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An Introduction to SPI on XBee

 

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Introduction

Purpose and Scope

The purpose of this document is to specify how SPI is implemented on the XBee product line. It can be used by technical publications to create product manuals, by technical support to assist customers, and by engineering, to ensure uniformity of implementation for future products. It may be used outside of Digi after product releases. As of the date of this writing, SPI has been implemented on three platforms, with more platforms expected to follow. These platforms are:

Most aspects of the SPI operate the same across product lines. Where differences exist, they will be specified herein.

Conventions

Underbars in signal names are used to separate words. When a word in the signal name is preceded by a lower case ‘n’, then the signal is asserted low (0 Volts).

Definition of Terms

Assert – Apply a voltage level to match its name. For example to assert Sleep_Rq means to request the module to sleep.
BPS – Bits per Second (where Kbps means 1000 bits per second, and Mbps means 1,000,000 bits per second).
OTA – Over the Air, i.e. using RF communications
Negate – Apply a signal to match the opposite of its name. For example, to negate Sleep_Rq means to request the module not to sleep, or wake up.
Serial – Data is transmitted and received one bit at a time. Contrast serial with parallel which transmits multiple bits at the same time by using multiple data lines. Note that in practice, a serial interface normally also implies an asynchronous interface as implemented by a UART. However, in the context of this document, serial means one bit at a time and is used to refer to either the asynchronous UART interface or the synchronous SPI interface.


Operation

This section specifies how SPI is implemented on the XBEE, what the SPI signals are, and how full duplex operations work.

XBee Implementation of SPI

The XBee module will operate as a SPI slave only. This means that an external master will provide the clock and will decide when to send and receive. The maximum SPI clock rate for each supported XBee product is given below:

XBee Series

Maximum SPI clock rate

S2C

5Mbps

S6

3.5Mbps

S8

3.5Mbps

Data is transmitted and received with most significant bit first using SPI mode 0. This means the Clock Polarity (CPOL) and Clock Phase(CPHA)are both 0. Mode 0 was chosen because it’s the typical default for most microcontrollers.

SPI Signals

The table below names the SPI signals and specifies their pinouts on the through-hole (S6) and surface mount (S2C and S8) modules. It also describes the operation of each pin:


Signal Name

Signal direction (XBee input or output)

Through-hole pin and applicable AT command

Surface mount pin and applicable AT command

Description

SPI_MOSI (Master Out/Slave In)

Input to XBee

Pin 11, ATD4

Pin 16, ATP6

The SPI master outputs data on this line at the SPI_Clk rate after it selects the desired slave. When the SPI slave (XBee module) is configured for SPI operations, this pin is expected to be an input.

SPI_MISO (Master In/ Slave Out)

Output from XBee

Pin 4, ATP2

Pin 17, ATP5

When SPI_nSSel is asserted (low) and SPI_Clk is active, the module outputs the data on this line at the SPI_Clk rate. When SPI_nSSel is negated (high), this output should be tri-stated such that another slave device can drive the line. However, on the S2C the tri-stating of this line is not supported, and therefore other slaves cannot be connected to the same SPI bus as the S2C. Tri-stating SPI_MISO is fully supported on the S6 and S8 modules.

SPI_nSSel (Slave Select)

Input to XBee

Pin 17, ATD3

Pin 15, ATP7

The SPI master outputs a low signal on this line to select the desired slave. When the SPI slave (XBee module) is configured for SPI operations, this pin is expected to be an input with an internal pull-up resistor. Be sure not to configure a pull-down resistor on this pin because it will prevent the SPI from working properly.

SPI_Clk (Clock)

Input to XBee

Pin 18, ATD2

Pin 14, ATP8

The SPI master outputs a clock on this pin, whose rate must not exceed the maximum supported by the XBee module. (See the table under XBee Implementation of SPI for the applicable maximum clock rate.) When the SPI slave (XBee module) is configured for SPI operations, this pin is expected to be an input.

SPI_nATTN (Attention)

Output from XBee

Pin 19, ATD1

Pin 12, ATP9

The SPI slave (XBee module) asserts this pin low whenever it has data to send to the master. After all data has been sent to the master, the SPI slave negates the pin (high). When this pin is configured for SPI operations, it is configured as an output. It is intended to signal the SPI master to assert SPI_nSSel and start SPI_Clk so that it can receive the data the XBee module has to send.

