AN1283 Microchip Wireless MiWi™ Media Access Controller – MiMAC Authors: Yifeng Yang • The learning curve for MiMAC can be flattened Pradeep Shamanna and applied to all Microchip transceivers across Derrick Lattibeaudiere different frequency bands and modulations. It Vivek Anchalia significantly reduces the development risk for Microchip Technology Inc. wireless application developers by providing end users the capability of changing different trans- INTRODUCTION ceivers at any stage of software development. Choosing a transceiver in the firmware is a The primary function of wireless communication protocol process that is transparent to the application is to transmit or receive information, or both, between developers by modifying the configuration param- two nodes. The Media Access Controller (MAC) layer eters made available by MiApp. For more infor- provides the basic channel access, addressing and data mation on MiApp, refer to the Application Note transmission/receiving functionalities, on top of the “AN1284 Microchip Wireless MiWi™ Application Physical (PHY) layer that handles raw data. In the Programming Interface – MiApp” (DS00001284). standard Open Systems Interconnection (OSI) model, it serves as the Data Link Layer (DLL). Due to the wide MiMAC FEATURES variety of possible implementations in the PHY layer, the The MiMAC implements the following features: MAC is the lowest possible layer to standardize in the • Easy to learn, implement, and support software for communication protocols. • Flexible enough to be implemented on This application note defines the Microchip MAC layer, microcontrollers (MCUs) and RF transceivers MiMAC, for communication protocols and transceivers from Microchip supported by Microchip for short range, low-data rate • Powerful enough to address most short range, and low-power wireless applications. low-data rate applications Implementing MiMAC benefits wireless application • Simple but strong, security module with its developers in multiple ways: Security modes for transceivers that do not have • Traditionally, wireless communication protocol a hardware security engine stacks are complicated to implement and difficult • Concise but powerful, programming interface to use. With the new definition of MiMAC, it is between MiMAC and all Microchip proprietary possible to make the protocol stack available for wireless communication protocols widely different RF transceivers. • Minimum impact to the firmware footprint FIGURE 1: BLOCK DIAGRAM OF MICROCHIP WIRELESS MiWi™ STACK ENVIRONMENT Application User A ppl ic ati on Configuration MiApp MiWi™ P2P\Star MiWi™ Mesh Protocol Configuration Future Microchip Proprietary Wireless Protocols ... Interchangeable Wireless Communication Protocols MiMAC MRF24J40 Transceiver RF Transceiver MRF89XA Transceiver Configuration Future Microchip RF Transceivers ... Interchangeable RF Transceivers 2009-2017 Microchip Technology Inc. DS00001283B-page 1 AN1283 Microchip Application Programming MiMAC OVERVIEW Interface (MiApp) The MiMAC layer consists of three major components. In addition to standardizing the MiMAC layer, Microchip The first and second components are defined for Micro- also aims to standardize the interfaces in the application chip proprietary RF transceivers that have limited hard- layer. The standard interface in the application layer is ware support in the MAC layer. The third component is called Microchip Wireless Application Programming defined for all Microchip RF transceivers. The three Interface (API) or MiApp. The definition of MiApp components are as follows: enables all Microchip proprietary wireless protocols to 1. MiMAC Frame Format be interchangeable with little or no change in the software application code. The frame format defines how the packet appears over-the-air. Basically, the MiMAC MiMAC standardizes the interfaces between the Micro- frame format determines the capability and chip wireless protocols and Microchip RF transceivers. efficiency of the MiMAC specification. It serves MiMAC makes all Microchip RF transceivers inter- as the foundation for the other two parts in the changeable with little or no change in the software MiMAC architecture. application code. 2. MiMAC Security Module Both MiMAC and MiApp enable wireless application developers the maximum flexibility to choose the RF For all wireless communication, the message is transceivers and wireless communication protocols at transmitted through the open air. It is relatively any stage of software development, thus reducing easier to intercept information from wireless com- development risk to the minimum. munication than from wired communication. Therefore, security may be a serious consider- ation for many applications. The MiMAC security Microchip Wireless