BASIC Stamp (OWL2) to TLC2543

analog to digital converter

(c) 1998 , 2003, 2005 EME Systems, Berkeley CA U.S.A.
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Contents (updated 12/22/2005)

TLC2543 Analog/digital converter

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The TLC2543 is the analog to digital converter EME Systems uses in the OWL2 series data logger. Many of the sensor data processing programs described on this web site call this (or similar) ADread routine to get their data.  

The TLC2543 is an advanced analog to digital converter chip from Texas Instruments, and it uses their proprietary switched capacitor technology to achieve high accuracy. Please refer to the TLC2543 data sheet for detailed information.  It has eleven voltage inputs. The resolution is 12 bits, so there are 4096 steps dividing the voltage range from (ref-) to (ref+). The chip requires an external reference for (ref-) and (ref+).  If the reference is 4.096 volts, then the result from the ADC will read directly in millivolts. If the reference is 5.000 volts, it will read 4096 increments of 1.2207 millivolts per bit.   A reference of 5.12 volts would be read 4096 increments of 1.25 millivolts.   The demo routine below assumes a 5 volts reference and shows the math to convert the reading into even units of millivolts, or into temperature, as it might do for an LM34 temperature sensor that generates 10 millivolts per degree Fahrenheit.

The chip is quite robust.  A resistor  of around 330 ohms can be added in front of each input to protect it from ESD and mis-wiring. These are shown in the schematic diagram as protection between the inputs and the outside world.    Additional input protection involving diode clamps can improve the level of protection.

wiring the TLC2543The TLC2543 has an SPI interface, which will be connected to four BASIC Stamp pins and using the SHIFTOUT and SHIFTIN commands. The TLC2543 has four control lines that concern us.  

The first three (SDO, SDI and CLK) can be shared with other SPI serial peripherals. The CS\ line is the only one that must be dedicated to this chip only.  When CS\ is at a low level, this TLC2543 is selected for action.   When CS\ is at a high level, the chip does not accept commands and its SDO output is at a high impedance level.   It is not necessary to poll the chip's end-of-conversion flag (EOC pin 19), because the ADC works much faster (<10 microsecond conversion time) than the Stamp. EOC is shown with no connection in the schematic.  Do not attempt to save lines by connecting the SDO to SDI.  That trick will not work with this part, because of the way it transfers data in and out simultaneously.

In the schematic diagram, the four control lines are shown using Stamp pins p12 to p15, and that is what the demo program below will assume.   There is no restriction on which Stamp pins are used, so long as they are defined correctly for the program instructions.   

The chip requires a 5 volt power supply, and a voltage reference connection to the (ref+) and (ref-)  pins.  For this example, the reference is connected directly to the power supply, so the range is 0 to 5 volts for 0 to 4095 counts.

The OWL2pe data logger from EME Systems uses a precision 4.096 volt reference, with respect to common, so the analog input range is zero to 4.096 volts at one millivolt per bit.  It talks to the TLC2543 on AUXIO pins x0, x1, x2 and x14. And it brings nine of the TLC2543 inputs to the edge of the module for applications, and uses two of them internally for monitoring the Vin power supply voltage and the internal board temperature.

Current drawn from the power supply is around 1 milliamp when the chip is active.  The chip has a sleep mode  that drops the current drawn to about 4 microamps in order to minimize battery drain when it is not in use. A command must be sent to the chip to put it into sleep mode, and that command is simply to address channel 14, which is not a real input channel.  It is understood by the chip as the command to sleep.   

The following program runs a demo, to scan 11 input channels, convert the count to millivolts, and to display the result on the debug screen.  Note that this program names the sdi and sdo from the reference point of the BASIC Stamp.    The sdo output from the Stamp connects to the sdi input of the TLC2543 and the complement, Stamp sdi to TLC2543 sdo.

'{PBASIC 2.5}
' ADread subroutine.
' On command, returns 12 bit digital data from one of 11 analog channels.
' The value of ADch from 0 to 10 selects external analog inputs.
' ADch=14 puts the converter into its sleep mode.
' converter has 5 volt reference, connection to the Vdd power supply, for 1.2207 mV per bit

sclk PIN 15 ' clock out from BS2 to AD2543
sdo PIN 14 ' data out from BS2 to AD2543 sdi
sdi PIN 13 ' data in from AD2543 sdo to BS2
ADcs PIN 12 ' AD2543 chip select, active low

ADch VAR Nib ' selects AD external channel 0-10
result VAR Word ' result, 12 bit A/D conversion
 
demo: ' to show off the subroutine below.
DIRS=$FFFF ' makes all Stamp pins outputs to start
OUTS=$1000 ' makes all pins except ADC chip select low to start
DO
 GOSUB ADwake ' makes a dummy conversion to initialize converter
 FOR ADch =0 TO 10 ' specify one of 11 input channels
 GOSUB ADread ' get millivolt data from that channel
 result = 14464 ** result + result ' convert count to millivolts.
 DEBUG DEC ADch,": ",DEC result,REP 32\5,cr ' display, use extra spaces to clear garbage
 NEXT
 GOSUB ADsleep
 NAP 7
 DEBUG home ' repeat the demo
LOOP
 
