Sharp pulse on ADC input and NAP

[ardhuru]

Hi, I have a requirement as follows;

Wait for response on an ADC input; the trigger signal expected is around 1 volt, width of 2 ms. The moment it arrives, output a 500 ms pulse on another pin. Kind of a pulse stretcher (was thinking of using a CMOS 555, but want to avoid components and make it flexible). Very weak power supply available, so maximum consumption to be limited to 40 uA or so.

I tried this code, and it works perfect, except of course the consumption (no load) is around 800 uA.

TEST:;NAP 0
ADCIN 2, LEVEL
IF LEVEL > 5 THEN
DEBUG #LEVEL,13,10
HIGH LED
PAUSE 500
LOW LED
ENDIF
GOTO TEST

I snicked in a NAP 0 command, and it stops working; triggers once in a few events.

The NAP 0 works fine if I manually feed a wider pulse to the input trigger and consumption drops to 30 uA), so I guess its the narrow pulse that doesnt like the NAP command. Since the input transition is way less than 5 volts, I am using the pin as an ADC input.

So my question is, how can this be done best? I'd like to avoid using any extra passive components if I can.

Thanks and Regards,

Anand Dhuru

[mister_e]

I would use a wake on comparator interrupt or pin change with a permanent sleep instead
@ SLEEP

a PIC10F204/206 can wake from a comparator or any pin change, really cheap, would be a perfect choice. Would require only a few lines of code.
http://ww1.microchip.com/downloads/e...Doc/41239D.pdf

Is it mandatory to supply 5V to your PIC? What's the requirement for the Output pulse amplitude?
Why i'm asking, refer to figure 13-2 and later.

[ardhuru]

Thanks for the fast response, Steve. Funny, I'm using a 12F675, but did think it was an overkill and a 10FXXX would be a fine fit.

Also thought the @ SLEEP would be the answer, but I have to admit interrupts (specially the ADC triggerred ones), are not my Forte. Could you possibly point me in the right direction?

Actually, the supply is 3.3, and the PIC merely needs to pull down an external pin thats held high (also at 3.3) with a very weak pull-up.

For those interested, this is a wired remote for DSLRs that uses a smartphone as a triggering device with s/w thats available free (for Androids for sure, dont know about other platforms)

Regards,

Anand Dhuru

[Bruce]

I have not tested this since I don't have a 12F675, but something like this should work. You might need to tweak the internal CVref settings.

Hook your sensor output to GPIO.1 Vin-. When your sensor outputs 1V (or anything > CVref) it should wake-up from sleep.

If you don't have PBP3 just get rid of the config line, and use what you have.

#CONFIG
  __config  _INTRC_OSC_NOCLKOUT & _BODEN_OFF & _MCLRE_OFF & _WDT_OFF & _CP_OFF
#ENDCONFIG
 
DEFINE OSCCAL_1K 1 ' load internal osc calibration value
 
LED VAR GPIO.2      ' LED output
CmpFlag VAR PIR1.3  ' alias comparator interrupt flag bit
CoutBit VAR CMCON.6 ' CMCON.6 is set while voltage on Vin- > CVref
 
ANSEL = 0          ' disable A/D
GPIO = 0           ' LED off on POR
TRISIO = %00000010 ' GPIO.1 = input for Vin- comparator, rest outputs
VRCON = %10100111  ' internal CVref enabled, low range.
                   ' CVref = (VR3:VR0 / 24) * VDD.
                   ' with (VR3:VR0) = 7 we have (7/24) * 3.3V
                   ' so internal CVref = 0.9625V
                   ' a voltage > 0.962V on Vin- (GPIO.1) will set COUT,
                   ' set the comparator interrupt flag, and wake us up.
CMCON = %00010100  ' Comparator w/o Output and with Internal Reference
 
PIR1 = 0           ' clear flags before enabling comparator int
PIE1 = %00001000   ' comparator interrupt enabled.
INTCON = %01000000 ' enable peripheral interrupts for comparator
                   ' global interrupts disabled, so there's no jump to
                   ' an interrupt vector. We're just waking-up from sleep.
 
TEST:
  WHILE CoutBit    ' wait for COUT to clear. This clears when the voltage on
  WEND             ' Vin- falls below the internal CVref
  CmpFlag = 0      ' clear comparator interrupt flag bit before entering sleep
  @ SLEEP          ' should sleep until comparator trips
  HIGH LED
  PAUSE 500
  LOW LED
  GOTO TEST
  end

[ardhuru]

Bingo, Bruce! My mistake, the trigger amplitude is more like 0.5 volts, so I just edited the VRCON bits and it works like a charm. But, I measure around 200 uA in standby now. Still a HUGE improvement over my earlier 800 uA, but with the NAP I was getting a draw of about 30 uA. Am I doing something wrong?

All above readings are on my breadboard at 5 volts, while testing. When its finalized the circuit will work on 3.3 volts, so I guess the consumption would be reduced significantly.

Regards,

Anand

[HankMcSpank]

I have a 12LF1822 running at about 8-10ua in Sleep (the PIC 'LF' variants consume signifiantly less power in sleep, so considering moving over to a PIC that can be had in LF variant. Also, as you point out, the lower the voltage the better)

It's imperative before putting the PIC to sleep, to get all your IO pins & PIC peripherals in the right 'state' to consume the least power while sleeping - I find the best way is studying the associated ciruit it interfaces with, then a massive amount of trial & error while checking the current draw on my DVM! (ie switch off all the uneeded PIC internal peripherals, IO pin TRIS states have the most impact along with pins being set up as analague or not...whilst there are rules of thumb - http://ww1.microchip.com/downloads/e...hapter%202.pdf - I've found the results in the real world to be somewhat random!)