This code was developed to read values from a import (Chinese) digital dial indicator. However, the same protocol is used for digital calipers, tire gauges, micrometers, ... Generically, these all are referred to as a measuring device.
- 1 Measuring Device (Dial Indicator, Digital Calipers, Tire Gauge, ...)
- 1 Arduino UNO (or equivalent)
- 1 Breadboard
- 2 Inverting Level Shifters (for clock and data)
- 2 2N2222 NPN BJT Transistors (or equivalent)
- 2 10kΩ Resistors
- 2 1kΩ Resistors
- 1 Voltage Divider
- 1 510Ω Resistor
- 1 220Ω Resistor
- Connecting Wires
- Voltage Divider
- Place the 510Ω resistor between +5V and a column on the breadboard
- Place the 220Ω resistor between Ground and the same column on the breadboard
- This column, where the two resistors meet, has the right voltage for the measuring device
- Prepare Inverting Level Shifters (two times)
- Identify the emitter, collector, and base of the BJT transistors from the coresponding datasheet.
- Place a transistor in three columns of the breadboard
- Connect the emitter to ground with a jumper wire
- Connect the collect to +5V with a 1kΩ resistor
- Connect the 10kΩ resistor from the base to the other side of the breadboard on the same column
- Repeat for the second Inverting Level Shifter
- Connect the Measuring Device
- Connect a wire from the device ground to the breadboard ground
- Connect a wire from the device 1.5V to the output of the voltage divider
- Connect the device clock output to the same column as the 10kΩ resistor (other end of the resistor connecting to the base of the transistor)
- Connect the device data output to the same column as the 10kΩ resistor on the second transistor
- Arduino UNO Connections
- Connect Arduino 5V to breadboard 5V
- Connect Arduino ground to the breadboard ground
- Connect Arduino PIN 2 (INT0) to the collector of the transistor with the clock line
- Connect Arduino PIN 4 to the collector of the transistor with the data line
When running, enable the Arduino serial monitor. The values of the measuring device will be sent over the serial port whenever the value changes.
The measuring devices have a four pin port used for the protocol. From left to right, looking at the port, the pads are: 1.5V, Clock, Data, Ground.
The data is transferred in 24-bit data packet (20 bits of data and 4 bits of metadata). Each data packet consists of 4 bit nibbles. There is a small time gap between nibbles and a much larger time gap between data packets. The data values are valid on the trailing edge of the clock pulse. The data arrives least significant bit first and metadata nibble last.
If the unit is mm (unit bit not set), then the measurement value needs to be divided by 100 to get the proper floating point value. If the unit is in inches (unit bit set), then the measurement values needs to be divided by 2000 to get the correct floating point value. For negative values, the sign bit will be set.
Investigating the data from the device with a USB oscilloscope, I determined the important timings are 605 µs (the maximum time between two bits during the 24-bit data packet) and 110 ms (the time between 24-bit data packets). I also saw that the data signal is available during the trailing edge of the clock pulse.
In order to read the data during the trailing edge of the clock pulse, I set up an external interrupt on pin 2 (INT0) of the Arduino. The interrupt is responsible for reading the data pin and storing the value in a variable.
To know when the data is complete, all 24 bits read, it is not sufficient to count the number of bits read as there is no guarantee that data is from a single data packet. So a periodic interrupt is needed to detect when the device is in the data packet gap. If a periodic interrupt sees no data change in two sequential calls, then it can assume it is in the gap as long as the period of the interrupt is greater than 605 µs and less than half 110 ms. These boundaries ensure that the interrupt will not run twice in the nibble gap and will run at least twice in the data packet gap.
From the Time/Counter Table
below, choosing an 8-bit counter with 1024 prescaler would result in a
16.4 ms period of the overflow interrupt. I chose timer2,
ISR(TIMER2_OVF_vect), since it would not affect delay(),
millis(), ... When the interrupt detects it is in the data packet gap
(no change in subsequent calls), then if it has a full 24 bits, it
will move the data to another variable and set a data_ready boolean
for the loop() function to detect. If it doesn't have all 24 bits,
it means the data collection is out of sync, so it clears the data.
The loop() function just tests this boolean until it sees it has
something to do. If the data has changed since last iteration, it will
recalculate the human readable values and output it to the Serial
monitor.
This schematic includes the voltage divider used to provide power to the measuring device as well as the level shifter-inverting circuits created with the two NPN (2N2222) BJT transistors.
Here are some screenshots from a USB oscilloscope showing the clock and data pulses along with their timings.
This is a python program that connects to the Arduino running this code
and then displays it in a terminal window using curses. Below are
sample screenshots.
Timer/Counter frequency and timing with prescaler factors:
F_CPU = 16 MHz (62.5 ns):
Prescaler Freq. Period Fovf. 8 Period Fovf. 16 Period
1 16 MHz 62.5 ns 62.5 kHz 16 µs 244 Hz 4.1 ms
8 2 MHz 500 ns 7.81 kHz 128 µs 30.5 Hz 32.8 ms
32* 500 kHz 2 µs 1.95 kHz 512 µs
64! 250 kHz 4 µs 977 Hz 1.02 ms 3.81 Hz 262 ms
128* 125 kHz 8 µs 488 Hz 2.05 ms
256 62.5 kHz 16 µs 244 Hz 4.1 ms 954 mHz 1.05 s
1024 15.6 kHz 64 µs 61 Hz 16.4 ms 238 mHz 4.19 s
Phase Correct PWM frequencies and timing with prescaler factors:
F_CPU = 16 MHz (62.5 ns):
Prescaler Freq. Period Fovf. 8 Period Fovf. 16 Period
1 16 MHz 62.5 ns 31.4 kHz 31.9 µs 122 Hz 8.19 ms
8 2 MHz 500 ns 3.92 kHz 255 µs 15.3 Hz 65.5 ms
32* 500 kHz 2 µs 980 Hz 1.02 ms
64! 250 kHz 4 µs 490 Hz 2.04 ms 1.91 Hz 524 ms
128* 125 kHz 8 µs 245 Hz 4.08 ms
256 62.5 kHz 16 µs 123 Hz 8.16 ms 477 mHz 2.1 s
1024 15.6 kHz 64 µs 30.6 Hz 32.6 ms 119 mHz 8.39 s
Fast-PWM Compare Match Register Values for Duty Cycles:
Duty Cycle Comp-8 ^Comp-8 Comp-16 ^Comp-16
0.0% 0 255 0 65535
10.0% 25 230 6553 58982
25.0% 63 192 16383 49152
50.0% 127 128 32767 32768
75.0% 191 64 49151 16384
90.0% 229 26 58981 6554
100.0% 255 0 65535 0
*: Only available on Timer2
!: Default prescaler set by Arduino IDE on Timer0 for delay(), millis()...