Bench Talk

(Source: Analog Devices)

Simple is better. Simplicity allows for fewer things to go wrong. It provides fewer things to figure out from a design standpoint. Additionally, it saves on cost. This blog discusses an alternative and simplified I²C/SPI communication solution when working with power I/O-constrained systems.

Traditionally, I²C and SPI have employed multiple wires. However, it is possible to deliver power and data to operate I²C and SPI endpoints, such as humidity or temperature sensors, using a single wire connection and ground. Specifically, Analog Devices’ 1-Wire^® technology offers a robust solution when working with I/O-constrained systems where there might be only one or a few pins available on the host. The Analog Devices DS28E18 1-Wire^® to I²C/SPI Bridge is an excellent example of a bridge device that leverages 1-Wire to address a standard set of system challenges like wiring limitations, communication distance, and protocol conversion (Figure 1).

Figure 1: The chart shows challenges associated with wiring limitations, communication distances, and protocol conversion. (Source: Analog Devices)

This one-wire interface technology is offered through Analog Devices and has been around since the 1980s. A single dedicated connection delivers power and data, enabling various applications such as medical sensors, accessory identification, and remote or local environmental sensing. The 1-Wire solution offers the benefits of operating SPI or I²C devices with a single-contact interface, eliminating the need for an external power source, and the flexibility of 1-Wire and I²C/SPI master operational modes for these applications.

Two contacts operate this interface. With the 1-Wire single connection and a ground connection, designers can communicate at two different speeds, 11.7kb/s and 62.5kb/s, in overdrive mode. A microcontroller host attaches to a remote SPI sensor through a 1-Wire interface to the DS28E18 bridge using only two connections, the 1-Wire I/O and ground (Figure 2).

Figure 2: The diagram illustrates the system-level configuration. (Source: Analog Devices)

One of the unique features of the DS28E18 communications bridge is that it can harvest up to 10mAs of current to power up the externally connected I²C/SPI endpoints. This device can also drive the I²C and SPI endpoints up to 1MHz and 2.3MHz. The DS28E18 communications bridge comes in a small 2x3mm TDFN package and operates at 3.3V (±10%) within environmental conditions of -40°C to +85°C.

Besides the 1-Wire interface and capabilities, the DS28E18 encompasses three main blocks (Figure 3) that are essential to interface with the I²C/SPI endpoints:

• A command sequencer that serves as a bridge between 1-Wire commands and I²C/SPI commands
• An SRAM block with data on it to ultimately operate the device
• An I²C/SPI/GPIO interface controller

Figure 3: DS28E18 block diagram showing the three main blocks essential to interface with the I²C/SPI endpoints. (Source: Analog Devices)

The Command Sequencer processes the buffer data and stores it at the specified address in SRAM (128 bytes at a time) and returns a CRC16 for the host processor to validate data transmission. The sequencer minimizes the host’s communication overhead by having the most commonly used commands stored in the SRAM. The DS28E18 provides a 512-byte buffer in SRAM that can be loaded with multiple I²C or SPI commands. Once loaded, the host controller sends an order to execute the sequence, provide power, and collect data from attached I²C or SPI peripherals. A subsequent 1-Wire command reads collected sensor data.

Three types of commands, which reside in the blue highlighted elements (Figure 4), operate this device. These commands are:

• 1-Wire ROM Function Commands
• DS28E18 Device Function Commands
• DS28E18 Sequencer Commands

Figure 4: The highlighted block diagram illustrates where commands that operate the DS28E18 reside. (Source: Analog Devices)

The host initiates communication to identify and select the DS28E18 bridge device using 1-Wire ROM level function commands. Once selected, device function commands interact with the sequencer. Figure 4 lists the 1-Wire ROM and device function command available for the DS28E18. Refer to the DS28E18 Technical Documentation for detailed information.

The DS28E18 has a 144-byte command buffer that utilizes 16-bytes for device function command operations and 128-bytes to transfer formed packets with sequential commands into a 512-byte SRAM sequencer. The formed packets installed in the SRAM sequencer can get called to write and read I²C/SPI data to attached slaves. The maximum length of a sequence is 512-bytes. The I²C/SPI slave response is recovered using a Read sequencer command upon completion of a sequence.

The result byte returned indicates success or any error encountered, such as receiving a NACK. If the byte indicates an error, two additional bytes are returned indicating the error position in the sequence.

The sequencer’s utility commands provide various functions such as delays and power gating to an endpoint device via the SENS_VDD pin. The delay can be employed in a sequence to allow additional time for an I²C/SPI endpoint device to perform a conversion or allow for settling after power is applied to the endpoint. The delay ranges from 1ms to 32s. The power provided to the endpoint is harvested from the 1-Wire interface. This means that the host must enable a strong pullup for the entirety of the sequence. The DS28E18 can deliver up to 10mA of current.

The DS28E18's GPIOs, I²C, and SPI interfaces multiplex across four pins (Figure 5). The I²C interface can operate at 100kHz, 400kHz, or 1MHz, while the SPI can be configured to operate at 100kHz, 400kHz, 1MHz, or 2.3MHz. The GPIOs are not available when configured as SPI.

Figure 5: GPIO/I²C/SPI Pin Multiplexing and Interface Control (Source: Analog Devices)

To get hands-on experience, order the Analog Devices DS28E18EVKIT Evaluation System.

Marco Antonio Ramirez Castro
Marco A. Ramirez Castro is a level entry Product Applications Engineer at Analog Devices. Graduated with a bachelor’s degree in Electrical Engineering from the University of Texas at El Paso, Marco has had the opportunity to not only develop himself professionally by attaining multiple internships with different Fortune 500 companies, but also on a more personal level by joining the recognized MAES/SHPE Latino organization as a board officer and Vice President of UTEP’s Chapter where he learned important leadership skills leading to various awards such as SHPE’s regional chapter of the year, SHPE’s Chapter Excellence award, and MAES/SHPE Chapter’s Community Service award. With his goals in mind, Marco plans to keep growing in this industry and is excited to learn and be part of this community of amazing engineers with the hopes of becoming a great one himself.

Marco Antonio Ramirez Castro authored the Just 1-Wire to Operate I²C/SPI Endpoints blog, which is repurposed here with permission.