6 Serial Communication

USB Bridge, UART, I²C and SPI Peripherals.

6.1 USB Bridge


The MCP2221A is a USB-to-UART/I²C serial converter. It bridges the gap between a computer's USB port and the I²C and UART interfaces on the Explorer. When the on-board USB-C® is disconnected, the USB bridge is held in reset. You can keep the USB bridge in reset using the MCU on the CNANO by pulling IO-37 on the Explorer low. Tip: When not in use, it is recommended to disconnect the bridge reset from the CNANO socket by removing the jumper cap on IO-37.

The USB bridge can be disconnected from the main I²C bus on the Explorer using slide switch S500. When disconnected, the USB bridge and the power monitor IC are isolated on their own I²C bus.

Important: The USB bridge is only able to act as an I²C host.

The UART connection to the CNANO socket is shared between the USB bridge and the mikroBUS™ socket. Use slide switch S501 to select the connected UART.

6.2 UART Peripherals


The CNANO’s default UART pins are shared between the USB bridge and the mikroBUS™ socket. Only one of the connections is active at any time. Select which pins are connected to the CNANO socket using slide switch S501.

6.3 I²C Peripherals

6.3.1 I²C Bus

The Explorer features an I²C bus with several attached devices and external connectivity options. All connected devices are compatible with Standard Mode (100 kHz) and Fast Mode (400 kHz) I²C. The bus is designed to meet the rise-time requirements for both modes. The SDA and SCL signals are connected to the COM section of the CNANO socket.

Important: Connecting I²C extension boards with pull-ups will impact bus performance.


The Explorer’s I²C bus can be split into two separate buses using the slide switch S500. Each bus has its own set of pull-ups - I²C pull-ups A and B. When the I²C bus is split, the USB bridge and the power monitor ICs are isolated from the main I²C bus, with the USB bridge acting as the host to communicate with the power monitor IC.

Important: The USB bridge can function only as an I²C host.

Table 6-1. I²C Electrical Characteristics
Parameter	Value
I²C Pull-up A	2.7 kΩ
I²C Pull-up B	5.1 kΩ
I²C bus with Pull-ups A and B
I²C bus capacitance¹	130 pF
Rise-time²	~200 ns
I²C bus with Pull-up A isolated
I²C bus capacitance¹	95 pF
Rise-time²	~220 ns
I²C bus with Pull-up B isolated
I²C bus capacitance¹	35 pF
Rise-time²	~150 ns
Note: ¹Bus capacitance calculated using measured rise time with equation 1 from the I²C-bus specification and user manual. ²Rise time is defined as the time it takes for the I²C bus to rise from 0.3V_DD to 0.7V_DD.

6.3.2 Temperature Sensor


MCP9808 is a digital temperature sensor connected to the Explorer’s I²C bus. The alert pin is connected to the CNANO socket IO-24. Accuracy: ±0.25°C (typical) from -40°C to +125°C ±0.5°C (maximum) from -20°C to +100°C User selectable measurement resolution: 0.0625°C 0.125°C 0.25°C 0.5°C User programmable temperature limits: Temperature window limit Critical temperature limit

Info: 7-bit I²C address: 0x1C. The I²C address can be changed using pins A0-A2 and follows the address format ‘0011, A2, A1, A0’.

6.3.3 Secure Element


ATECC608B is a secure element in the Microchip CryptoAuthentication™ family and is connected to the Explorer’s I²C bus. Supports advanced Elliptic Curve Cryptography (ECC) Protected storage for up to 16 keys, certificates or data Internal random number generator Secure boot support

Info: 7-bit I²C address: 0x58.

Info: The ATECC608B-MAH4I used on the Explorer is pre-provisioned for the Microchip Trust Platform and configured to support some of the most common IoT use cases. For more details, visit: https://www.microchip.com/en-us/products/security/trust-platform/trustflex.

6.3.4 Proximity and Ambient Light Sensor


VCNL4200 is a proximity sensor (PS) and ambient light sensor (ALS) connected to the Explorer’s I²C bus. Programmable emitter current 16-bit resolution ALS 12-bit/16-bit resolution PS Detection distance of up to 1.5m Interrupt function available for ALS and PS with upper and lower thresholds

Info: 7-bit I²C address: 0x51.

Figure 6-6. Proximity and Ambient Light Sensor

6.3.5 2 Mb Serial EEPROM


AT24CM02 is a 2 Mb Serial EEPROM connected to the Explorer’s I²C bus. 1024 pages of 256 bytes each Random, byte and sequential read modes Byte and page write modes Built in error detection and correction Hardware write protection

Info: 7-bit I²C address space: 0x54 - 0x57.

Info: The write protect pin is available through pin header footprint J209 and can be remapped to the CNANO socket.

6.3.6 I/O Expanders

The Explorer features two MCP23008 I/O expanders connected to the I²C bus.

8-bit bidirectional I/O port
I/O pins default to input
Configurable interrupt output pin

I/O Expander 1 is set up to control the LED row containing eight yellow LEDs.

See chapter LED Row and I/O Expander 1 for more information.

I/O Expander 2 is set up to read the status of the joystick and tactile or touch buttons.

See chapter Buttons, Joystick and I/O Expander 2 for more information.

6.3.7 OLED Display Module


The Explorer features an OLED display connected to the I²C bus. SSD1306 display driver 1.30” screen size with 128x64 pixel resolution Fast response time (≦ 10 µs) Typical brightness: 120 cd/m² Info: 7-bit I²C address: 0x3D. Alternative I²C address 0x3C can be used by connecting pin A0 to GND.

6.4 SPI Peripherals

6.4.1 SPI Bus

The Explorer features an SPI bus with several peripherals and a MikroBus socket attached. The individual Chip Select (CS) pins and the mutual SPI bus pins are routed to the CNANO socket.

Info: A 47 kΩ pull-up resistor (R221) is mounted on the MISO line close to the microSD card socket, causing the MISO line to idle high.

6.4.2 Digital-to-Analog Converter


MCP4821 is a single-channel 12-bit Digital-to-Analog converter (DAC) with internal voltage reference. The DAC is connected to the SPI bus on the Explorer. SPI with 20 MHz clock support 2.048V Internal voltage reference (V_ref) DAC output, limited by supply voltage (3.3V) Synchronous update of DAC output using the LDAC pin

Table 6-2. Programmable Gain
Gain Selection	Ideal Output Range	LSb Size
2.048V	0.0-2.0475V	0.5 mV
4.096V	0.0-3.299V	1.0 mV

Tip: The DAC output is connected to the speaker circuit by default. It can be remapped using pin header J202.

6.4.3 4 Mb Serial EEPROM


25CSM04 is a 4 Mb Serial EEPROM connected to the Explorer’s SPI bus. SPI with up to 8 MHz clock support 2048 pages of 256 bytes each Byte or sequential reads Byte or page writes Built-in Error Correction Code (ECC) Configurable software and/or hardware write protection

Tip: Enable the hardware write-protected feature by pulling the WP pin low.

6.4.4 microSD Card Socket


The Explorer features a microSD card socket connected to its SPI bus. Only pins required for an SPI connection to a microSD card are connected to the CNANO socket. All pins of the microSD card socket are available on pin headers close to the microSD card socket. The pin headers allow devices with a dedicated high-speed multimedia interface to fully utilize all data lines of a connected microSD card.