FPC/Lazarus serial oscilloscope This oscilloscope project consists of two parts: a microcontroller that read analog signoals via its ADC peripheral, and an application that runs on a computer that requests data over a serial connection with the microcontroller.
A serial connection needs to be established between the application and the microcontroller. The serial port cab be selected by clicking on the dropdown list nad selecting the appropriate serial device. If the actual serial device is not listed, the full name can be typed into the dropdown text box. The baud rate is determined by the settings used in the microcontroller firmware - typically 115200. The baud rate can be edited to suit the microcontroller firmware.
Data acquisition is started by clicking on Running. After a data frame has been received, a new data acquisition request is sent to the micrcontroller. This continues until Running is unselected. If the SingleShot option is selected, a single data frame is requested and displayed after enabling the Running mode. Once the data frame is received Running is automatically disabled, so that the received data can be inspected.
There are two optional trigger modes: Rising and Falling. If for example the Rising trigger is selected, then data samples are evaluated until a data sample below the trigger level is followed by a data sample above the trigger level. Data capture is delayed until a trigger condition is met, a certain number of data samples have been evaluated, or until approximately 4 seconds have expired. Note that the trigger is only applied on the first selected channel. Valid trigger values are between 0 and the maximum range for the current ADC resolution. Internally the trigger value is rounded down to the nearest multiple of 4.
A reticule ("crosshair") tool is available which will snap to the nearest data point and display its coordinates. Activate this by pressing the Alt button while moving the mouse pointer close to the signal trace. Note that the mouse pointer needs to be within a certain distance of a data point before the reticule will snap to it.
This allows changing the data sampling frequency. Increasing the prescaler value results in lower data sampling frequency. Note that the atmega328p datasheet recommends a limit of 200 kHz sampling frequency by the ADC (about 65 microseconds acquisition time) to maintain maximum accuracy. Selecting small prescaler values may therefore result in inaccurate results.
Different voltage references can be selected for the ADC. The options include Vcc, 1.1V and Aref. The attinyx5 series has another internal reference voltage which can be selected: 2.56V (without an external bypass capacitor).
One or more of the available ADC channels can be selected for sampling. If more than one channel is selected, the channels will be sampled sequentially. Note that on low pin count microcontrollers (such as the attinyx5 series) some of the ADC capable pins are used for other functions such as serial pins or frequency output and will not be listed.
The ADC of the classic AVR microcontrollers typically have a resolution of 10 bits. These 10 bit values are stored and transmitted by packing two values into 3 bytes. To increase the number of samples that can be stored in one data frame, the data can be stored as 8 bit values. On the flash constrained microcontrolles (attiny24 and attiny25) only the 8 bit data option is available.
The baud rate defaults to 115200, but can be changed by defining BAUD, e.g. via command line: -dBAUD=38400.
The firmware code needs to know the system clock frequency for certain timing calculations. The system clock frequency should be specified via the F_CPU define,e.g. via command line: -dF_CPU:=16000000.
The code is factored to be compatible with the following microcontroller families:
- atmegaxx8 (atmega48, atmega88, atmega168, atmega328p)
- attinyx4 (attiny24, attiny44, attiny84)
- attinyx5 (attiny25, attiny45, attiny85)
The firmware was tested on atmega328p, attiny24, attiny45 and attiny85 microcontrollers.
A square wave signal of 500 Hz is generated by the firmware. The pin number for this output is shown in the table below.
| Controller | TX | RX | Square wave pin |
|---|---|---|---|
| atmegaxx8 | PC0 | PC1 | PD3 |
| attinyx4 | PB2 | PB1 | PA7 |
| attinyx5 | PB2 | PB1 | PB0 |
Some signals were generated on a signal generator similar to this one (https://scienceprog.com/avr-dds-signal-generator-v20/)
A decent quality trace of the two signals.
A poor quality trace caused by to high a sampling frequency. At too high sampling frequencies the ADC charge capacitor cannot be fully charged/discharged between sample cycles, leading to smearing of samples. In this case the alternating sampling caused channel A0 to affect A1's reading and visa versa.
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