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(I/O Port) How do you calculate MCU port drive capacity ? [2006/01/06]

Latest Updated:01/06/2006


(I/O Port)  How do you calculate MCU port drive capacity ? [2006/01/06]


Here is an explanation using port P60 "H" output voltage as an example (can also be applied to "L" output voltage). VOH of the electrical characteristics port P60 and port P60 IOH to VOH characteristics (P-channel drive) of standard characteristics in the 3803 Group are shown here.

Table : 3803Group Electrical characteristics (VCC = 2.7 to 5.5 V, VSS = 0 V, Ta = -20 to 85 degrees C, unless otherwise noted)

Symbol Parameter Test conditions Limits Unit
Min. Typ. Max.
VOH "H" output voltage
P00-P07, P10-P17, P20-P27,
P30, P31, P34-P37, P40-P47,
P50-P57, P60-P67
IOH = -10 mA
VCC = 4.0-5.5 V
VCC-2.0     V
IOH = -1.0 mA
VCC = 2.7-5.5 V
VCC-1.0     V

Ex.1) "H" output voltage at Vcc = 5V and 10mA

  1. From electrical characteristics, the minimum value of this MCU's "H" output voltage (VOH) is Vcc-2.0 = 5.0-2.0 = 3.0[V]. Under conditions requiring VOH to be 3.0V or above, there is no problem with this value. The above specifications mean that "even at Vcc = 5V and 10mA, "H" output voltage will never be below 3.0V", which signifies the guaranteed range.
  2. Please refer to the port standard characteristics example if the minimum value of 3.0V VOH is too low. The standard characteristics example shows measured results using standard samples. The graph shows that VOH is held about 4.5V with [Ta = 25℃] and 4.4V with [Ta = 90℃] in these samples at Vcc = 5V and 10 mA. Take value margins into account when considering whether your target VOH can be achieved. Decide on margin levels according to your system. This standard characteristics example is only a measurement example using standard samples provided as a reference, not a guarantee.
  3. If the value obtained from the standard characteristics example is less than your target VOH, consider using an external driver, etc.

Ex.2) When usage conditions do not match electrical characteristic measurement conditions

When usage conditions do not match electrical characteristic measurement conditions, for example, if the user wants 5mA current at 3.0V, consider obtaining the port resistor value from VOH and IOH in electrical characteristics, or estimating a curve matching usage conditions using a standard characteristics example graph, etc.

Refer to the electrical characteristics of the appropriate MCU or output port standard characteristics examples for individual MCU specifications. Check applicable ports for each applicable device.


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