What should I pay attention to when using AM335x series chips?

ARM's research and development journey is long and interesting. Its well-known development advantages and its helplessness when faced with problems make people love and hate ARM. What is your relationship with ARM?

Figure 1 AM335x functional block diagram

With its powerful communication functions such as 6-channel UART, 2-channel CAN-Bus, 2-channel USB OTG, and support for 2-channel Ethernet interfaces, as well as industrial-grade performance assurance, it has been widely used in multiple industries such as portable navigation systems, industrial automation, and human-machine interfaces.

With the widespread application of Cortex-A8 processors, the customer base has increased rapidly, and the problems encountered by customers are also strange. Today we will understand why some customers have experienced the inexplicable burning of Cortex-A8 processors?

In order to find out the cause and solve the problem, ZHIYUAN R&D engineers conducted a lot of analysis on the burned out modules and found that the burned out modules were all caused by the power domain. So what caused the power domain to burn out?

The R&D personnel analyzed whether some IO ports of the module were over-voltage or had reverse current, which caused the IO ports to burn out, and in serious cases, the CPU to burn out. With the analysis direction, the next step was to immediately consult the data and conduct experiments to confirm it.

Figure 2 IO voltage range specified in the M335x data sheet on TI’s official website

Figure 3 IO voltage range specified by Freescale i.MX28 series chips

In order to further confirm the difference between the two, the R&D personnel conducted a comparative experiment on the modules. After multiple tests, they found that the IO voltage resistance value of the Freescale ARM9 chip CPU is up to 5V, while the IO voltage resistance value of the TI Cortex-A8 platform M335x series chip CPU is only 3.6V! I believe this is also an important reason why the CPU of the Cortex-A8 platform burns out.

So what issues should we pay attention to when using the AM335x series chips?

1. Precautions for IO use

When GPIO is used as input, the maximum voltage must not exceed 3.3V and the minimum voltage must not be lower than -0.2V. When GPIO is used as output, due to the limited driving capability of IO, the maximum output current of a single IO is 4mA, and the maximum output current of all IOs does not exceed 50mA. It is recommended to add a triode or MOS tube when driving power devices such as LEDs, optocouplers, and relays. When driving low-power devices such as LEDs and optocouplers, it can also be designed to be a current sink output mode.

It is recommended to add 100R current limiting resistors when using Cortex-A8 IO, especially the reserved GPIO.

2. Power supply design considerations

The 3.3V power output of the baseboard is enabled only after the 3.3V power supply of the core board is powered on, to prevent the 3.3V of the baseboard from affecting the power-on timing of the core board through IO when the core board is powered on, causing the core board to fail to start or be damaged.

Figure 4 CPU power-on timing description

It is recommended that the 3.3V power supply of the baseboard use a power supply with an enable terminal, and connect the enable terminal to the PWR_OK of the core board (PWR_OK is connected to the 3.3V power supply of the core board) or use GPIO for enable control. A pull-down resistor should be added to make the power supply in a disabled state when powered on to avoid system failure caused by uncertain IO port status when powered on.

3. Notes on startup settings

EMU0 ~ EMU4 are boot configuration pins, which can be used as normal GPIO, but these two pins should be in floating or high impedance state when powered on. Wrong configuration will cause the system to fail to boot. Therefore, it is recommended not to use them as input pins during design.

The lcd_data[0..15] of the M335x and A335x core boards are the boot item selection pins of the AM335x processor. If these pins are not used as LCD data lines, it should be noted that the power-on state cannot affect the startup of the processor and can only be used as output pins.

The GPMC_D[7..0] data bus is already used internally by NAND Flash. When the GPMC bus is needed externally, the GPMC communication protocol must be strictly followed to prevent affecting system startup.

These three points are where errors are prone to occur in system applications. I will focus on introducing them here, hoping to provide a reference for your future designs.

For 16 years, ZLG Zhiyuan Electronics has been committed to the research of various technologies in the fields of high-end measurement and analysis instruments, embedded industrial control and the Internet of Things. We work hard on technology just to provide you with good products and high-quality technical services.

Figure 5 ZLG Zhiyuan Electronics Industrial Internet Intelligent Control Solution