AN2010 - USBid
Contents
1. value Ox7ecO 0111111011000000 mask Oxlc80 0001110010000000 List of unwanted messages that will be accepted by the current filter patter Message CAN I Ox257 List of wan Message CAN I 0x392 0x245 0x256 0x376 reS D Accepted by filter 1 ted messages accepted by different filters D Accepted by filter 0 2 1 3 You MUST place a space between the option switch and its value e g im 1 NOT im1 You can increase the number of iterations by changing the t 100 parameter AN2010 MOTOROLA Software Example 2 8 Software Example 2 AN2010 MOTOROLA The information for a network may not be stored in a convenient textual format as shown above In particular the wanted and the network messages may be stored in a single list in some other standard application This example explains how to extract the information required from these lists and configure the Filter Generator to read the new lists correctly Suppose the information is stored in Microsoft Word format as a table Message Name Message Id Source node Receiving nodes Temperature 1553458A Sensori Dashboard HVAC Temperature 1553558A Sensor2 Dashboard Temperature1 1555658A Sensor3 HVAC DriverTemp 12694002 ControlPanel HVAC PassengerTemp 12694003 ControlPanel HVAC WindowPosition 17334467 Door1 HVAC DoorOpen 17334465 Door Dashboard WindowPosition 17334477 Door2 HVAC DoorOpen 17334475 Door2 Da2. on the network then the microcontroller CPU would spend time analysing messages only to discover that many were not of interest to that particular node In networks where the node was interested only in a small subset of the A MOTOROLA Motorola Inc 2000 AN2010 Application Note total messages available then a significant amount of processing time could be lost performing worthless evaluations To allow the processor to avoid doing this Motorola provides filters on the msCAN module that will prevent certain unwanted messages from reaching the input buffers and thus requiring the CPU s attention The user must configure these filters to best suit the message traffic on the network to which the node belongs Although this is not a trivial task it is performed at design time and the extra effort spent in the design phase will translate to improved runtime CPU utilisation 3 How filters work In the simplest case filters work by applying a logical comparison to individual bits in the CAN message identifier For each filtered bit the comparison can be either Test if bit is logic 1 or Test if bit is logic 0 or Ignore value of bit Consider the following example Suppose that the filter examines the eleven bits of a standard CAN message identifier For each bit the filter will check if the Id contains a logic 1 represented by 1 a logic 0 represented by 0 or either logic 1 or log
3. which Ids are in extended format the data field immediately following the Id must be a 1 If the data is a standard Id the data field must contain 0 Extended and standard messages Ids can both be used in the same file If the information is stored in Microsoft Excel format then follow the above sequence except that steps 5 and 6 can be executed as a single step using the save as comma delimited text CSV 9 Software Example 3 AN2010 MOTOROLA In the examples so far the filter configuration has been left to the Filter Generator tool In these cases the tool itself will decide on the optimum configuration of filter size However the user can define the exact filter configuration to use by specifying the number of filters and their size This may be a useful feature to take advantage of where there is a visible pattern obvious to the user or perhaps where a minor change is made to the network and the filter configuration should remain as before The nf and s options are used to define the configurations From the earlier description of the filter options on the 68HC08 and 68HC12 families the possibilities available to configure the tool are Family Option Configuration 68HC08 nf4 s8 4 x 8 bit filters 68HC08 nf2 s16 2 x 16 bit filters 68HC08 nf1 s32 1 x 32 bit filters 68HC12 nf8 s8 8 x 8 bit filters 68HC12 nf4 s16 4 x 16 bit filters 68HC12 nf2 s32 2 x 32 bit filters 11 Application N
4. Order this document by AN2010 D Motorola Semiconductor Application Note AN2010 Using The Motorola msCAN Filter Configuration Tool by Steven McAslan Body Electronics Software Systems East Kilbride Scotland 1 Introduction The Controller Area Network CAN communication standard is widely used in automotive and industrial applications to signal between nodes on a network One of the reasons for its success in such areas is the availability of CAN modules integrated onto microcontrollers intended for embedded applications The Motorola msCAN is one such module that is available on both the 8 bit M68HC08 family of devices and the 16 bit M68HC12 family As well as providing a robust implementation of the CAN standard the msCAN is able to reduce the overhead on the microcontroller CPU by filtering out unwanted CAN messages before they reach its input buffers This application note describes the use of a software tool that provides automated configuration for the msCAN filter based on the messages that are present on the CAN network Used in conjunction with the Filter Generator s user s manual it provides practical examples and tips on how to best use the tool 2 The benefits of filters The CAN standard allows for a very large number of nodes on a single network However each individual node will typically only require to receive and transmit a subset of the messages on the network If the msCAN were to receive every message present
