High impedance inputs

[luke2]

Posted on November 03, 2016 at 06:25

I have an STM32F303 and I'm trying to connect 8 pins into an active bus to latch values via DMA. I need the pins to be high impedance inputs as soon as the device is powered, as it may be booted connected to an active bus and should not interfere with it. I'm not entirely clear about the different GPIO input modes and how to ensure they start in the correct mode. Do I need to use pull-up/pull-down GPIO input type with 'no pull' pupd, or do I use open drain mode (is this relevant for inputs)? Does anybody know what kind of impedance I can expect from these modes? Also when I'm booting how do I ensure the pins stay high impedance during the period it is booted until it is configured, so they don't corrupt the bus signal? Do pins start in high impedance mode as the MCU is booted?

Any suggestions would be much appreciated.Luke

[John F.]

Posted on November 03, 2016 at 09:14

I would use a separate IC to buffer the conenction. for example the 74LVC244A whic buffers eight lines. This part offers ''Ioff ''which Supports Live Insertion, Partial-Power-Down Mode, and Back-Drive Protection. In other words, it won't load your external bus if it's powered down and its inputs won't be damaged by voltages on the external bus (up to 5V). It won't try to drive the microcontroller inputs until it's powered up and the outputs are enabled.