From cff038e463891af8c9c2c6e31d02bf081abfb470 Mon Sep 17 00:00:00 2001 From: Philipp Klaus Date: Wed, 14 Nov 2018 11:37:58 +0100 Subject: [PATCH] pt100: readme improved --- pt100/README | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 58 insertions(+), 2 deletions(-) diff --git a/pt100/README b/pt100/README index 4121412..1f04fd1 100644 --- a/pt100/README +++ b/pt100/README @@ -24,7 +24,9 @@ The first digit is the board address: The first board connected directly to the Configuration Memory -------------------- + | Address | Content | +| ------ | ------ | | 0x0 - 0x7 | Per-channel offset calibration. The relation is about 0x10 for a change of 0.08 degrees centigrade.| | 0x9 | Current and gain calibration. Nominal value is 0xd260. The value is calculated as 2**29/gain/current[nA] | | 0xa | Conversion rate. Gives the time between starting two conversion (of all channels) in units of 2ms | @@ -39,8 +41,62 @@ The display shows temperatures of even channels in the left column, odd channels Programming ----------- -Programming the ATmega 324PA on the board is done using `make program`. It uses the dragon_jtag programmer option of avrdude. The programmer has to be connected to pinheader P1. -The VTG pin on the dragon is an input (target voltage) and it provides power to the dragon's level shifters. So power the Pt100 board from en external 3.3V supply on the header P2. You may generate the 3.3V from the the additional VCC/GND on the dragon with an LDO. +Programming the ATmega 324PA on the board is done using `make program`. It uses the dragon_jtag programmer option of avrdude. The programmer has to be connected to pinheader P1 (next to the label for switch two, S2). +The VTG pin on the dragon is an input (target voltage) and it provides power to the dragon's level shifters. So power the Pt100 board from an external 3.3V supply on the header P2 (pin 1 is GND, pin 2 is the supply voltage) if the "jumper" W1 is set. If W1 is open, connect the 3.3V power to P13 (right next to P2); again: pin 1 is GND, pin 2 is the supply voltage. You may generate the 3.3V from the the additional VCC/GND on the dragon with an LDO. + +How it works +------------ + +The REF200 (U4) sinks a current of 200uA on combined pins labeled with the signal Ccurr. +This is switched by a 4051 multiplexer (analog switch) to the center pin of one of the seven sensor connectors (C1...C8). +This center pin is the single lead of the 3-wire Pt100 sensor. +The current goes through the sensor and comes from one of the other leads labeled VccPT. +At the same time, a mirrored current (also 200uA) is sunk by the REF200 on the Tcurr signal. +It is connected via the 4051 switch to the third sensor lead via the so-called offset resistor (R3...R10). +It doesn't go through the Pt100 itself, instead it just goes through the wires soldered together just before the Pt100. +The current is also sourced from the one lead connected to VccPT. +Thus, the wire connected to VccPT sees a current of 400uA and the other two wires see a current of 200uA. +Now we connect a very precise voltmeter and measure the potential (differential voltage) between the two sensor pins C (C1...C8) and T (T1...T8): +First, we switch the inputs using two 4051s (U3, U6) to our instrumentation amplifier INA333 (U8) on the pins 2,3. +The absolute potential of those Tsig,Csig lines is slightly pulled towards AVCC/2 by the voltage divider R12,R13. +The voltage is amplified and output on pin 6. +R14 (2.05kOhm) sets the gain to G = 1+(100/2.05) = 49.78. +R20 generates a bit of load on the output and the output is then given to the ADC ADS1118 (U1) on pin 4 (Ana0 / AIN0). +The gain of the ADC is set to a FSR (full-scale range) of +- 0.256V giving a LSB (least significant bit) size of 7.8125uV. + +Calibration +----------- + +* 120 Ohm (actual: 120.000 Ohm) ~ 51.578 °C +* 100 Ohm (actual: 100.015 Ohm) ~ 0.038 °C + +General Hardware Settings +------------------------- + +powering: + +VCC -> external power +VCC33 -> either external or connected to VCC via W1 +Vuc -> filtered VCC33 +AVCC -> filtered VCC +VccPT -> filtered AVCC + +P12 is an additional ADC channel to be connected externally. + +Hardware configurations +----------------------- + +w/ display: +If a display is used, the board has to be powered with both: 5V and 3.3V. +In this case, the ADC and the micro controller will be powered from xV??? + +w/o display, no daisy-chaining +An external FT232RL board is plugged into the board streching from P2 over P13 and P11. +It supplies VCC with 5V and VCC33 with 3.3V. + +w/o display, daisy-chaining +The board is powered with 5V via P2 only. The jumper W1 is set and connects Vcc33 with VCC making it actually 5V(?). -- Jan Michel +Philipp Klaus -- 2.43.0