![\"encoder\"](\"http:\/\/heppg.de\/ikg\/wordpress\/wp-content\/uploads\/2017\/02\/encoder.png\")
<\/a><\/p>\nIn raspberry pi forum, someone asked for the performance of a python program to decode these signals.<\/p>\n
The code used to validate python program is<\/p>\n
import RPi.GPIO as GPIO\r\nimport time\r\n\r\nGPIO.setmode (GPIO.BCM)\r\nGPIO.setwarnings(False)\r\npin_A = 22\u00a0 \u00a0\r\npin_B = 27\r\n\r\nEncoder_Count = 0\u00a0\u00a0\u00a0\u00a0\u00a0 # Encoder Count variable\r\n\u00a0 \u00a0\r\ndef do_Encoder(channel):\r\n\u00a0\u00a0\u00a0 global Encoder_Count\r\n\u00a0\u00a0\u00a0 if GPIO.input(pin_B) == 1:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Encoder_Count += 1\r\n\u00a0\u00a0\u00a0 else:\r\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Encoder_Count -= 1\r\n\r\nGPIO.setup (pin_A, GPIO.IN, pull_up_down=GPIO.PUD_UP)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 # pin input pullup\r\nGPIO.setup (pin_B, GPIO.IN, pull_up_down=GPIO.PUD_UP)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 # pin input pullup\r\n\r\nGPIO.add_event_detect (pin_A, GPIO.FALLING, callback=do_Encoder)\u00a0\u00a0 # Enable interrupt\r\n\r\nlastCount = 0\r\nt0 = time.time()\r\nwhile(1):\r\n\u00a0\u00a0\u00a0 t0 += 20\r\n\u00a0\u00a0\u00a0 time.sleep(t0 - time.time())\r\n\u00a0\u00a0\u00a0 print (\"{e:d} diff={d:d}\".format(e=Encoder_Count, d = Encoder_Count-lastCount))\r\n\u00a0\u00a0\u00a0 lastCount = Encoder_Count\r\n\r\n<\/pre>\nAs a driver for these signals I use an arduino DUE.<\/p>\n
The results for the python code on a Raspberry Pi 3 are<\/p>\n
freq\u00a0 \u00a0 \u00a0 \u00a0exp\u00a0 \u00a0 \u00a0measure\r\n\u00a0 \u00a010Hz\u00a0 \u00a0 200\u00a0 \u00a0 \u00a0 \u00a0 \u00a0200\r\n\u00a0 100Hz\u00a0 \u00a02000\u00a0 \u00a0 \u00a0 \u00a0 2000\r\n\u00a0 500Hz\u00a0 10000\u00a0 \u00a0 \u00a0 \u00a010000\u00a0 +-2\r\n\u00a01000Hz\u00a0 20000\u00a0 \u00a0 \u00a0 \u00a020000 +-25\r\n\u00a02000Hz\u00a0 40000\u00a0 \u00a0 \u00a0 \u00a040000 -40\r\n\u00a03000Hz\u00a0 60000\u00a0 \u00a0 \u00a0 \u00a060000 -660\r\n\u00a05000Hz 100000\u00a0 \u00a0 \u00a0-100000 +-20, but received negative values, so python too slow to get matching level<\/code><\/pre>\nUp to 3.000 pulses, the results are quite reliable. For a 1.000RPR encoder, these are 3 rotations per second.<\/p>\n
Usually I recommend to use an atmel328-processor as a slave processor to handle time sensitive operations. With an arduino UNO, 16MHz, the results are precise up to 15.000Hz.
\nThere is no complete arduino needed. A bare atmel328, clocked from GPIO4, using level shifters for clock and serial line, would be a cost effective solution.
