Demystifying MIPI SLIMbus

SLIMbus is a multi-drop, time division multiplexed serial bus. It has one clock and one data line, with CMOS signalling and no analog PHY. It is targeted for low bandwidth connectivity between the AP/modem and audio/Bluetooth/haptic. SLIMbus was originally specified by the MIPI Alliance in 2007. Arasan's total IP solution delivery demystifies the adoption of SLIMbus.

According to Ajay Jain, director, Mobile Connectivity Products, Arasan Chip Systems, the SLIMBus system overview includes a host component (e.g., apps processor), a device component (e.g., a broadband modem), and a SLIMbus device component (e.g., audio processor, Bluetooth modem). The logical implementation of SLIMbus system feature is realized through devices within SLIMbus IPs.

The AP/modem have software infrastructure and an active manager device that manages the SLIMbus. Any component can have a framer device activated to drive the SLIMbus CLK. Each component can have one or more generic devices to buffer and transmit/receive audio and other data.

The physical layer enables TDM. The data line NRZI is encoded. The active framer can drive clock gears 1 to 10 for power management. There is an interleaving of control and data on the SLIMbus frames.

As far as device evaluation and enumeration are concerned, each component initializes its devices in correct order under the direction of the interface device. The active framer drives the SLIMbus CLK and framing channels with default values. All components perform frames, superframes and message synchronization. All active devices report presence and characteristics with broadcast messages. Arasan provides the software stack to perform SLIMbus.

The SLIMbus allows a finite set of channel rate multipliers (data segments/superframes). If SLIMbus CLK frequency, it allows channel rate multiplier of audio data rate. Other transfer protocols may be preferred in certain cases, e.g., flow control required, pushed or pulled protocol. All transfer protocols are programmable through the Arasan software stack.

Each port-port connection needs to be mapped onto a SLIMbus data channel. There is two-channel audio on SLIMbus data channels 6 and 7. A subframe length of 32 slots is assumed. The SLIMbus is amazing, yet complex. There are a finite set of parameters. Arasan's IPs have addressed the low-level complexities of implementation.