Limitations of LinkIt 7697

Characteristics of ADC Pins

The input voltage of ADC pins on LinkIt 7697 ranges from 0V to 2.5V and ADC modules are with 10K Ohm input impedance. If your peripheral requires a 3.3V or 5V input, you need to map the voltage with external voltage divider circuits.

Hardware SPI Limitation

MT7697 does provide hardware support for SPI master. However, be aware of several limitations as described below:

1. It only supports half-duplex. In other words, MT7697 cannot receive data from MISO while sending data to MOSI.

2. Current driver interface requires the sending and receiving bytes to be of the same length. That is, if the driver sends 4 bytes of data on MOSI line, it always receives 4 bytes of data from MISO line.

Due to the above hardware and driver limitations, the built-in Arduino SPI library of LinkIt 7697 uses a software implementation instead of providing mappings to hardware feature. Moreover, since it's S/W implemented, you can change the pinmux definition to other set of GPIO pins as the SPI pins. Check the file in packages/LinkIt/hardware/linkit_rtos/0.7.11/libraries/SPI/src/RHSoftwareSPI.h and modify the definitions in line 36~38 to preferred pins:

#define RH_MOSI_PIN (11)	// P11 for MOSI
#define RH_MISO_PIN (12)	// P12 for MISO
#define RH_CLK_PIN  (13)	// P13 for CLK

If you want to use hardware SPI to connect to your peripherals, please use the LinkIt API directly.