Update README.md

Author: NanaVan

README.md

@@ -1,2 +1,127 @@
-# ArduinoTriggerSystem
-Catching the TTL trigger signla using Arduino Yun.
+# Catching the TTL trigger
+
+
+
+Our group need to build a new DAQ system for planned nuclear decay experiments, which require a stable and reliable procedure to remote-control all network-connected devices (a spectrum analyzer, a IQ recorder and a server) to successively collecting large amount of data. However, the original trigger system of IQ recorder can not satisfy the requirment. It can only work steadily when executing the _recording mode_ each time the trigger signal comes.
+
+***An independent trigger system to catch the TTL trigger signal is in high demand!***
+
+
+**Goal**
+
+Catching the TTL trigger signal and sednding the *triggered* message to server, so that the server can execute the IQ recorder to collect data.
+
+**material**
+
+ * Arduino Yun: ATmega 32U4 (16MHz) and built-in Ethernet support
+ * resistances, PCB, soldering iron (tin)
+
+**Content**
+ * designing concept
+ * circuit diagram
+ * software 1.0: resetting prescaler
+ * software 2.0: using analog comparator
+
+## Designing concept
+
+The TTL trigger signal: 
+ * falling edge
+ * width: 50 $\mu$ s
+ * amplitude range: 0~4V
+
+![image](/Pic/triggerSignal.png)
+
+Design index:
+ * sampling frequency > 100 kHz
+ * amplitude < 4V, triggered
+
+Process:
+
+1. measuring TTL signal: when amplitude < 4V, triggered
+2. netcat: using `nc` ShellCommand to send the message to server
+
+
+## Circuit diagram
+
+A simple circuit is for measuring the voltage of the resistance (1k$\Omega$), which is connected with another resistance (the same 1k$\Omega$) in series. The total voltage of this circuit is the input voltage between TTL and GND. The voltage between ADC input A0 and GND is what we measure.
+
+![image](/Pic/Circuit.png)
+
+## software 1.0: resetting prescaler
+
+This idea was used in our beam test 2016.
+
+The main idea is to resetting prescaler. The `analogRead()` usually needs 100$\mu$s to read the analog input. Since the default prescaler is 128.
+
+ADC clock = 16 MHz / 128 = 125 kHz
+
+One ADC transfer needs 13 ticks. The final sampling rate
+
+125 kHz / 13 = 9600 Hz
+
+***It is far from our design index!***
+
+Since we do not need a very high accuracy of the voltage. We reset the prescaler to instead 4 of 128. The ideal sampling rate is 308 kHz (3.24$\mu$s per point). The test result is 9.42 $\mu$s per point.
+
+**Code**
+```Arduino
+// Define various ADC prescaler
+const unsigned char PS_2 = (1 << ADPS0);
+const unsigned char PS_4 = (1 << ADPS1);
+const unsigned char PS_8 = (1 << ADPS1) | (1 << ADPS0);
+const unsigned char PS_16 = (1 << ADPS2);
+const unsigned char PS_32 = (1 << ADPS2) | (1 << ADPS0);
+const unsigned char PS_64 = (1 << ADPS2) | (1 << ADPS1);
+const unsigned char PS_128 = (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
+
+// To change the prescaler modify setup
+ADCSRA |= PS_16; // 16 prescaler
+ADCSRA |= PS_8; // 8 prescaler
+ADCSRA |= PS_4; // 4 prescaler
+```
+
+**Reference link**
+
+[Advanced Arduino ADC - Faster analogRead()](http://www.microsmart.co.za/technical/2014/03/01/advanced-arduino-adc/)
+
+[The Analog to Digital Converter(ADC)](http://www.avrbeginners.net/architecture/adc/adc.html)
+
+**Problem**
+
+The new setting has a high risk of shortening life of the Arduino. Since the suggested prescaler is not least than 16. 
+
+In our one-week test experiment, 2016, the trigger was working fairly fine all the time. Nevertheless, bad luck comes in 2017.
+
+
+## software 2.0: using analog comparator
+
+This idea will be used in our beam test 2017.
+
+The main idea is using analog comparator, comparing the input values on the positive pin AIN+ and negative pin AIN-. When the voltage on the AIN+ is higher than the voltage on the AIN-, the Analog Comparator output, ACO, is set.
+The comparator can trigger a separate interrupt, exclusive to the Analog Comparator.
+
+In this case, the AIN+ we choose is the Bandgap reference (1.1$\+-$0.1V) and the AIN- is ADC input A0 (ATmega 32U4 mapping pin ADC7).
+
+**Code**
+```Arduino
+ACSR |= ((1<< ACBG)| // AIN+ set to bandgap reference voltage 1.1(1)V
+ (1<< ACI)|
+ (1<< ACIE)| // Analog Comparator interrupt is activated
+ (1<< ACIS1)|); // Comparator interrupt on falling output edge
+ADCSRB |= (1<<ACME); // AIN- set to ADC7(A0)
+ADCSRA &= ~(1<<ADEN); 
+ADMUX |= ((1<<MUX2)|(1<<MUX1)|(1<<MUX0));
+DIDR1 |= (1 << AIN0D); // disable the digital input buffer
+```
+
+**Reference link**
+
+[ATmega 32U4 datasheet](http://www.atmel.com/Images/Atmel-7766-8-bit-AVR-ATmega16U4-32U4_Datasheet.pdf)
+
+[Using the Arduino Analog Comparator](http://www.gammon.com.au/forum/?id=11916)
+
+**Result**
+
+No one trigger missing!
+
+The DC Characteristics shows the typical propagation delay is 500 ns at VCC = 4.0V, TA = -40 to 85 Celsius degree.