5. d is used with a 16 bit filter size The 11 bit Id itself the extended Id indicator IDE bit and the RTR bit are all handled as required leaving three bits in the 16 bit filter undefined If these lower 3 mask bits are set to 0 then the filter comparison will continue beyond the Id section of the message into the next sections of the message that may be DLC data etc To avoid this problem the least significant three bits of the 16 bit mask register CIDMR1 3 must be set to 1 to force the filter to ignore message bits that are not part of the Id The Motorola msCAN Filter Generator provides a powerful and flexible way of optimising the filter configuration on an mSCAN module This in turn can greatly reduce the CPU overhead for handling CAN messages not intended for a particular node 13 Software Availability AN2010 MOTOROLA To receive further information about the msCAN Filter Generator tool and to obtain the software itself send an email to software systems helpline sps mot com with msCAN Filter Generator in the subject line 15 Motorola reserves the right to make changes without further notice to any products herein Motorola makes no warranty representation or guarantee regarding the suitability of its products for any particular purpose nor does Motorola assume any liability arising out of the application or use of any product or circuit and specifically disclaims any and all liability including without limitat
6. e wanted message list is wanted txt and the network list is network txt Each file contains standard CAN 11 bit message identifiers The content of each file is shown below File c messages wanted txt 0x245 0x256 0x392 0x376 File c messages network txt This must contain all the messages in wanted txt too 0x067 0x068 0x069 0x123 0x245 0x256 0x257 0x376 0x392 0x393 Execute the filter generator software by typing the following command in a DOS window on a single line mscanfg nf 1 w c messages wanted txt m c messages network txt iw 1 im 1 t 100 d The filter generator will configure a single 32 bit filter to best match the data 100 times and then produce a detailed output description in the DOS window Note that some warning messages will also be created when using the above operating mode these can be ignored at this stage The filter generator will display the calculated filter values in the DOS window For this example the filter generator produces the following output Application Note It took 100 loops to find the optimal filter MSCAN Filter List generated by the MSCAN Filter Generator NOTE Het Number of filters 2 Filter size 16 Number of wanted messages 4 Number of unwanted messages 6 Number of unwanted but accepted messages 1 Rejection ratio 83 Filter values Filter 0 value 0x4ae0 0100101011100000 mask 0x0260 0000001001100000 Filter 1
7. emp 0x12694003 WindowPosition 0x17334467 WindowPosition 0x17334477 Dashboard HVAC HVAC HVAC HVAC 1 Sensorl 1 Sensor3 ControlPanel 1 ControlPanel 1 Doorl HVAC 1 Door2 HVAC 9 Run the filter generator using the following command mscanfg nf 2 w c messages wanted2 txt m c messages network2 txt iw 2 im 2 t 1000 d p T For this example the filter generator produces the following output It took 1000 loops to find the optimal filter MSCAN Filter List generated by the MSCAN Filter Generator tt Number of filters 2 Filter size 32 Number of wanted messages 6 Number of unwanted messages 3 Number of unwanted but accepted messages 0 Rejection ratio 100 Filter values Filter 0 value Oxbbde88ee 10111011110111101000100011101110 mask Ox2ac408ea 00101010110001000000100011101010 Filter 1 value Oxaabecb14 10101010101111101100101100010100 mask 0x00244000 00000000001001000100000000000000 List of unwanted messages that will be accepted by the current filter pattern Message CAN ID Accepted by filter AN2010 10 MOTOROLA NOTE Software Example 3 List of wanted messages accepted by different filters Message CAN ID Accepted by filter 0x17334467 0 0x17334477 0 0x12694003 2 0x1555658a Hl The e option tells the Filter Generator that the file contains message Ids that are in extended format To tell the Filter Generator
