Implementation of SMBus Master/Slave Protocol Application Brief Challenge The System Management Bus (SMBus) interface is used by Smart Batteries to pass Smart Battery Data (SBD) to external devices and systems. Zilog Z8 Encore! MC microcontrollers provide low-cost options for use in smart battery rechargers. Given: A 7.2 V Inspired Energy Smart Battery, model ND2017A24. Implement an SMBus interface between the battery and a Zilog microcontroller. Solution Use the tools in a Z8 Encore! MC motor control microcontroller (MCU) development kit to build the Smart Battery SMBus interface. Result Easy, low-cost implementation of an SMBus interface that integrates with inexpensive Zilog microcontroller-based battery chargers. Features and Functions Our simple master/slave SMBus interface provides the following features and functions: Simple, fast construction using readily available components. Smart Battery Data (SBD) output to the development kit console port. Supports the following SMBus protocol version 1 elements: Start condition. Repeat start condition. Read. Write. ACK or Not ACK. Stop condition. Hardware Circuit Details The SMBus protocol uses two-wire interface through which different system chips can communicate with each other and with the rest of the system. It is based on the principles of operation of I2C. SMBus uses the I2C physical layer for communication, making it possible for SMBus devices and I2C devices to communicate using the same bus. Refer to the simplified block diagram shown in Figure 1. We elected to use the Z8 Encore! MC motor control microcontroller development kit to design and test our master/slave SMBus protocol implementation. We chose the commonly available 7.2 V Inspired Energy Smart Battery, model ND2017A24, to use for testing purposes. This Smart Battery is SMBus version 1.0 compliant. The on-chip I2C hardware of Z8FMC16100 MCU is used along with its GPIO to implement the SMBus time-out feature. The I2C clock line is tied to one of the GPIO lines. The logical status of the clock at this GPIO is monitored. After the transaction is started on the I2C bus (SMBus), a timer is used to count the clock LOW period and detect SMBus time-out. Though we have not implemented them in this application brief, the two optional SMBus signals SMBSUS# and SMBALERT# are shown in the Figure 1. To implement SMBSUS#, configure a GPIO as an output port. This is pulled LOW by the SMBus host (Master/Slave Z8FMC16100) as it enters
2 power-saving mode. Once the host is in power-saving mode, no transactions occur on the SMBus. By sampling this line, other SMBus hosts on the network know that the network is in suspended mode. To activate the network, a SMBus slave device can alert the host using the SMBALERT# line. To implement SMBALERT#, configure a GPIO to be interrupt-driven so that it can drive the line. The SMBus slave device can then signal the master using SMBALERT#. Note: When using the FMC16100 in SMBus applications, Vdd must always be applied to the MCU in order to fulfill the SMBus leakage current requirement. Software Implementation The FMC16100 MCU in the Z8 Encore! MC development kit is configured to use the SMBus protocol to read information from the Inspired Energy Smart Battery. The MCU collects the information and makes it available through its UART to the development kit s console port, where it can be read using the HyperTerminal application on a PC. The Smart Battery contains circuitry which provides the following information via the SMBus: Charging voltage. Charging current. Temperature data. Charge complete status. When the Smart Battery charge drops below a predetermined level, it transmits data over the SMBus at regular intervals. It sends a start sequence that is identified by the FMC16100 MCU on the development kit. Once the start sequence has been transmitted, the Smart Battery sends its address and waits for acknowledgment (ACK) from the FMC16100 MCU. When the MCU acknowledges, the Smart Battery continues its communication sequence. If the MCU fails to acknowledge, the Smart Battery ends data communication and generates a STOP condition on the SMBus. All transactions on the SMBus are controlled by the FMC16100MCU. It is sometimes necessary to generate a delayed ACK/NACK in slave mode during the address and data byte transaction. We manage this by setting the IRM bit in the I2C Mode Register. The following APIs are used to implement the SMBus protocol: Generate Start/Stop condition. Send/Receive Byte. Write/Read Byte. Slave SMBus addressing. Bill of Materials The Z8 Encore! MC Motor Control Development Kit is required to implement this application. A 7.2 V Inspired Energy Smart Battery, model ND2017A24, is also required. Summary The Z8FMC16100 MCU contains its own I2C hardware, which is also used by the SMBus protocol. Implementation is therefore simplified. We also have used the MCU s GPIO is to implement other SMBus protocol requirements. Since the FMC16100 MCU supports Master/Slave I2C operation with arbitration support, it is possible to connect multiple Master/Slaves to the SMBus network. Project Enhancements A Power Management Bus (PMBus) protocol can also be implemented on the same SMBus physical layer. The FMC16100 MCU can therefore be used as a PMBus controller to configure, control and monitor other devices such as PWM controllers, DC/DC controllers, AC/DC converters, and so on.
3 Simplified Block Diagram Figure 1. Simplified Block Diagram of SMBus Master/Slave Interface Implementation
4 Applications and Support Tools The Z8 Encore! MC motor control microcontrollers are supported by the Opto-Isolated USB Smart Cable. The microcontroller is also supported by the ZDS II Z8 Encore! MC integrated development environment (IDE) with ANSI C-Compiler, available on www.zilog.com. Related Documentation More information about the Z8 Encore! MC FMC16100 described in this application brief is available in the following documents: Z8FMC16100 Series Flash Motor Control MCU Product Brief, PB0166 Z8 Encore! Motor Control Flash MCUs Z8FMC16100 Series Product Specification, PS0246 ez8 CPU User Manual, UM0128
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