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Keywords: SSI, simple serial interface, embedded measurement, uart, source code
APPLICATION NOTE 5947
USING THE SIMPLE SERIAL INTERFACE FOR
EMBEDDED MEASUREMENT DEVICES: 78M6610+LMU,
78M6610+PSU, AND THE MAX78630+PPM
Abstract: The simple serial interface (SSI) is a binary protocol for communicating with Maxim embedded measurement devices
(78M6610+LMU, 78M6610+PSU, and MAX78630+PPM).
Overview
The simple serial interface (SSI) is a binary serial protocol implemented for UART communications on Maxim Integrated’s embedded
measurement devices (EMD). This application note addresses the features and structure of the protocol by providing a detailed
description of the available commands and corresponding source code examples. The key features of the protocol are:
Simplicity: The protocol can be implemented on host processors without absorbing excessive system resources.
Robustness: Each packet contains a header, byte count, payload, and checksum. If the header, byte count, and checksum of a
command are not all correct, the recipient can reject the packet.
Efficiency: Data is exchanged in binary encoding rather than character-coded values, and commands do not need to be echoed
to the host. Additionally, multiple commands can be sent in a single packet to further reduce the communications overhead.
Supports multidrop systems: Target device addressability supports the connection of multiple target devices on a single serial
bus.
SSI is a packet-based protocol that implements master-slave or command-response communications. Two types of packets are used
in the SSI: master packets, which are issued by a host device; and slave packets, which are issued by a slave EMD in response to a
command from the host or master.
Download the corresponding source code
.
Serial Interface Selection
Access Maxim EMDs by the master through three serial interfaces: UART, SPI, and I2C. SSI uses the UART interface and the EMD
must be configured accordingly to use SSI. This is done through pullup or pulldown resistors on the appropriate pins of the device.
Refer to the device data sheet for a description of configuring the serial interface of the device.
Device Address
The device address (SSID) of an EMD is set through the combination of the DevAddr register and one or two pins of the device
depending on the package of the device. Refer to the device data sheet for an explanation of setting the SSID of the device.
Master Packets
A master packet always contains the following:
A one-byte header, 0xAA
A one-byte packet length indicator, which is the count of all the bytes in the packet including itself
A payload, which can consist of multiple commands
A checksum, which is the two’s complement of the modulo-256 sum of all the preceding bytes in the packet, starting with the
header byte and including the last bye of the payload
From the description above, there are a few characteristics that pertain to all valid master packets:
A master packet is always 4 or more bytes long.
A master packet cannot be greater than 255 bytes long.
The field byte count is one byte.
The maximum length of a packet is limited by the slave device’s command buffer. Refer to the slave device’s data sheet for
the maximum command length for that device.
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