8. ic 0 represented by x In this simple example there are only three possible CAN messages The filter will accept two of the messages and reject one Filter value 0001x1001x0 Message 1 00011100110 Message 2 00110100110 Message 3 00010100100 In this case the filter will reject Message 2 since its third most significant bit is not 0 001 rather than 000 In effect the filter is simply a convenient way of defining the set of messages that the CPU must receive In the above example there are two bits whose value is ignored and 9 whose value is fixed This gives a set of four identifiers that the filter will pass 00010100110 00010110110 00011100110 00011110110 AN2010 2 MOTOROLA The msCAN filters 4 The msCAN filters AN2010 MOTOROLA The msCAN provides the features described above and adds further flexibility by allowing the user to configure the filter in a number of ways For the M68HC08 family the filter may act as a single 32 bit value that operates across the full 29 bits of an extended CAN message identifier In this case the filter identifies two sets of messages one set that it receives and one set that it rejects Alternatively the filter may be split into two This allows the msCAN to examine only the first 16 bits of a message identifier but allows two separate filters to perform the checking In our simple example this would allow Message 2 to be accepted by the extra filter now ava
9. ilable Filter value A 0001x1001x0 Filter value B 00x101x01x0 Message 1 00011100110 Message 2 00110100110 Message 3 00010100100 Now the msCAN will accept all three messages Filter A will accept Messages 1 and 3 as before and filter B will accept Message 2 For the configuration that uses two filters there are four sets of messages accepted by Filter A rejected by Filter A accepted by Filter B rejected by Filter B In practice it is unimportant which filter accepts the message messages accepted by either will be placed in the input buffer This means that a message may be accepted by more than one filter In our example both Filter A and Filter B accept Message 3 The filter may be further split into four 8 bit filters that examine only the first 8 bits of a message identifier In this case the filter can accept or reject complex sets of message identifiers only however where the variation is in the first eight bits of the identifier For the M68HCO08 family the possible filter combinations are 1 x 32 bit filters 2 x 16 bit filters 1 x 16 bit filter 2 x 8 bit filters 4 x 8 bit filters In the case of the M68HC12 msCAN module there is a second 32 bit filter that can be configured in the same way as the first filter In other Application Note words for the M68HC12 family the msCAN has the following combinations 2 x 32 bit filters 1 x 32 bit filter 2 x 16 bit filters 1 x 32 bit filter 4 x 8 bit filters 2
10. ion consequential or incidental damages Typical parameters which may be provided in Motorola data sheets and or specifications can and do vary in different applications and actual performance may vary over time All operating parameters including Typicals must be validated for each customer application by customer s technical experts Motorola does not convey any license under its patent rights nor the rights of others Motorola products are not designed intended or authorized for use as components in systems intended for surgical implant into the body or other applications intended to support or sustain life or for any other application in which the failure of the Motorola product could create a situation where personal injury or death may occur Should Buyer purchase or use Motorola products for any such unintended or unauthorized application Buyer shall indemnify and hold Motorola and its officers employees subsidiaries affiliates and distributors harmless against all claims costs damages and expenses and reasonable attorney fees arising out of directly or indirectly any claim of personal injury or death associated with such unintended or unauthorized use even if such claim alleges that Motorola was negligent regarding the design or manufacture of the part Motorola and Ad are registered trademarks of Motorola Inc Motorola Inc is an Equal Opportunity Affirmative Action Employer How to reach us USA EUROPE Motorola Litera
11. ote To demonstrate the effect of forcing filters to certain sizes Software Example 2 can be re run with the filter size forced to 16 mscanfg nf 4 s 16 w c messages wanted2 txt m c messages network2 txt iw 2 im 2 t 1000 d p e This produces a result that is not so efficient It took 1000 loops to find the optimal filter MSCAN Filter List generated by the MSCAN Filter Generator tt Number of filters 4 Filter size 16 Number of wanted messages 6 Number of unwanted messages 3 Number of unwanted but accepted messages 2 Rejection ratio 33 Filter values Filter 0 value Oxb99e 1011100110011110 mask 0x0000 0000000000000000 Filter 1 value Oxaaba 1010101010111010 mask 0x0000 0000000000000000 Filter 2 value Oxaa9e 1010101010011110 mask 0x0000 0000000000000000 Filter 3 value 0x935a 1001001101011010 mask 0x0000 0000000000000000 List of unwanted messages that will be accepted by the current filter pattern Message CAN ID Accepted by filter 0x1553558a 2 0x17334475 0 List of wanted messages accepted by different filters Message CAN ID Accepted by filter 0x17334467 0 0x1555658a 0x17334477 0x12694003 0x12694002 0x1553458a NO WWOF AN2010 12 MOTOROLA Software Example 4 10 Software Example 4 AN2010 MOTOROLA This example examines more advanced features of the Filter Generator In particular it looks at the options for genera
