Circuit basics(4): three-state logic

High impedance (hi-Z, tri-stated, floating) means the output is not being driven to any defined logic level (neither logical high, nor low).

The use of high impedance: allow multiple circuits to share the same output line. When you enable the output (pull the line OE to low, or write to a reg), the output will drive it respective loads (0, or 1). Otherwise, it is in high-impedance.

  • A pull-up resistor can be used to try to pull the wire to high and low voltage levels. When devices are inactive, they tri-state their outputs. When all the devices on the bus have done this, the pull-up resistor will pull the line up. If a dvice wants to communicate, it drives the line low.
Example
the device document says it “employs high-impedance CMOS on all signals pins. Voltage on any signal pin that exceeds the ranges can induce destructive latch-up (a type of short circuit in IC)”.
All BT656_D(x) pins are connected to imx_CSI1_DAT(12~19). At the rightmost, it is 3V3DC connected. The decoder uses 3.3V, 1.8V.
sch_1
shc_3
To pull the line/pin in a defined status/level when there is no signal output,
  • For non-used pins (maybe used by other component), e.g MX_CSI1_DAT(1), R10845 is NB (for pull down), R10872 is 4K7(pull up), which effectively pull up the line (3.3V).
  • For pins being used: MX_CSI1_DAT(12), R10184 is 10K,
    • which effectively pulls down the line when there is no signal coming;
    • when the device outputs signals (drive 3.3V or 1.8V on the pin), the line will be high???
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