The evaluation board is designed around the uD6350 IC to test its performance and demonstrate its functionality. The main components of the evaluation board are shown in Figure 1. It consists of two 3.3V and 2.5V LDO regulators, 32-bit ARM Cortex M0 microcontroller, uD6350 IC, two logic shifters and four pushbuttons. The board runs from a single 3.7V lithium ion battery. The power input of the board has a reverse voltage protection circuit to avoid damage from inccorect power supply polarity. Two LDO regulators step down the 3.7V supplied by the battery to 3.3V for the microcontroller and to 2.5V for the uD6350.
Based on the demo selected, the microcontroller sends a series of 32-bit instructions to uD6350 using SPI bus, which consists of SCLK, SDI and SLAT signals. In addition, the microcontroller also provides RESET and CLK2 (optional) signals to the u6350. Finally, a 3.3V-to-2.5V logic level shifter translates these five 3.3V logic signals from the microcontroller to 2.5V logic signals and drives respective input pins of uD6350.
There are two optional jumpers for setting behavior of the chip, PWM_EN and CLK2_SEL, on the board. PWM_EN jumper enables or disables the PWM feature of uD6350. Whereas enabling CLK2_SEL jumper selects CLK2 pin as the clock source for the scan generator block. This allows the scan generator to run at a different frequency than the main SPI SCLK frequency. For more details, please refer to the System Overview section.
There are six test pins on the board for probing SCN0, SCN19, PSCN0, PSCN19, PWM and RNCTR debug outputs of uD6350, and one SDO pin which is the serial data out signal from E6350 . Optional 2.5V-to-3.5V logic shifters can be installed on the board to convert and buffer these 2.5V debug outputs to 3.3V outputs. The SDO pin can be used to daisy chain multiple uD6350s to expand micro LED array size. Furthermore, the 3.3V SCN19 signal from the logic shifter is connected to an input of the microcontroller and can be used as end of column signal for syncing serial data transfer while the display is scanning.
Schematics and PCB for the evaluation board is created using KiCAD. The final 2 layer PCB of the evaluation board and the breakout board for the custom 20x20 LED array are shown in Figure 2 and Figure 3.
Figure 4 shows the custom microLED die of size 15mmx 15mm. This die is placed on top and center of the microLED breakout board shown in Figure 3. The pads of the die are wirebonded to the microLED breakout PCB pads.
Figure 5 and 6 shows the front and the back of the complete assembled evaluation board.
Test software and demos
The test software is written in C. The pseudo code for the software is shown Listing 1. Four display demos are stored in the microcontroller and can be selected by clicking on buttons 1-4. Artwork for the demos is stored in a 20 x 32-bit array. The final executable demo is loaded to the microcontroller via JTAG programmer. Figure 5 shows typical serial instructions sent by the microcontroller at 1 MHz SCLK frequency.