DSO 150 : (Slow) Timebase

Slow Capture


So far, we've captured up to 1ms/division  just by selecting the right ADC scaler & rate ( and dropping some samples).

What about slower capture ? i.e. 5mS/div or 100ms/div ?

One option would be to do oversampling i.e. capture much more data at 1ms/div  and drop a lot of them

That would require large amount of RAM, for nothing.

The other option is to use a timer.

The scheme is as follows :
* Start a timer and ask for N ADC samples
* When the timer elapses, average the N samples and start a new ADC sampling sequence
* When we have the required number of points, done !

It was a bit tricky to do because there were more timer interrupts than expected (for reason unknown so far).

Triggers


The trigger is based on a OP-Amp comparator, with its output connected to PA8.
The relevant part of the JyeTech schematic  is that one :


When input (after scaling) is higher than Vref, PA8 is "One"
When input (after scaling) is lower than Vref, PA8 is "Zero"

So we have to set Vref (taking scaling into account) and the proper interrupt type on PA8  and we have our trigger.

Vref is connected to TL_PWM (PB8) through a low pass RC filter (~ 16 Hz cut-off frequency).
TL_PWM is meant to be used as a PWM, so that the low pass filter will give the duty_cycle *Vcc.
We just need  the PWM frequency to be much higher than 16 Hz, not a problem.

       Vref=Vcc*DutyCycle

Since it is a 16bits PWM, the max range is 0...65535 noted as  64k
       Vref=Vcc*R/64k


The tricky part is that we have to invert the scaling to get the Vref we want i.e.


Pwm DutyCycle = (TriggerVoltage*InputGain+CalibrationZero)*64k/Vcc

CalibrationZero is  ~1.5v, we can get a better value from the calibration offset.




Comments

Post a Comment

Popular posts from this blog

Component tester with STM32 : Part 1 ADC, Resistor

Image
I was always  fascinated by the so call "transistor testers" that identify and report pin out, features of pretty much whatever you throw at it. They are based on Atmel 328p chip, the same as arduino nano. So why not do the same thing with a STM32/Bluepill ? The STM32 is faster, has a better ADC accuracy, more memory, can do float etc... and is not more expensive. Warning : I'm doing this for fun & to learn, it might contain error & plain misunderstanding So, i watched a couple of video on the general principle on how they work. This is not a "port" but a rewrite from ~ scratch. When something is weird, i check what the original one does for comparison. Measuring resistance So first thing first :  Resistance & Capacitance. These 2 will unlock the other ones. So how do we measure resistance ? What we do is a resistor divider with a know resistance value: Measure = Vcc*(RtoTest/(RtoTest+Rknown)) As far as the ADC goes, it means ADC=
Read more

Fixing the INA3221

Image
It 's not too complicated to change the board we have seen  previously . You 'll need to cut some tracks, i did it with a dremel and i'm not very handy, so you'll do better. Red =Cut tracks Blue = Add wire 1- You'll need to separate the bottom connector from the ground, cut the 3 connectors BOTH SIDES. 2- Remove the small red area below the middle resistor to isolate each channels from each other Last, connect right side of each connector to the now isolated pad. Mine looks like this after the operation (ugly i know): Be wary of the small track on the left connector that leads to the left side of channel3 resistor. I cut it while isolating the right pad. Now it works, we can go back to the charger.
Read more

INA3221, weird wiring

Image
Continuing my "smart" battery charger, i've finally received the INA3221 module. It is the purple one you can find on ebay for ~ 5 bucks I soldered a couple of pins to be able to connect it so that i could test it and something immediately caught my attention. It is supposed to be a high side current/voltage sensor, typically in a setup like that : So you can measure the voltage supplied to the load, and the current going through it. Now, let's have a look at the module, look at the top of the following pic, where the sense connectors are. CH3/GND/CH2/GND/CH1/GND ? Wait ? What ? Yes, all the IN- pins are connected to ground, so it is a bottom current only sensor. Additionally, the - side of the shunts are ALSO connected to the ground, no only the connectors. The actual layout is like that : That raises two problems :  The voltage is meaningless as we are only measuring Current x ShuntValue, not the supply volt
Read more