Avnet will be publishing an errata for the Rev B and Rev C MicroZed assemblies.
PLEASE NOTE: This does NOT affect operating the board in standalone mode. The issues were discovered as we were finalizing testing of the I/O Carrier. If you have already built up your own Carrier or are duplicating MicroZed on your own chip-down design, please be aware of the following issues.
- Silicon Labs CP2104 USB-UART device Vbus cannot exceed Vdd by 3.6V or more (unfortunately, this is not currently documented in the CP2104 datasheet). Since MicroZed was designed in self-powered mode (internal CP2104 regulator bypassed), it is possible that a MicroZed could be plugged into a Carrier, with a USB-UART cable plugged in, and the Carrier powered-down. The USB-UART Vbus will be ~5V in this case, while Vdd (tied to the MicroZed 3.3V rail) will be floating. This causes the part to latch up, and we have seen cases where the internal regulator will power up and start regulating 3.35V back onto the MicroZed 3.3V rail. Eventually, the CP2104 will fail in this mode.
- The Marvell 88E1512 Ethernet PHY includes a couple LED control signals. These control signals are driven from the Vddo pins of the PHY, which on MicroZed are tied to 1.8V. We connected these control signals to the Bel Fuse RJ45 with integrated LEDs, and then we biased those LEDs with 3.3V. In the case where a Carrier signals the MicroZed to power down, the 1.8V rail will go down before 3.3V if the USB-UART cable is plugged in. While the 1.8V rail is down and the 3.3V rail is still up, leakage could occur from the 3.3V rail, through the Ethernet LEDs, into the PHY, and onto the 1.8V rail.
- Work-around for Rev B and Rev C – In both cases, the issue can be completely avoided by unplugging the USB-UART cable. In standalone mode, this is the only means of powering down the board, which is why this is not an issue in standalone mode. In SOM/Carrier mode, the user must unplug their USB-UART cable first before powering down the Carrier. If you don’t, you will find that the MicroZed does not fully power down and you may eventually damage your board. We will continue to sell the Rev C MicroZed boards as evaluation kits to be used in standalone mode, and we plan to provide a warning inside the Carrier kits when we start to ship them related to usage with Rev B/C. Engineers going to production with the MicroZed SOM should wait for the updated MicroZed, which we expect mid-November.