Hardware Reference
In-Depth Information
Figure 2-1. MCP23017 pinout
GPIO Output Current
Another factor in choosing a peripheral chip is its output drive capability. How well can
the GPIO pin source or sink current? As covered in Chapter 10 of Raspberry Pi Hardware
Reference (Apress, 2014), the Raspberry Pi's own GPIO pins can source/sink up to
16 mA, depending on configuration. The MCP23017 chip specifications indicate that it
can source or sink up to 25 mA.
We still need to remember that if the MCP23017 is powered from the Raspberry
Pi's 3.3 V regulator on header P1, the total current budget must not exceed 50 mA.
This budget includes the Pi's own GPIO pin current usage. If, on the other hand, the
MCP23017 is powered from a separate 3.3 V power supply, this limitation is eliminated.
There are still reasons to budget current, however. The chip must not consume more
than 700 mW of power. This implies a total current limit as follows:
P
V
I
=
VDD
DD
07
33
212
.
.
=
=
mA
 
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