tests/drivers_mcp23x17: test app for MCP23x17 driver

tests/driver_mcp23x17/Makefile

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+include ../Makefile.tests_common
+
+# default expander is MCP23017
+DRIVER ?= mcp23x17_i2c
+
+USEMODULE += $(DRIVER)
+USEMODULE += benchmark
+USEMODULE += shell
+
+include $(RIOTBASE)/Makefile.include

tests/driver_mcp23x17/Makefile.ci

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+BOARD_INSUFFICIENT_MEMORY := \
+    arduino-duemilanove \
+    arduino-leonardo \
+    arduino-nano \
+    arduino-uno \
+    atmega328p \
+    atmega328p-xplained-mini \
+    #

tests/driver_mcp23x17/README.md

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+# Microchip MCP23x17 I2C I/O expanders test application
+
+## Overview
+
+This test appliation demonstrates the usage of the MCP23x17 driver interface
+and can be used to test each MCP23x17 expander I/O pin with shell commands.
+
+The application bases on the test application for GPIO peripheral drivers
+which is under following copyright:
+
+Copyright (C) 2014,2017 Freie Universität Berlin
+Author Hauke Petersen <hauke.petersen@fu-berlin.de>
+
+## Compilation
+
+Used MCP23x17 I/O expander or interface variants have to be defined by
+the variable `DRIVER` using the corresponding pseudomodules `mcp23017`,
+`mcp23s17`, `mcp23017_i2c` and/or `mcp23017_spi`, for example:
+
+```
+DRIVER='mcp23x17_i2c mcp23x17_spi' BOARD=... make -C tests/driver_mcp23x17 flash term
+```
+
+**Please note:** If `DRIVER` is not defined, `mcp23x17_i2c` is used by default.
+
+The default hardware configuration parameters are defined in
+`$(RIOTBASE)/drivers/mcp23x17/include/mcp23x17_params.h`. They can
+be overridden either by placing a `mcp23x17_params.h` file with changed
+configuration parameters in the directory of the test application or by
+defining them in the `CFLAGS` variable in the make command, for example:
+
+```
+CFLAGS="-DMCP23X17_PARAM_RESET_PIN=GPIO_PIN\(0,7\)" \
+USEMODULE=mcp23x17_reset \
+BOARD=... make -C tests/driver_mcp23x17 flash term
+```
+
+To use external interrupts for the MCP23x17 I/O expander pins, the MCP23x17
+low-active push/pull interrupt signal has to be enabled. For this purpose
+add the module `mcp23x17_irq` and define the GPIO pin to which the
+combined `INTA`/`INTB` interrupt signal is connected, e.g. by overriding the
+default parameter `MCP23X17_PARAM_I2C_INT` for the default I2C device or
+`MCP23X17_PARAM_SPI_INT` for the default SPI device.
+
+```
+CFLAGS="-DMCP23X17_PARAM_I2C_INT=GPIO_PIN\(0,6\)" \
+USEMODULE=mcp23x17_irq \
+BOARD=... make -C tests/driver_mcp23x17 flash term
+```
+
+Using module `mcp23x17_irq` uses by default an event thread with medium
+priority. An event thread with high priority can be used by enabling
+the moduleW `mcp23x17_irq_highest`.
+
+**Please note:** Since interrupts are handled in the context of a separate event thread,
+enabling interrupts requires more RAM.
+
+## Usage
+
+The test allows to use commands as known from GPIO test application for
+MCP23x17 I/O expanders:
+
+```
+> help
+Command              Description
+---------------------------------------
+init_out             init as output (push-pull mode)
+init_in              init as input w/o pull resistor
+init_in_pu           init as input with pull-up
+init_od              init as output (open-drain without pull resistor)
+init_od_pu           init as output (open-drain with pull-up)
+init_int             init as external INT w/o pull resistor
+enable_int           enable gpio interrupt
+disable_int          disable gpio interrupt
+read                 read pin status
+set                  set pin to HIGH
+clear                set pin to LOW
+toggle               toggle pin
+bench                run a set of predefined benchmarks
+```
+
+The number of the first MCP23x17 I/O expander port used by the test
+application is defined by the macro `MCP23X17_PORT_0`, which is 16 by
+default. This value can be overridden during compilation, for example:
+
+```
+CFLAGS="-DMCP23X17_PORT_0=8" \
+BOARD=... make -C tests/driver_mcp23x17 flash term
+```
+
+Using the port number defined by `MCP23X17_PORT_0` and following port
+numbers, you can apply the command to the MCP23x17 expander ports. For
+example, the following command initializes I/O pin 7 of the first MCP23x17
+expander:
+
+```
+init_out 16 7
+```
+
+Commands with port numbers less than `MCP23X17_PORT_0` refer to GPIO
+peripheral ports. Thus, both the I/O pins of the MCP23x17 expanders as well
+as the GPIO peripheral pins of the MCU can be addressed by all commands.
+
+Thus it is possible to connect a GPIO pin of the MCU with MCP23x17
+I/O expander pins for testing.