Build Instructions

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• 1

  Using The Compute Module 3

  The Raspberry Pi Compute Module 3 is a very powerful core board for prototyping some interest gadgets. Detailed introduction can be found here. And some useful information can be found here.

  The module uses a DDR2 SODIMM type connector, which is a slightly harder to use. In addition all the GPIO pins of the BCM2837 core BANK1 and BANK0 are lead out.

  To start using the compute module, we need to provide several different voltages: 1.8V, 3.3V, 2.5V and 5.0V. Among them, 1.8V and 3.3V are used to power some peripherals which need about 350mA each. The 2.5V power line drives the TV-out DAC and it can be tied to 3.3V as we do not need TV-out feature. The 5.0V should be connected to the VBAT pins and it powers the Core. The VBAT input accepts voltages range from 2.5V to 5.0V and just make sure the power supply can outputs up to 3.5W. VCCIO pins (GPIO_XX-XX_VREF) can be connected to 3.3V as we use a 3.3V CMOS level. The SDX_VREF pin should also be connected to 3.3V.

  All the HDMI, DSI, CAM pins are not used here, just leave them float. Remember to tie the EMMC_DISABLE_N pin to 3.3V as we will use a TF card as the hard drive instead of the USB boot feature.

  The Pi need to know what is function of each GPIO. Here I provide some files, put "dt-blob.bin", "bcm2710-rpi-cm3.dtb" and "config.txt" in the boot folder of a newly flashed TF card. The put the "dcdpi.dtbo" in the /boot/overlay folder. The dt-blob.bin defines the default function of each GPIO. I change the GPIO14/15 to normal GPIO and move the UART0 function to GPIO32/33 as we need GPIO14/15 to interface with the LCD module. I also tell the Pi to use GPIO40/41 as pwm function and make them to be the right and left audio output. The dcdpi.dtbo is a device-tree overlay file and it tells the Pi that we will use GPIO0-25 as DPI function. Finally, we write "dtoverly=dcdpi" to aware the Pi to load the overlay file we provided.

• 2

  Interfacing the LCD Module

  As different DPI/RGB interface LCD module may be used in this console, here we take the module used in my own build as example. And if you chose a different one, check the pin definition of your module and just make the connections according to the pin names as showed in the example.

  There are two interfaces on the LCD module: an SPI and a DPI. The SPI is used to configure the initial settings of the LCD driver IC and we can connect them to any unused GPIO. Only connect the Reset, CS, MOSI(SDA/SDI) and SCLK(SCL) pins, the MISO(SDO) pin is not used. To initialize the LCD driver, here we use the BCM2835 C Library to drive the GPIOs and outputs a certain initialize sequence provided by the module supplier. The source file can be found at the end.

  Install the BCM2835 C Library on another Raspberry Pi 3 according to the instructions here. Then use the command "gcc -o lcd_init lcd_init.c -lbcm2835" to compile the source file. Then add a new line in the /etc/rc.local file before "exit 0": "/home/pi/lcd_init"(assume you have put the compiled application under /home/pi folder). It should be emphasis that the source file is only used for the certain module I used and for a different LCD module, just ask the supplier for a initialize sequence and modify the source file accordingly. This process is quite tricky because at this point nothing can be seen from the screen, that's why I strongly suggest you do this on a RPI-CMIO board as it leads out all the GPIOs so you can debug it with uart or wlan.

  The following part is easy, just connect the left pins of the LCD module according to here. Depends on what kind of LCD module you have got, chose the RGB mode wisely. For me, here I chose the DPI_OUTPUT_FORMAT_18BIT_666_CFG2(mode 6). Alter the "dpi_output_format=0x078206" line according to your choice. And if your LCD module uses a different resolution, adjust the "hdmi_timings=480 0 41 60 20 800 0 5 10 10 0 0 0 60 0 32000000" refering to the file here.

  If all the settings are correct, on the next boot up of your Pi, you should see the display on the screen after a 30-40s black(from power to system loads your SPI initialization script).

• 3

  The Key Pad and Audio

  A game console needs keys and buttons. Here we need 10 ALPS SKPDACD010 switches as the up/down/right/left, LR and A/B/X/Y buttons. And normal 6x6 surface mount keys are used for other buttons like start/select and volume-up/down.

  There are two ways to interface the buttons with the Raspberry Pi. One way is connecting the buttons directly to the GPIOs on the Pi and another way is connecting the buttons to an MCU and interface with the Pi through USB HID protocol. Here I chose the second one, because we need an MCU to deal with the power on sequence anyway and it is safer to keep the Pi away from human touch.

  So, connect the keys to the STM32F103C8T6 and then connect the MCU to the Pi with USB. An example of the MCU program can be found at the end. Alter the pin definitions in hw_config.c and compile it with the MCU's USB library found here. Or you can just download the hex file directly to the MCU as long as you share the same pin definitions in the schematic at the end.

  As for the audio outputs, the official schematic of the Raspberry Pi 3 B gives a good way of filtering the pwm wave and the same circuit should work perfectly here. One thing that should be point out is that remember to add the "audio_pwm_mode=2" line at the end of config.txt in order to lower the noise of the audio output.

  In order to drive the speaker, a speaker driver is needed. Here I chose the TDA2822 and the circuit is the official BTL circuit. Note that the phone jack PJ-327 has a auto detach pin on the right output. When there is no headphone plugged in, the pin 3 is connected to the right channel. And as soon as the headphone is plugged in, this pin is detached from the right channel. This pin can be used as the speaker input pin and the speaker will mute when headphone is plugged in.

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