12. shboard The table contains all of the information for messages in a very simple network This example examines the configuration of the filter generator for the HVAC node The CAN message Ids are in extended format and these require an additional piece of information to be read correctly There are 9 steps to follow 1 Copy the table to another document 2 Insert an additional column in the table after the Id column to indicate extended Ids used place 1 in each row 3 Delete the header row or comment it by placing a character in front of it 4 Add the hexadecimal delimiter in front of each Message Id 5 Convert the table to text format separated by commas Application Note 6 Save the file in text format as network2 txt Te Te Te mperaturel mperaturel mperaturel 0x1553458A 0x1553558A 0x1555658A 1 1 1 Se Se Se nsorl nsor2 nsor3 Dashboard HVAC Dashboard HVAC 0x12694002 1 PassengerTemp 0x12694003 1 ControlPanel WindowPosition 0x17334467 1 Doorl HVAC DoorOpen 0x7334465 1 Doorl Dashboard WindowPosition 0x17334477 1 Door2 HVAC DoorOpen 0x17334475 1 Door2 Dashboard HVAC HVAC DriverTemp ControlPanel 7 Delete lines in the table that do not have the HVAC as a receiver 8 Save the file in text format as wanted2 txt Temperaturel 0x1553458A Temperaturel 0x1555658A DriverTemp 0x12694002 1 PassengerT
13. ssage CAN ID Accepted by filter List of wanted messages accepted by different filters Message CAN ID Accepted by filter 0x1553458a 0 0x12694002 2 0x17334477 1 0x17334467 C function to initialize MSCAN filters void msCANFiltersInit void Be sure that SFTRES bit is set CIDAC 0x0 unsigned long unsigned long unsigned long CIDAR4 unsigned long CIDARO Oxbbde88ee CIDMRO Ox2ac408ea Oxaabecb14 CIDMR4 0x00244000 return 14 AN2010 MOTOROLA Other Considerations 11 Other Considerations 12 Conclusion Although the Filter Generator tool has a wide range of options to examine message data it does so in a static sense In other words it takes no account of the number of times each message is broadcast on the network This means that although a filter may appear to be highly successful by accepting only a single message out of many rejected if this accepted message is the most common on the network the CPU overhead in dealing with it may still be large In this case it may be more effective to modify the filter generator configuration options to force this regular message to be rejected and to accept others that are less often transmitted Due to the hardware implementation of the msCAN filter itself some standard Ids may be inadvertently rejected because the filter does a pattern match on more than the message Id itself This can occur when a standard I
14. ting a C language configuration file using an alternative algorithm and other minor details The filter generator can produce a C language function that will configure the filter registers of the msCAN module use the f option The C file produced contains an ANSI C function with the following prototype void msCANFiltersInit void The Filter Generator includes an alternative algorithm for selecting a suitable filter configuration This second option is faster than the main choice but may require more iterations to achieve the same result as the main choice use the da option A typical use may be as a first pass on a new network configuration to determine the complexity of the filter selection task The CAN message Id includes an RTR bit that indicates a Remote Transmission Request Like the Extended CAN ld field this data is determined for each individual Message Id and is placed as a separate field after the extended field in the input files If RTR is present then the filter will operate on this bit also In the examples so far the Filter Generator tool has displayed its results in the DOS window This is helpful when examining different options and making judgements on the use of the tool However where the output will be used to configure an msCAN module a text file is a much more useful outcome Use the o option to specify the name of the output file Finally the Filter Generator can produce a detailed map of the opera
15. tion of the selected filter configuration for the wanted messages use d option This allows the user to see which filters are accept most messages and this in turn may be used to optimise the specification of Message Ids used within the CAN network All of our examples shown here use the d option For the Software Example 2 a C language output file called filter c can be specified In addition a detailed description of the performance of the selected configuration can be produced by executing the following command mscanfg nf 2 w c messages wanted2 txt m c messages network2 txt iw 2 im 2 t 500 d p e o c messages filter c 13 Application Note This produces the following output message It took 500 loops to find the optimal filter Filter information written to c messages filter c and detailed output and C function MSCAN Filter List generated by the MSCAN Filter Generator tt Number of filters 2 Filter size 32 Number of wanted messages 6 Number of unwanted messages 3 Number of unwanted but accepted messages 0 Rejection ratio 100 Filter values Filter Filter Oss Ts value Oxbbde88ee 10111011110111101000100011101110 mask OQOx2ac408ea 00101010110001000000100011101010 value Oxaabecb14 10101010101111101100101100010100 mask 0x00244000 00000000001001000100000000000000 List of unwanted messages that will be accepted by the current filter pattern Me