Full Duplex Operation

The official specification for SPI includes the four signals SPI_MISO, SPI_MOSI, SPi_Clk, and SPI_nSSel. Using these four signals, the SPI master cannot know when the slave needs to send and the SPI slave cannot transmit unless enabled by the master. For this reason, the SPI_nATTN signal is available in the design. This allows the SPI slave to alert the SPI master that it has data to send. In turn, the SPI master is expected to assert SPI_nSSel and start SPI_Clk, unless these signals are already asserted and active respectively. This, in turn, allows the XBee SPI slave to send data to the master.

SPI data is latched by the master and slave using the SPI_Clk signal. When data is being transferred the MISO and MOSI signals change between each clock. If data is not available then these signals will not change and will be either 0 or 1. This will result in receiving either a repetitive 0 or 0xFF. The means of determining whether or not received data is valid is by packetizing the data with API packets, without escaping. Valid data to and from the XBee is delimited by 0x7E, a length, the payload, and finally a checksum byte. Everything else in both directions should be ignored. The bytes received between frames will be either 0xff or 0x00. This allows the SPI master to scan for a 0x7E delimiter between frames.

SPI allows for valid data from the slave to begin before, at the same time, or after valid data begins from the master. The interesting case is when the master is sending data to the slave and the slave has valid data to send in the middle of receiving data from the master. This allows a true full duplex operation where data is valid in both directions for a period of time. During this time, the master and slave must simultaneously transmit valid data at the clock speed so that no invalid bytes appear within an API frame, causing the whole frame to be discarded.

An example follows to more fully illustrate the SPI interface during the time valid data is being sent in both directions. First, the master asserts SPI_nSSel and starts SPI_Clk to send a frame to the slave. Initially, the slave doesn’t have valid data to send the master. However, while it is still receiving data from the master, it has its own data to send. Therefore, it asserts SPI_ nATTN low. Seeing that SPI_nSSel is already asserted and that SPI_Clk is active, it immediately begins sending valid data, even while it is receiving valid data from the master. In this example, the master finishes its valid data before the slave does. The master will have two indications of valid data: The SPI_nAttn line is asserted and the API frame length is not yet expired. For both of these reasons, the master should keep SPI_nSSel asserted and should keep SPI_Clk toggling in order to receive the end of the frame from the slave, even though these signals were originally turned on by the master to send data. During the time that the SPI master is sending invalid data to the SPI slave, it is important no 0x7E is included in that invalid data because that would trigger the SPI slave to start receiving another valid frame.

 

Low Power Operation

In general, sleep modes work the same on SPI as they do on UART. Describing all the various sleep modes is beyond the scope of this document. However, due to the addition of SPI mode, there is an option of another sleep pin, as described below:
By default, DIO8 (Sleep_Rq) is configured as a peripheral and is used for pin sleep to awaken and to sleep the module. This applies regardless of the selected serial interface (UART or SPI).

However, if Sleep_Rq is not configured as a peripheral and SPI_nSSel is configured as a peripheral, then pin sleep is controlled by SPI_nSSel rather than by Sleep_Rq. Asserting SPI_nSSel by driving it low either awakens the module or keeps it awake. Negating SPI_nSSel by driving it high puts the module to sleep.

Using SPI_nSSel for two purposes (to control sleep and to indicate that the SPI master has selected a particular slave device) has the advantage of requiring one less physical pin connection to implement pin sleep on SPI. It has the disadvantage of putting the module to sleep whenever the SPI master negates SPI_nSSel, even if that wasn’t the intent. (When using deep sleep, wakeup time from an unintended sleep may not be acceptable.) Therefore, if the user has full control of SPI_nSSel so that it can control pin sleep, whether or not data needs to be transmitted, then sharing the pin may be a good option in order to make the Sleep_Rq pin available for another purpose. Obviously, if the module is one of multiple slaves on the SPI, then the module would sleep while the SPI master talks to the other slave, but this is probably acceptable in most cases.

