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MCP25020I/P Datasheet(PDF) 22 Page - Microchip Technology |
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MCP25020I/P Datasheet(HTML) 22 Page - Microchip Technology |
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22 / 66 page  MCP2502X/5X DS21664D-page 22 © 2007 Microchip Technology Inc. 4.3.1 INFORMATION REQUEST MESSAGES Information Request Messages (IRM) are messages that the MCP2502X/5X receives into Receive Buffer 0 (matches Filter 0) and then responds to by transmitting a message (output message) containing the requested data. IRMs can be implemented as either a Remote Transfer Request (RTR) or a Data Frame message by configuring the MTYPE bit in the OPTREG2 register. TABLE 4-1: MESSAGE FUNCTION 4.3.1.1 RTR Message Type When RTR message types are selected (OPTREG2.MTYPE) and a node in the system wants information from the MCP2502X/5X, it has to send a remote frame on the bus. The identifier for the remote frame must be such that it will be accepted through the MCP2502X/5X’s mask/filter process (using RXF0). The RTR message type (remote frames) is the default configuration (MTYPE bit = 0). Information Request “RTR” messages must not only meet the RXMASK/RXF0 criteria but must also have the RTR bit of the CAN ID set (since the filter registers do not contain an explicit RTR bit). If a message passes the mask/filter process and the RTR bit is a ‘0’, that message will be ignored. Once the MCP2502X/5X has received a remote frame, it will determine the function to be performed based upon the three LSb’s (RXB0SIDL.SID2:SID0 for standard messages and RXB0EID0.EID2:EID0 for extended messages) of the received remote frame. Additionally, a predefined Data Length Code (DLC) must be sent to signify the number of data bytes that the MCP2502X/5X must return in it’s output message (see Table 4-2 and Table 4-3). 4.3.1.2 Data Frame Message Type When a non-RTR (or data frame) message type is selected and a node in the system wants information from the MCP2502X/5X, it sends an Information Request in the form of a data frame. The identifier for this request must be such that it will be accepted through the MCP2502X/5X’s mask/filter process (using RXF0). Information request messages in the data frame format must not only meet the RXMASK/RXF0 criteria, but must also have the RTR bit of the CAN ID cleared (since the filter registers do not contain an explicit RTR bit). If a message passes the mask/filter process and the RTR bit is a ‘1’, that message will be ignored. Once the MCP2502X/5X has received a data frame information request, it will determine the function to be performed based upon the three LSb’s (RXB0SIDL.SID2:SID0 for standard messages and RXB0EID0.EID2:EID0 for extended messages) of the received data frame. Also, Bit 3 of the received message ID must be set to a ‘1’. In addition, the data length code (DLC) must be set to a zero. Refer to Table 4-2 and Table 4-3 for more information. Regardless of the message format, all messages except the Read Register message can use either standard or extended identifiers. The Read Register message has one additional requirement; it must be an extended identifier. This is discussed in more detail in Table 4-1 and Table 4-3 for more information. Name Description Read A/D Registers Transmits a single message containing the current state of the analog and I/O registers, including the configuration Read Control Registers Transmits several control registers not included in other messages Read Configura- tion Registers Transmits the contents of many of the configuration registers Read CAN error states Transmits the error flag register and the error counts Read PWM Configuration Transmits the registers associated with the PWM modules Read User Registers 1 Transmits the values in bytes 0 - 7 of the user memory Read User Registers 2 Transmits the values in bytes 8 -15 of the user memory Read Register* Transmits a single byte containing the value in an addressed user memory register Write Register Uses a mask to write a value to an addressed register Write TX Message ID0 (TXID0) Writes the identifiers to a specified value Write TX Message ID1 (TXID1) Writes the identifiers to a specified value Write TX Message ID2 (TXID2) Writes the identifiers to a specified value Write I/O Configuration Registers Writes specified values to the three IOCON registers Write RX Mask Changes the receive mask to the specified value Write RX Filter0 Changes the specified filter to the specified value Write RX Filter1 Changes the specified filter to the specified value * The Read Register command is available when using extended message format only. Not available with standard message format. |
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