ADread: ' entry point to give result as count from 0 to 4095
 LOW ADcs ' select chip
 SHIFTOUT sdo,sclk,msbfirst,[ADch<<8\12] ' mode, left justify ADch
 SHIFTIN sdi,sclk,msbpre,[result\12] ' get result, 12 bits
 HIGH ADcs ' deselect chip
RETURN
 
ADsleep: ' entry point to put TLC2543 to sleep
 LOW ADcs ' select chip
 SHIFTOUT sdo,sclk,msbfirst,[$e\4] ' command=$e
 HIGH ADcs ' deselect chip
 LOW sdi ' keep this pin from floating in sleep
RETURN

ADwake:
 ADch=$b
 GOTO ADread


In this situation the scale factor is 1.2207 millivolts per bit.  The count value from 0 to 4095 returned by the TLC2543 has to be multiplied times that scale factor to convert to volts.  In Stampese, one way to do this uses the ** operator as follows:

 result = 14464 ** result + result ' convert count to millivolts.

The factor 14464 comes from the fact that 14464/65536 is a close approximation to the decimal fraction 0.2207.   The ** operator internally multiplies times 14464 (to 32 bits of precision) and then implicitly divides that product by 65536.    Adding in result again makes the overall multiplication time 1.2207.    More details on this magic on the math pages.

 If the reference voltage is 4.096 volts, there will be no conversion necessary from counts to volts (assuming that the reference itself is right on accurate!). With a 5.120 volt reference, the operation result = result */320 can be used to convert from counts to millivolts as the scale factor is 1.25 millivolts per count. Note that 320/256 is equal to 1.25, and the division by 256 is implied in the Stamp */ operator. 

A note about the ADwake subroutine. To wake up the chip, send a command to address any channel lower than than channel 14. The first reading returned will sometimes be in error, due to internal settling delays, so throw it away. The same thing goes for the first reading after the chip is powered up.   Throw the first reading away and take a second one to get it right.  Note that this program uses the ADwake routine at the top of the loop, where it will be executed prior to the first time through the loop and also after each NAP.  Note that to achieve minimum battery current, the above program would also have to attend to the other Stamp pins in an appropriate manner.  The program assumes that those other Stamp pins are uncommitted.  The ADwake routine addresses channle $b,an internal reference point at Vrev/2, but it could just as well address any of the input channels.

Circuit layout makes a big difference with the performance of the ADC. The reference must be stable, and there must not be ground bounce in series with the analog or reference inputs, and the power supply must be stable and well bypassed.  

If a device connected to an input has high source resistance or if you need lowpass filtering on an input, it is possible to hang a capacitor, say 0.33 microfarad, on the analog input pin, but there are issues to consider.  An explanation of how the TLC2543 works inside will clarify this.    When a channel is addressed, a 60 picofarad capacitor in series with a 1kohm resistor (approximate model internal to the TLC2543!)  is connected to that input channel abruptly just after the address is decoded, that is, after the first four bits of the SHIFTOUT command from the Stamp.   Then that input is "sampled" during the last 8 clock pulses of the SHIFTOUT.  After that, the sample is held while the actual conversion takes place.   The "sampling" period is determined by the SHIFTOUT clock rate of around 16 kilohertz (BS2).  So the sampling period is around 0.0005 second (0.005 = 8 * 1/16000).   The voltage transfered to that 60 pf capacitor inside the TLC2543 at the end of that interval should equal the input voltage you want to measure, within one part in 4096.  For example, the time constant of 60 pf with 1 megaohm of input resistance is 0.00006  second, which is only 10 times less than the sampling period, so it would not settle completely.   It is better to keep the input resistance below 10k ohms so that the input has time to settle to the correct value within the 0.0005 seconds.   There are other reasons too, to keep the input resistance below 10kohms, one being leakage between the channels.    The sampling interval is relatively long due to the slow clock speed of the BASIC Stamp.   The faster Stamps would require a correspondingly lower input source resistance, because they have a faster SHIFTOUT clock.   Now suppose a 0.3 uf capacitor charged to some voltage is hung on the input.    When that is connected abruptly to a 60pf capacitor inside the TLC2543, the charge will be distributed in the ratio of the two capacitors, which is 0.0002.    (0.0002 = 60E-6 / 0.3).   That could mean up to a 1 millivolt worst case change in the voltage sampled at the input, which is the same order of magnitude as the resolution of the ADC.    A larger capacitor would not shift as much due to being connected to the 60pf sampling capacitor, but remember that the capacitor has to be recharged to the correct voltage before the channel is sampled again.   I hope that clarifies the considerations that go into filtering and input source impedance.

One symptom of too high an input source resistance or a bad RC time constant at the input, is that the reading on one channel appears to affect the reading on a neighboring channel.   Remember though that channels that are not connected to anything at all will still give readings, and those readings will tend to drift toward the readings on neighboring channels.    Do not connect the inputs to voltages more than 0.3 volt below Vss or above Vdd, as this will lead to bad readings on all the channels, not only the one that has the excess voltage input.   If excess voltages might be present in a system, use diode clamps or other means to limit the voltage seen at the converter inputs.

The actual conversion is done by the TLC2543 using an internal high speed conversion clock, and takes only about 10 microseconds maximum.    That is much less than the time it takes for the Stamp to get ready to read out the data using its SHIFTIN command.   Inside the TLC2543, it uses switched capacitor technology to redistribute the charge acquired by the 60 pf input capacitor, in a clever successive approximation scheme that is explained in the TI data sheet.  

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