[FAQ] What Analog Switch / Multiplexer Should I use for SPI Applications (3/4 Wire SPI, QSPI, or OSPI) ?

When looking for a SPI, QSPI, or OSPI analog switch / multiplexers, it's important to understand some of the specifications of the various SPI standards. The frequencies listed below are some common ranges, but not necessarily the absolute maximum:

Multi-Wire, Multi-Point,  Push-Pull Communication Protocol

Serial Peripheral Interface (SPI 3 and 4 wire):

• Voltage Levels: 1.8V, 3.3V, 5V
• Typical Frequency: Device Specific, normally <100MHz

Quad Serial Peripheral Interface (QSPI)

• Voltage Levels: 1.8V, 3.3V
• Typical Frequency: Device Specific, normally 100MHz – 120MHz

Octal Serial Peripheral Interface (OSPI)

• Voltage Levels: 1.8V, 3.3V
• Typical Frequency: Device Specific, can be up to 200MHz

Even though there are three different protocols, beyond the voltage levels and bandwidths differences, they are very similar in operation. All three groups of protocols can be broken down the same way. These are multi-wire protocols, and each one of them can separate the signals into 3 groups: data signals, clock signal, slower control signals. The protocol works by having one device controlling all communication on the bus shared by multiple devices. The controlling device is known as a controller and the other devices are known as peripherals.

However, why would a switch or multiplexer be needed in a multi-point system that uses addressing and chip select lines to control what device is talking? The answer is two-fold. First, the SPI protocol specifies 1 Controller, and in some systems controllers are sharing peripherals. To change which controller is controlling the bus, the bus needs to be switched between the two controllers. Another reason is that each device for 4-Wire, QSPI, and OSPI requires its own chip select line, which can quickly use up available GPIO pins. However, a switch or multiplexer can be used to expand the GPIOs to multiple devices. When selecting a switch for the latter, the only concerns are voltage levels and configuration as the chip select is going to be one of the slower lines of the protocol. However, the former requires a bit more analysis.

            The SPI protocols do not have a “specified frequency” as such, but it is a product of the devices on the bus and their limitations. As a rule of thumb, the max frequencies are going to be higher for OSPI and QSPI as they are nominally 100MHz or greater while 3 and 4 wire SPI are going to be generally <100MHz. A very important note is that SPI frequency is referring to the fundamental frequency of a square wave, so the switch’s bandwidth needs to be at least 1.5 – 3.0 times the SPI frequency to pass without much attenuation.

            With frequency and voltage levels discussed, the next important idea is how many wires are being switched. This is a simple answer, 3-wire SPI is 3, 4-Wire SPI is 4, QSPI is 6, and OSPI is 12.

Please see below for recommendations for SPI switches/multiplexers for the 2 controller-based systems.:

Best,

Parker Dodson