xPIClog

Power Supply

The circuit does not need any power switch. The current consumption in sleep mode (measured at power terminal X9) is ~5uA:

• 1.5 uA MCP1703
• 1.0 uA R36/R60
• 0.5 uA NC7S14 + MCP6441
• 2.0 uA PIC, sleep, RTC enabled (not all peripherals off in V0.8)

MCU

• real-time clock and calendar
• "lot" of RAM (for a PIC ;-)
• 10 channel, 12 bit A/D converter
• internal PLL up to 48MHz for fast SD-card access
• an UART 8)

While operating (sampling) or turned off, the PIC is kept in sleep mode, with RTC enabled.
It wakes up on one of the following conditions:

• RTC alarm (new measurement)
• external trigger (RC4/RP15)
• push button (RC3/RP14); stop acquisition or power on

Beside reading a push button (including JP7 for an external one), lighting a LED (JP8 for external connection) and monitoring a trigger input pin, the PIC can:

• measure 8 external A/D channels
• measure its internal band gap reference
• measure battery voltage via R36, R40 and IC6
• access a SD-card (standard size), including power-down feature
• shutdown the analog input stage IC1,2,4,5 via T2 (POWOUT)
• switch external 3.3V supply at X1-2
• measure switchable terminal voltage at X1-2
• switch external GND connection at X6-1
• communicate via CMOS level UART at X10 (115200bits/s, 8N1)
• ...

For in-circuit programming, three test points MCLR (VPP), PGC and PGD are available. Necessary GND and +3V3 connections are available at terminal X8-1/X6-2/X1-1 (GND) and X7-1 (+3V3).

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(*1*) Note that the schematic contains a MS1V-10 (10p-type). This probably requires C12 and C14 to be raised up to 12pF.

SD-card

Note that the PIC programming pins PGD and PGC share the SD-card's chip select and clock line. The card has to be removed during programming.

Analog Input Stage

Note that the inputs are left floating.
The absence of any pull resistors maintain a maximum of flexibility for external connections and offer the highest input impedance possible.

The complete gain stage can be powered down via T2, saving additional ~8uA (1uA per amplifier).

By default, the circuit is only protected against reverse polarity (e.g.: D2)!
Although the input currents are limited by the 20k resistors, +UB might rise (MCP6032's internal protection diodes) and, at some point, will start to source current into the +3V3 rail via

• T2 substrate diode
• every MCP6032 output and the PIC's internal clamp diodes.

Switchable Power Supply

This can be used to turn on (or off) external components or circuits that otherwise would draw too much current.
Notice that external equipment should not draw more than 10-30mA.

R30 adds a little safety and protects xPIClog against short circuits, but of course creates an additional voltage drop, hence the output voltage will decrease with rising current demand.
To maintain precision, the terminal voltage can be monitored via AN4.

Serial Interface

Changes

- LED now on in MENU MODE (half brightness)
- continue to write to card after overflow (previous write error)

Push Button Operation

• push the button (LED lights up)
• continue to hold button until LED turns off
• xPIClog is now in SLEEP MODE, reducing current consumption to a minimum

POWER ON (from SLEEP MODE):

• push the button (LED lights up)
• release the button (LED turns off)
• xPIClog is now in MENU MODE, wasting ~10mA until you decide to change mode

START ACQUISITION (requires MENU MODE):

• tip the button (LED flashes rapidly)
• xPIClog is now in ACQUISITION MODE

STOP ACQUISITION:

• push or tip the button (LED lights up during that)
• xPIClog writes pending data to SD-card
• xPIClog is now back in MENU MODE

Commands and Configuration

  help             - what you're reading right now...
  start            - start acquisition
  stop|ESC         - stop acquisition (only valid if not sleeping)
  show             - show acquisition parameters
  fname=<filename> - set log and config file name (max 8 chars/digits)	
  fname?           - show log and config file name
  load             - load acquisition parameters from SD-card file '<fname>.cfg'
  save             - save acquisition parameters to SD-card file '<fname>.cfg'	
  time=<hh:mm:ss>  - set time	
  time?            - query time	
  date=<dd:mm:yy>  - set date	
  date?            - query date	
  sleep=<0|1>      - sleep during acqusition? 0=NO, 1=YES	
  sleep?           - show sleep flag	
  srate=<n>        - set sampling rate to <n>	
  srate?           - query sampling rate	
  smeans=<n>       - set number of samples for mean calc (1..16)	
  smeans?          - query number of samples for mean calc	
  sunit=<s|m>      - set sampling rate unit; s=SECS, m=MINS	
  sunit?           - query sampling rate unit	
  sep=<ch>         - set log file separator character; e.g. ';', ',', ...	
  sep?             - query log file separator character	
  logdate=<0|1>    - write date to logfile? 0=NO, 1=YES	
  logdate?         - query date log flag	
  logtime=<0|1>    - write time to logfile? 0=NO, 1=YES	
  logtime?         - query time log flag
  dati2nd=<0|1>    - show date/time behind channels in log file? 0=NO, 1=YES
  dati2nd?         - show if date/time is behind channels in log file
  logxcor=<0|1>    - log UBext and Vdd core to log file? 0=NO, 1=YES
  logxcor?         - show UBext and Vdd core log flag
  usecard=<0|1>    - use SD-card? 0=NO, 1=YES	
  usecard?         - show if logging to SD-card is enabled
  bat?             - show battery voltage in mV (channel 9)
  measraw=<ch>     - measure and show raw A/D channel value <ch> 1-8 (0-11)
  measvol=<ch>     - measure and show voltage at channel <ch> 1-8 (0-11)
  bgcorr=<n>       - set band gap correction in mV (-100..100)
  bgcorr?          - show band gap correction term
  version?         - show firmware version string
  off              - turn unit off (sleep)
			

TO BE CONTINUED...

Connecting xPIClog

TO BE CONTINUED...