2 Protocols Overview

SPI

Serial Peripheral Interface is a serial communication protocol with a simple architecture consisting of a main controller and one or more peripheral devices.

The controller generates the clock signal (SCLK) and chip-select () signal to enable or disable specific peripheral devices. Standard SPI communication uses two unidirectional data lines, Send Data Out (SDO) and Send Data In (SDI), allowing the controller and peripheral ICs to simultaneously write and read data, supporting full-duplex communication. 3-wire SPI is an alternative that utilizes only one bidirectional data line to both transmit and receive data.

A primary advantage of SPI over other protocols is faster and more precise data transfer. Data is synchronized with a clock signal, enabling high-frequency communication with accurate timing for reads and writes. SPI's continuous data streaming also eliminates the need for interrupts or complex addressing schemes, reducing latency.

In data centers, due to the large amount of data stored, require high-speed data access as well as system monitors for environmental and electrical conditions. SPI can be used to interface between temperature, pressure, and voltage sensors. SPI communication reduces latency between the sensors and controllers, minimizing the potential for system damage or failure. It is also relevant in automotive ADAS systems, which rely on real-time monitoring and sensing with cameras, radar, lidar, and infrared sensors, where low latency is essential to ensure rapid reaction time.

Since each device requires an individual chip-select line, SPI poses a potential challenge for high-density systems with many peripheral devices and a limited number of GPIO pins. Current solutions tend to use GPIO expanders or larger MCUs, increasing BOM count and solution size.

To overcome this, a multiplexer, such as the TMUX1204 (4:1), can switch one chip-select line between multiple peripheral devices. As shown in Figure 3 below, it reduces four pins to only one GPIO, with a fast transition time of 14 ns and size as small as 2.5 mm². For larger systems, the TMUX1208 (8:1) has identical switching time and electrical characteristics but can replace up to eight pins.

Another high-density solution multiplexers provide is reducing ADC count in industrial or data center sensor networks system designs. When communicating with sensors such as voltage, pressure, and temperature sensors, an individual ADC for each signal leads to a significant increase in area and cost. Multiplexers can switch several analog outputs to a single ADC, with a 16:1 multiplexer, such the precision multiplexer MUX506, effectively replacing 16 individual ADCs.

Precision multiplexers are rated with low charge injection and leakage current to ensure signal integrity. This is especially relevant in data center applications for cooling systems and electrical monitors to ensure proper operation. Industrial systems also rely on many sensors for motor control, factory automation, and robotics. These require pressure and electrical sensing for packaging, manufacturing, and equipment operation. Figure 4 below illustrates a single-wire SPI system, using the TMUX1104 (4:1) to switch between three sensors, utilizing only one ADC. When not using all the channels, unused pins can be tied to ground to avoid potential noise or crosstalk.

QSPI

Quad-SPI follows a very similar protocol to standard SPI but achieves enhanced data rates with four half-duplex data lines. The additional data lines allow up to four times the amount of data to be transferred, effectively quadrupling the date-rate of SPI. QSPI is particularly applicable in data centers, serving as the communication interface between the main processors and external flash memory, which store FPGA firmware and BIOS system images. QSPI enables faster boot times and system startups compared to standard SPI. It also has low-latency applications in the automotive industry for real-time control and monitoring, such as infotainment systems and body module controls.

Although QSPI improves system throughput, it also introduces system limitations by requiring additional I/O pins, increasing both cost and area to accommodate signal routing and pin requirements. However, by multiplexing a SPI bus, one set of GPIO pins can enable communication between the main controller and multiple flash chips, as shown below in Figure 5. With high bandwidth, low capacitance, and excellent isolation, the TMUXL27518 (2:1 6-ch) is designed for expanding multimedia card host devices to multiple peripherals. It provides switching between primary and backup flash in the event of an outage or data corruption, ensuring consistent data accessibility under emergency conditions while providing a space and cost-effective solution.

OSPI

For applications requiring more data throughput, Octal-SPI provides eight half-duplex data lines, doubling the data-rate of QSPI. While OSPI use cases are similar to QSPI, it is typically found in designs requiring higher throughput and faster response times, such as MCU or FPGA systems in data or high-computing centers. OSPI is also growing in popularity with automotive applications due to its high data-rate signaling. Current uses include infotainment systems requiring high-resolution graphics and rapid stored-memory access, as well as ADAS systems for autonomous driving control and sensing. Data-intensive system backups also utilize OSPI for memory bank and fail-safe boot selection. Fail-safe protection is essential to ensure safety and reliability for both data centers and ADAS systems, which must recover quickly in the event of memory corruption or power loss.

Like QSPI, OSPI pin requirements can pose a challenge for high density applications. Signal multiplexing allows systems to efficiently switch between primary and backup selections for memory or power, reducing both GPIO count and system area, as shown in Figure 6. As similarly used for QSPI bus multiplexing, two SN3257-Q1 (2:1, 4-ch) muxes, can be used to switch between two OSPI memories.