Configurations module defines a low-cost block cipher with There are three layers of configurations for application strong security strength. The MiMAC security protocol stacks and RF transceivers: module also defines multiple Security modes to work with the block cipher for different • Application Configurations – this may change requirements from the applications. between devices in the same application according to their hardware design, role in the 3. MiMAC Universal Programming Interface application or network, or both. Wireless applica- The MiMAC universal programming interface tion developers tend to do the majority of the serves as a driver between all Microchip RF configurations in the application layer. transceivers and Microchip proprietary wireless • Protocol Stack Configurations – this can fine- communication protocols. The programming tune the behavior of the protocol stack. The interface enables the Microchip RF transceivers majority of the configurations in the stack level to work under any Microchip proprietary wireless aims to set the timing of the stack, specify the protocol; and also enables all Microchip propri- routing mechanism, and so on. etary wireless communication protocols to use • Transceiver Configurations – this defines the the Microchip RF transceivers. frequency band, data rate, and other RF related The transceivers supported by Microchip differ widely features of the RF transceiver. in features. Some transceivers have a well-defined Note: Any RF radio transceivers of sub-GHz and hardware MAC layer, including frame format or security 2.4 GHz bands can be used. However, the engine, or both. There may be hardware features that application note refers to the Microchip are built into the transceivers to comply with the speci- MRF89XA (sub-GHz) and MRF24J40 (2.4 fication. Microchip MRF24J40 is a good example of GHz) tranceivers. such transceiver as it complies with the IEEE 802.15.4™ specification. MiMAC does not intend to The default settings for both protocol stack and regulate the frame format or security engine, or both, if transceiver configurations works fine with the applica- already implemented in the transceiver hardware, as tion without any modification. The application configu- prior experiences demonstrate that the hardware fea- rations, however, tend to be changed to fit the needs of ture is often faster and consumes less system different wireless applications. Figure 1 demonstrates resources. the Microchip Wireless MiWi™ stack environment. DS00001283B-page 2 2009-2017 Microchip Technology Inc. AN1283 For those transceivers that have a built-in hardware MiMAC FRAME FORMAT support in the frame format or security engine, or both, it is recommended to use the hardware implementation The MiMAC frame format definition ensures that the on the transceiver and the MiMAC programming inter- application developer can easily learn and support face. various wireless applications. The universal packet for- mat also simplifies the sniffer implementation. It is For other proprietary RF transceivers, there is very possible to implement only one sniffer software running limited or virtually no MAC layer defined in the hard- on the PC while using different hardware transceivers ware. For these types of transceivers, all three major to sniff and send packets to the PC for interpretation. parts of the MiMAC specification are recommended. Since all packets have the same format in the MiMAC Microchip MRF89XA sub-GHz transceiver does not frame format definition, the interpolation in the MiMAC comply with the IEEE 802.15.4 specification. However, layer is the same across all RF transceivers from with a powerful MiMAC definition in the software, Microchip. Microchip enables those simple RF transceivers virtu- ally the same communication or networking capability The criteria to evaluate the frame format are based in the software as their siblings with much more from its capability and its efficiency. Typically, the MiWi complexity in silicon. protocol specification uses the same command code as defined by the IEEE802.15.4, MAC frame format. Each of the three major parts in the MiMAC specifica- Figure 2 shows the MAC frame format for the tion are described in the subsequent sections. IEEE802.15.4 defined protocols. Compared to IEEE 802.15.4, the industrial standard for short range, low-data rate and low-power wireless PAN, the MiMAC frame format provides essentially the same capability with more efficiency. Hence, a typical minimum IEEE 802.15.4 frame is 9 bytes in the MAC header, whereas MiMAC unicast can be as short as 2 bytes. Figure 3 shows the details of the MiMAC frame format. FIGURE 2: IEEE802.15.4 MAC FRAME FORMAT Frame Sequence Destination Destination Source Source NAME Payload Control Number