\nThe sketch uses interrupts for the event inputs:<\/p>\n
const byte ip2 = 2;\r\nconst byte ip3 = 3;\r\nvolatile long counter = 0;\r\n\r\nvoid ip2Int() {\r\n\u00a0 byte iip2 = digitalRead(ip2);\r\n\u00a0 byte iip3 = digitalRead(ip3);\r\n\u00a0 if ( iip2 == 1 ) {\r\n\u00a0\u00a0\u00a0 if ( iip3 == 1 ) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter --;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0\u00a0\u00a0 else\r\n\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter ++;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0 }\r\n\u00a0 else \/\/ ip2 == 0\r\n\u00a0 {\r\n\u00a0\u00a0\u00a0 if ( iip3 == 1 ) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter ++;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0\u00a0\u00a0 else\r\n\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter --;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0 }\r\n}\r\n\r\nvoid ip3Int() {\r\n\u00a0 byte iip2 = digitalRead(ip2);\r\n\u00a0 byte iip3 = digitalRead(ip3);\r\n\u00a0 if ( iip2 == 1 ) {\r\n\u00a0\u00a0\u00a0 if ( iip3 == 1 ) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter ++;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0\u00a0\u00a0 else\r\n\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter --;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0 }\r\n\u00a0 else \/\/ ip2 == 0\r\n\u00a0 {\r\n\u00a0\u00a0\u00a0 if ( iip3 == 1 ) {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter --;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0\u00a0\u00a0 else\r\n\u00a0\u00a0\u00a0 {\r\n\u00a0\u00a0\u00a0\u00a0\u00a0 counter ++;\r\n\u00a0\u00a0\u00a0 }\r\n\u00a0 }\r\n}\r\n\r\nvoid setup() {\r\n\u00a0 Serial.begin(115200);\r\n\u00a0 Serial.println(\"encoder\");\r\n\r\n\u00a0 pinMode(ip2, INPUT);\r\n\u00a0 pinMode(ip3, INPUT);\r\n\r\n\u00a0 pinMode( 13, OUTPUT);\r\n\u00a0 pinMode(pin_sync, INPUT );\r\n\u00a0 \r\n\u00a0 attachInterrupt(digitalPinToInterrupt(ip2), ip2Int, CHANGE);\r\n\u00a0 attachInterrupt(digitalPinToInterrupt(ip3), ip3Int, CHANGE);\r\n}\r\n\r\nvoid loop() {\r\n\u00a0 long c0 = 0;\r\n\u00a0 noInterrupts();\r\n\u00a0 c0 = counter;\r\n interrupts();\r\n\r\n Serial.println(c0);\r\n delay(10000);\r\n}<\/pre>\nThis code runs up to 15.000Hz, at 20.000 Hz it fails.<\/p>\n
An obvious optimization is to change the interrupt routine and avoid the digitalRead-function for the atmel328-processor.<\/p>\n
void ip2Int() {\r\n\u00a0 uint8_t pd = 0b00001100 & PORTD; \/\/ pins 2,3 are PD2, PD3\r\n\u00a0 if ( pd == 0b00001000 || pd == 0b00000100 ) {\r\n\u00a0\u00a0\u00a0 counter ++;\r\n\u00a0 }\r\n\u00a0 else {\r\n\u00a0\u00a0\u00a0 counter --;\r\n\u00a0 }\r\n}\r\n\r\nvoid ip3Int() {\r\n\u00a0 uint8_t pd = 0b00001100 & PORTD;\r\n\u00a0 if ( pd == 0b00001100 || pd == 0b00000000 ) {\r\n\u00a0\u00a0\u00a0 counter --;\r\n\u00a0 }\r\n\u00a0 else {\r\n\u00a0\u00a0\u00a0 counter ++;\r\n\u00a0 }\r\n}<\/pre>\nThis code is much faster and update rates till 50.000Hz work perfect.<\/p>\n
With handcrafted assembler code for the interrupt-routine I would expect to extend performance even more.<\/p>\n
Last option investigated was an arduino feather M0 board running at 48MHz.
\nThis produces good results up to 30.000Hz, at 40.000Hz there are failures. With the results from optimized atmel328-code, there is quite a lot of optimization expected.<\/p>\n
For the fast running signals, timing accuracy is crucial. To work around this problem, a sync pattern was used: an additional sync signal indicated start and end of measurement from generator to measurement device. Pulse sequence was 10 sec in each case.<\/p>\n