Electronic – Same pin as output and input

inputoutputpins

In the 7447 BCD to 7-segment decoder chip, pin 4 is BI'/RBO'. That pin acts both as input and output. I would like to understand how exactly the internal circuitry works to make this possible. On page 7 of this datasheet, there is a schematic showing the internal circuitry for the BI'/RBO' connection. Can somebody please explain how exactly this works?

Does the dashed line on the right carry the pin signal fed to the rest of the chip circuitry? If so, how can the signal there ever attain the logical "high" state? The two diodes in series will always clamp down the voltage there to be the sum of two diode forward voltages (i.e., max ~1.4V total).

Best Answer

Yes, the dashed lines show the connections to the rest of the IC.

the left half shows an open-collector output. you can't force it high without risking damage to the chip but it can be forced low, the right half represents the input circuit

see notes at the bottom of page 3

Related Solutions

Electronic – Configure a PIC pin for Input and Output

Change the pin configuration every time. It's the TRIS register that controls if it's an input (1) or output (0). And, when you change direction, you have to wait a moment (check the datasheet) like a cycle or two before you use that pin. a couple Nop(); will do fine.

And, make sure you write to the LAT register, and read from the R register.

e.g. for port D, pin 0.

//output
_TRISD0 = 0;
Nop();
Nop();
_LATD0 = 1;
_LATD0 = 0;


//input
_TRISD0 = 1;
Nop();
Nop();
int i = _RD0;

Electronic – Input and Output pin voltage specs on a datasheet

On page 38 of the datasheet you'll see for instance that the voltage on the PB should never exceed Vcc + 0.3 V. You'll find that this is true for almost all inputs on ICs: the input levels should remain between the rails. An exception is when you have for instance 5 V tolerant inputs on a 3.3 V powered microcontroller, but then that will be clearly indicated in the datasheet. (The manufacturer will consider this as an important feature, so he will make sure you notice that.)

To know if the high output will damage the input of another IC with a Vcc of 2.7 V you'll have to consult the datasheet of that IC. Probably the same limits will apply: input voltage should be no higher than the 2.7 V supply.

Best Answer

Related Solutions

Electronic – Configure a PIC pin for Input and Output

Electronic – Input and Output pin voltage specs on a datasheet

Related Topic