC?

2)

What voltages can we use on external VEREF+ pin ?

3) Regarding Pin mux

if i use eg. RSM package 32-pin MSP430FR2353TRSM and if i want use OA3O then this pin is only available on pin25 or can be set to other pin?

Br.

David.

Hi Srinivas,

why do you think that Veref+ can only accept 1.2V ?

I think it can be also 3.3V, because of table 4.1 page 18 in datasheet, because it says the power source for Veref+ is DVCC and we will use 3.3V for DVCC ?

Eg. the MSP430FR5994 has this specification on page 60

10) do you see any issue to use single ended crystal oscillator ECS-2520MV-240-BN on XIN pin ?

Best Regards,

David.

Dear Srinivas,

1)

waht is the proper termination of unused I/O pins on MSP430?

Best Regards,
David.