16. ture Distribution PO Box 5405 Denver Colorado 80217 1 303 675 2140 Mfax RUFAX0 email sps mot com TOUCHTONE 1 602 244 6609 http sps motorola com mfax US amp CANADA ONLY hitp sps motorola com mfax HOME PAGE hitp motorola com sps JAPAN Motorola Japan Ltd SPS Technial Information Center 3 20 1 Minami Azabu Minato ku Tokyo 106 8573 Japan 81 3 3440 3569 ASIA PACIFIC Motorola Semiconductors H K Ltd Silicon Harbour Centre 2 Dai King Street Tai Po Industrial Estate Tai Po N T Hong Kong 852 266668334 CUSTOMER FOCUS CENTER 1 800 521 6274 Mfax is a trademark of Motorola Inc Motorola Inc 2000 AA MOTOROLA AN2010 D
17. wo lists The first list contains all of the message identifiers that the node wishes to accept the second list contains all of the message identifiers used in the network The software will examine the accepted message list and propose a filter configuration that will accept all of these messages Next the software will compare the network list against the filter and determine which messages will be accepted and which will be rejected Finally the software will produce a listing of its results either to the screen or to a text file Particularly in the case where there are a large number of messages it may not be possible to select the best filter configuration on the first pass For this reason the software can automatically repeat the process up to 10 000 times and select the best configuration that it detected For many cases this simple mode will be sufficient to produce an effective filter configuration so before examining further options for the software consider the preparation of the message lists for use by the filter generator The software always reads the message lists from two text files The exact format of these files again is configurable but in the simplest case they consist of lists of hexadecimal numbers Consider the following example AN2010 6 MOTOROLA Software Example 1 7 Software Example 1 AN2010 MOTOROLA In this case the message list files are contained in a folder named messages on the C drive Th
18. x 16 bit filter 4 x 8 bit filters 4 x 16 bit filters 8 x 8 bit filters In practice the msCAN filter uses three sets of registers to provide the filter configuration Firstly the CIDAC register determines the configuration of the banks into filter sizes and number of filters Secondly registers CIDMRO 1 2 3 determine those bits on which the filter will operate by placing a 0 at the appropriate bit position in the filter register Finally registers CIDARO 1 2 3 determine the value of those bits determined by CIDMR0 1 2 3 For instance in the case of our simple example with the filter value of 0001x1001x0 The CIDMRO 1 2 3 register would be configured as 00001000010 and so all message identifier bits except bit 1 and bit 6 would be compared against the CIDARO 1 2 3 registers These would be configured as 00010100100 In this case bits 1 and 6 are set to 0 but since they are ignored it is equally valid to set them to 1 AN2010 4 MOTOROLA The Configuration Problem 5 The Configuration Problem AN2010 MOTOROLA This powerful flexibility brings with it a problem of its own Namely how to select the best configuration of filter size number AND filter values to most efficiently reject those messages that the node wishes to ignore Note that for any configuration it is essential that a required message is never rejected and in many cases this may mean that the filter has to accept messages that are not used b
19. y the node In this situation the microcontroller CPU must still perform acceptance testing on each message accepted by the filter although it would have to examine fewer messages than would be the case without a filter Even in systems with only moderate complexity this filter configuration may be extremely complex and time consuming for a node programmer to select In addition it may not be apparent without much further work to confirm whether a selected combination is the most efficient at reducing CPU overhead in message acceptance For the above reasons Motorola has developed an msCAN Filter Generator software tool The objective of the tool is to automate the selection of filter values for the msCAN module The tool can search through many combinations of filter values select the most efficient that it can detect and provide the values to the user in a form that can be readily used The rest of this application note provides examples to illustrate typical uses of the tool Note that some less significant details of using this tool are not discussed here but are fully described in the software user s manual Application Note 6 Using the Filter Generator The user can configure the filter generator software to operate in a variety of different modes however Software Example 1 begins by examining the basic operation The filter generator software creates the filter configuration by examining message identifiers contained in t
Download Pdf Manuals
Related Search