Design of Handheld Oscilloscope Based on FPGA

　　At present, digital storage oscilloscopes have gradually replaced traditional analog oscilloscopes with their unique advantages of small size, easy portability and high accuracy, and are developing towards smaller, low-cost and portable applications. In recent years, many researchers have made full use of the advantages of rich resources, flexible use and low development cost on FPGA chips to propose some virtual instrument solutions [1-2] and embedded solutions [3-8] for digital oscilloscopes. These designs use FPGA on-chip resources to implement digital oscilloscope data storage (RAM), trigger control, digital signal calculation and processing, display terminal drive and other functions, which can greatly reduce the cost and complexity of the oscilloscope. However, these designs often use microprocessors [3-7] (8051/ARM/NoisⅡ) as the core of system scheduling and data processing or use VGA displays as graphic output terminals [1,2,8], which cannot meet the requirements of low-cost handheld and portable use.

　　LCD12864 liquid crystal display module has the advantages of low voltage, micro power consumption, long life, ultra-thin, etc. It is more suitable for low-cost, portable electronic information products to display characters and graphics. Therefore, this paper uses LCD12864 liquid crystal module as a low-cost graphic display terminal for digital oscilloscopes; based on FPGA application technology, a low-cost handheld digital oscilloscope with arbitrary analog signal level, rising edge or falling edge trigger, vertical sensitivity and scanning speed adjustment, and direct readout of waveform parameters is designed.

　　1 System Design of Handheld Oscilloscope

　　The block diagram of the handheld oscilloscope system based on FPGA is shown in Figure 1. The system mainly includes an off-chip AD chip, a configuration chip EPCS16, an on-chip PLL module, a sampling clock configuration module, a key scanning module, a trigger control module, a dual-port RAM storage module [9-10], a sampling data graphics and transposition module, an LCD12864 display driver module, etc. The working process of the oscilloscope is roughly as follows: the 12 bit digital signal collected by the off-chip ADC128S022 is converted into a dot matrix data displayed on a column LCD screen after passing through the on-chip graphics module; in order to adapt to the LCD screen's row-by-row reading mode, the channel data transposition module is required to transpose the dot matrix data sampled in columns into data arranged in rows; the trigger module mainly controls the RAM's write data start or stop instructions according to the trigger level and mode set by the user (keyboard input), and generates the corresponding write data address at the same time; the numerical control sampling clock module can generate a variety of different sampling clocks to meet the measurement and display of signals of different frequencies; the LCD driver module mainly generates the RAM's read data address and LCD mode control instructions based on the LCD's working timing.
 

　　The key to the design of the handheld oscilloscope shown in Figure 1 is to organize the read/write of the dual-port RAM and the data acquisition of the off-chip A/D according to the working sequence of the LCD12864 display module; this ensures that the measured analog signal waveform is displayed correctly. The following is a more specific design description of the three key modules of the off-chip A/D module, the read/write control of the dual-port RAM, and the LCD display driver.

　　2 A/D conversion module

　　The system uses the 12-bit CMOS analog/digital conversion chip (ADC128S022) of Texas Instruments (TI) to implement data sampling of analog signals. The AD can realize 8-channel analog-to-digital conversion through channel selection signals and is powered by a single power supply. The power consumption is extremely low. Within the power supply range of 2.7~5.25 V, the power consumption is only 1.2~7.5 mW; the conversion rate can reach 50~200 KSPS, and data is exchanged with external devices through the serial peripheral interface (SPI). The chip uses a 16-pin ultra-small TSSOP package. These features are very suitable for use in small portable electronic products. The internal structure of ADC128S022 is shown in Figure 2.

　　3 Read and write control of dual-port RAM

　　Each bit of the dual-port RAM in the handheld oscilloscope design shown in Figure 1 corresponds to a pixel on the LCD 12864 screen, requiring a storage capacity of 8192 bits (1024×8 bits). The memory management needs to meet the requirements of real-time writing of collected data (64 bits) by column and reading data by row (8 bits) from the LCD module. [page]

　　To this end, first of all, the 64-bit graphic data needs to be divided into 8 segments and stored in 8 128×8-bit RAM units, so as to ensure that the read/write data bit width of each RAM storage unit is consistent. Secondly, the collected waveform data needs to pass through the row-column data transposition module to write the RAM data row by row. The row-column data transposition module adopts a pipeline structure, and under the joint action of the trigger start signal and the clock, the collected column dot matrix data is transposed into a row data format suitable for LCD screen display.

　　Figure 3 shows the read and write sequence of the dual-port RAM storage unit; after transposition, the row data is written in the horizontal "S" shape sequence shown in the figure, and when the data is read out, it is necessary to read the data in the vertical "S" shape sequence row by row. This is the control timing requirement of the LCD12864 module. The column-by-column word-by-word writing memory organization method can improve the RAM cache data refresh frequency, but it also creates a high design difficulty for the memory read and write address generation circuit.

　　4 LCD display driver module

　　To drive the LCD module to display the correct graphics, it is necessary to design a correct finite state machine (FSM) according to the control timing of the LCD screen and the user instruction set to complete the LCD module initialization, control commands and data writing operations; and generate the RAM read data address. Figure 4 shows the state migration of the LCD display driver module.

　　In the state transition diagram of the LCD module, after the system is powered on, it first performs an automatic reset for about 0.05 s (needs to be adjusted according to the clock frequency), and then enters the initialization process of the LCD module. Therefore, three conditional conversion paths are set in the state machine to realize the working mode switching of the LCD screen: initialization, display data and start line address writing. At the same time, a delay with configurable parameters is also set on the key path - while facilitating the debugging of the LCD module, the LCD module is always working in the screen writing mode, driving the LCD module to display dynamically in real time and generate the read data address of the dual-port RAM.

　　5 Design Verification

　　The hardware test and verification platform of the handheld oscilloscope is composed of the DE0_Nano development board (Altera FPGA Cyclone IVEP4CE22F17C6N), LCD module (KB12864KZK) and 4×4 keyboard; the running effect diagram of the oscilloscope design project after compilation and chip download configuration of the Quartus Ⅱ 10.1 FPGA development platform is shown in Figure 5, in which (a) is the screen of the handheld oscilloscope when it is turned on; (b) is the measurement effect of the triangle wave; (c) is the measurement effect of the sine wave; (d) is the effect of hiding the drop-down menu. The oscilloscope parameter setting adopts the 5-key input drop-down menu mode; when setting, the parameter adjustment menu is displayed at the bottom of the screen, and the menu is automatically hidden after the setting is completed and confirmed.

　　The experimental results show that the LCD12864 liquid crystal module can be used as the display terminal of the handheld oscilloscope, although the display resolution is low, the pixels of the graphics can be clearly seen; at the same time, this is also the unique advantage of using the LCD12864 screen - it can directly read the measured signal period according to the number of pixels of a period waveform and the sampling signal frequency. If a sampling rate of 100 kHz (period 10 μs) is used; the number of waveform points in a complete period on the display screen is measured to be 50, then the period of the measured signal is 500 μs.

　　6 Conclusion

　　The low-cost handheld oscilloscope designed in this paper uses LCD12864 module as the graphic display. The verification results on DE0_Nano FPGA development board show that the measurement of analog signals is fully realized; vertical sensitivity and scanning speed adjustment, and waveform parameter direct readout functions; this not only makes the oscilloscope cheap and portable, but also has the advantage of direct readout of the measured signal period.