July 20, 2009
about 2 years ago
Problem with your schematic: The pins which TXLED# and RXLED# are on are tied internally to VCCIO with 200k resistors. The LEDs themselves are connected to VCC. Because of this, connecting voltages lower than 3.3V such as 1.8V or 2.8V to VCCIO will not work as they are supposed to because VCCIO is being pulled up to 3V+. So for this schematic it is not possible to have less than 3.3V on TX and RX as they’re being pulled up to VCC (minus diode drop).
For anyone wanting 1.8V or 2.8V on VCCIO, to get around this R1 & R2 can be removed (meaning you lose the LEDs) or they can be removed from the pad connecting them to VCC and jumped to VCCIO.
about 4 years ago
I’ve tried to use this at 1.8V but there is a voltage always on the VCCIO which pulls up the voltage I try to use there. I remove the jumper to 3.3V (and I’ve checked, they are no longer connected) but there is still 3.3V on the VCCIO pin. I’ve done different ways of supplying the 1.8V to VCCIO with a 1.8V regulator with VCC tied to the regulator’s input voltage (as the datasheet suggests) or to the 1.8V of the system I’m using these with. Neither one works as the voltage is pulled up to around 3V by the 3.3V already on the pin. I can get the voltage down to about 1.8V by loading the pin down with a 1.2k resistor as a workaround. I’m wondering what I am missing that the VCCIO isn’t acting as it is supposed to. I’ve used many of these boards and they’re all the same. However, tying VCCIO to VCC does work as expected.
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