If neither pin is configured as a peripheral, then the module stays awake, being unable to sleep in SM1 mode.

Configuration

The three considerations for SPI configuration are:

  1. How is the serial port selected? (I.e. Should the UART or the SPI port be used?)
  2. If the SPI port is used, what should be the format of the data in order to avoid processing invalid characters while transmitting?
  3. What SPI options need to be configured?

Serial Port Selection

To enable the UART port, DIN (DIO14) and DOUT (DIO13) must be configured as peripherals. To enable the SPI port, SPI_MISO, SPI_MOSI, SPI_nSSel, and SPI_Clk must be enabled as peripherals. If both ports are enabled then output will go to the UART until the first input. Other rules for serial port selection are platform specific as follows:

Serial Port Selection on S2C

For the S2C, in the default configuration, the UART and the SPI ports will both be configured for serial I/O. However, input and output will use the UART as long as there is no SPI input. When the first SPI input happens, all subsequent I/O will use the SPI and the UART will not be usable without resetting the module.

If the configuration is changed so that only one port is configured, then that port will be the only one enabled or used immediately after reset for both input and output.

If neither serial port is enabled, the module will not support serial operations and all communications must be done OTA.

Serial Port Selection on S8

For the S8, in the default configuration, the UART and the SPI ports will both be configured for I/O. Initially, all serial data will go out the UART. But, as soon as input occurs on either port, that port is selected as the active port and no input or output will be allowed on the other port until the next reset of the module.

If the configuration is changed so that only one port is configured, then that port will be the only one enabled or used. If the parameters are written with only one port enabled, then the port that is not enabled will not even be used temporarily after the next reset.

S8 SPI works the same as S2C SPI with two minor variations:

If both ports are disabled on reset, the UART will be used in spite of the wrong configuration so that at least one serial port will be operational for the mdoule.

Serial Port Selection on through hole XBee that supports SPI (S6)

For the S6, in the default configuration, only the UART is configured for I/O. In order to use the SPI in a module using the default configuration, the user needs to hold DOUT low at the time the module is powered up or brought out of reset. This forces a configuration override of the SPI port so that it will be used rather than the UART. If the WR command is then issued through the SPI interface, DOUT will not need to be held low on the next module reset to force operation in SPI mode.

Since the SPI_nATTN line is critical for indicating that the SPI master needs to read, it will also be enabled when the other SPI lines are forced by holding down DOUT at reset, providing it was configured as disabled, which is the default configuration. When D1 (SPI_ nATTN) is configured as a peripheral, its high to low transition indicates to the SPI master that the SPI slave is initialized and ready for data.

Thereafter, operation of S6 is like S8:

Forcing UART operation

In the rare case that a module has been configured with only the SPI enabled and no SPI master is available to access the SPI slave port, the module may be recovered to UART operation by holding DIN low at reset time. As always, DIN forces a default configuration on the UART at 9600 baud and it will bring up the module in command mode on the UART port. Appropriate commands can then be sent to the module to configure it for UART operation. If those parameters are written, then the module will come up with the UART enabled, as desired on the next reset.

Data Format

The SPI will only operate in API mode with escaping disabled (AP=1). Transparent mode will not be supported. This means that the AP configuration only applies to the UART and will be ignored while using the SPI. The reason for this operation choice is that SPI is full duplex. If data flows in one direction, it flows in the other. Since it is not always possible to have valid data flowing in both directions at the same time, the receiver must have a way to parse out the valid data and to ignore the invalid data.

Officially, the invalid data is undefined for SPI. The only requirement is that the start of frame byte (0x7E) cannot be included in the invalid data as this would cause the receiver to begin parsing a new frame. But, in reality the S2C product will send 0xff for invalid characters and the S8 and S6 products may send all ones or all zeros.

SPI Options

Most host processors with SPI hardware allow the bit order, clock phase and polarity to be set. For communication with all XBee modules the host processor must set these options as follows:

This is SPI Mode 0 and MSB first for all XBee modules.  Mode 0 means that data is sampled on the leading edge and that the leading edge rises. MSB first means that bit 7 is the first bit of a byte sent over the interface.

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