78M6610LMU Datasheet, PDF (51/63 Pages) Maxim Integrated Products – Energy Measurement Processor for Load Monitoring Units

78M6610+LMU Data Sheet
SSI Protocol Description
The SSI protocol is command response system supporting a single master and one or more targets. The
host (master) sends commands to a selected target that first verifies the integrity of the packet before
sending a reply or executing a command. Failure to decode a host packet will cause the selected target to
send a fail code. If the condition of a received packet is uncertain, no reply is sent.
Each target must have a unique SSI ID. Zero is not a valid SSI ID for a target device as it is used by the
host to de-select all target devices.
With both address pins low on the 78M6610+LMU, the SSI ID defaults to 1 and is the “Selected” device
following a reset. This configuration is intended for single target (point-to-point) systems that do not
require the use of device addressing or selecting targets.
In multipoint systems, the master will typically de-select all target devices by selecting SSI ID #0. The
master must then select the target with a valid SSI ID and get an acknowledgement from the slave before
setting the target’s register address pointer and performing read or write operations. If no target is
selected, no reply is sent. The SSB/DIR/SCL pin is asserted while the device is selected. The sequence
of operation is shown in the following diagram.
Select Target
Device
Set Register
Address Pointer
Read/Write
Commands
De-Select
Target Device
Figure 29. SSI Protocol
Master Packets
Master packets always start with the 1-byte header (0xAA) for synchronization purposes. The master then
sends the byte count of the entire packet (up to 255 byte packets) followed by the payload (up to 253
bytes) and a 1-byte modulo-256 checksum of all packet bytes for data integrity checking.
Header
(0xAA)
Byte
Count
Payload
Checksum
Figure 30. Master Packet Structure
The payload can contain either a single command or multiple commands if the target is already selected.
It can also include device addresses, register addresses, and data. All multibyte payloads are sent and
received least-significant-byte first.
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