What is I/O drive capability
Time:2022-10-26
Views:1559
It is often heard that the drive capacity of an I/O of MCU is xxmA, so what exactly is the drive capacity? If the driving capacity of an IO is 5mA, can it not output more than 5mA of current? Why are the driving capabilities of IO different?
To answer the above questions, you need to understand the I/O structure inside the MCU,
This drawing is from STM32F103 reference manual)
When high level 1 is output, the internal VDD of I/O flows to the I/O pin through PMOS, as shown by the red arrow in the figure below,
At this time, the drive ability means that the I/O pin can normally output logic 1 when the output current is less than or equal to the maximum output current.
The source (S) of P-MOS tube is connected to VDD. When MCU outputs 1, P-MOS tube is connected and current flows from source (S) to drain (D). The current and VDS (source drain voltage) flowing through the P-MOS tube will be different depending on the external load size of the I/O pin. The larger the output current, the larger the VDS, and the smaller the I/O voltage. The output current when the voltage is as low as Voh indicates the driving ability.
When low level 0 is output, I/O pin flows to I/O internal VSS through N-MOS, as shown by the red arrow below
At this time, the drive ability means that the I/O pin can normally output logic 0 when the maximum input current is less than or equal to. The reason is the same. The greater the current flowing through N-MOS, the greater the voltage drop generated by N-MOS, and the greater the I/O port voltage. When the voltage reaches the highest voltage Vol of logic 0, the current at this time represents the driving ability.
After understanding the above, the answers to the three questions at the beginning of the article are also available,
1) I/O drive capability refers to the maximum current that can be output and absorbed on the premise that Voh (the minimum voltage value when the output pin is logic 1) and Vol (the maximum voltage value when the output pin is logic 0) are met simultaneously. Voh and Vol values can be found in the chip data book.
2) If the driving capacity of an I/O is 5mA, it does not mean that it can not output more than 5mA of current, but when the output current exceeds 5mA, its output voltage will drop and the voltage will be less than Voh.
3) The difference of I/O driving ability comes from the characteristics of MOS tubes. If you want to consider that the current capacity is large, the MOS tubes will be larger, the area required by the chip will be larger, and the cost will naturally be higher. Therefore, MCU usually only has a part of I/O with high drive capability, and not every I/O is made into a high drive capability.
So much has been said, which can be explained by the following simple circuit,
The IO pin of MCU is externally connected with a resistance to GND. As long as the resistance is not too small, the voltage of the IO pin is basically unchanged, only a slight drop. When the resistance is small and the current is large, the voltage will drop obviously.
If you are interested in it, you can actually test it, but you should be careful not to set the current too high to burn the IO.
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