AN-891 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com ADuC703x Series LIN Baud Rate Calculations by Aude Richard INTRODUCTION LIN FRAME HEADER The purpose of this application note is to familiarize the user with calculating the divisor values, COMDIV0, COMDIV1 and COMDIV2, for use with UART on the ADuC703x series devices from Analog Devices, Inc. This application note assumes that the user is familiar with Local Interconnect Network (LIN) 2.0 specifications. A standard LIN communication frame is shown in Figure 1. It is broken down into a break symbol, a Sync byte, the protected identifier, data and a checksum. This document is divided into three sections: • The Sync byte is shown in more detail in Figure 2. The Sync byte is 0xAA transmitted at the master’s desired baud rate. The standard method for determining the master’s bit rate is to measure the time from the first falling edge to the fifth falling edge. This value is divided by eight, which gives the desired bit rate. This value is then used to calculate the UART divisor values. The calculations are described in more detail in subsequent sections. This application note assumes that the user has set up the LHS MMR to time the full eight bits of the Sync byte (LHSCON1 = 0x62). FRAME SLOT FRAME INTERFRAME SPACE RESPONSE SPACE HEADER BREAK SYNC RESPONSE PROTECTED IDENTIFIER DATA 1 DATA 2 DATA N 06488-001 • LIN Frame Header. This section explains the LIN Frame header and the Sync byte. LIN Baud Rate Calculations. This section explains one approach to calculating the UART divisor values using the LIN Hardware Synchronization (LHS) functionality. LIN Baud Rate Calculations Example C Code. This section provides an example of C code implementation of the calculations explained in the LIN Baud Rate Calculations section. The break symbol signifies the start of the LIN packet. The Sync byte calibrates the slave’s baud rate. The protected identifier identifies the slave. The checksum is either a classic checksum, calculated over the data transmitted, or the extended checksum, which is calculated over the protected identifier and data. CHECKSUM Figure 1. LIN Frame START BIT STOP BIT Figure 2. LIN Sync Byte Rev. 0 | Page 1 of 4 06488-002 • • • • • AN-891 LIN BAUD RATE CALCULATIONS Using the LHS system, the user gets a value in LHSVAL0 after the Sync byte is received. The LHSVAL0 contains the equivalent of 8 TBIT. This value is used to generate the values for the UART dividers COMDIV0, COMDIV1, and the fractional divider COMDIV2. For more information on the UART refer to the relevant ADuC703x data sheet. To calculate the COMDIV0/COMDIV1 values using the standard baud rate generator, the following basic UART equation is used: DL = where M and N are COMDIV2 values. M+ Replacing Baud Rate, M+ DL is the value of COMDIV0 and COMDIV1. CD is the clock divider. N LHSVAL0 = 2048 DL × 2CD × 2 × 16 × 2 M+ N LHSVAL0 = 2048 DL × 2CD + 6 To reduce the need for complex math used in the calculation of the fractional divider, limit the values of DL (COMDIV0/COMDIV1) to the power of two. For example, if DL = 17, use DL = 16 = 24 in the calculation of N. This automatically adjusts the value of N to compensate for the error introduced by modifying DL. In terms of LHSVAL0, the desired baud rate is as follows: 5.12 MHz × 8 LHSVAL0 DL = 2 DL _ Power (LHSVAL0 is clocked from an internal 5.12 MHz clock and it is assumed that LHSCON1 is configured to measure 8 TBIT.) If the standard baud rate equation and the desired baud rate equation are combined, 20.48 MHz × LHSVAL0 N LHSVAL0 = 2048 2DL _ Power × 2CD + 6 M+ N LHSVAL0 = DL _ Power + CD + 6 2048 2 N= 211 × LHSVAL0 − 2048 2DL _ Power + CD + 6 N = 25 − DL _ Power − CD × LHSVAL0 − 2048 LHSVAL0 2CD + 6 Using only the standard baud rate generator equation gives the required values of COMDIV0/COMDIV1. For increased accuracy, use the ADuC703x fractional divider with the DL value (COMDIV0/COMDIV1) calculated previously for the standard baud rate generator. The equation using the fractional divider is as follows: Baud Rate = M+ If M is set equal to 1, 5.12 MHz × 2CD × 16 × 2 × 8 LHSVAL0 DL = CD 2 × 16 × 2 × 2 DL = M+ Baud Rate × 2 CD × 16 × 2 where: DL = 20.48 MHz × LHSVAL0 N = 2048 5.12 MHz × 8 × DL × 2CD × 16 × 2 This simplifies to 20.48 MHz Desired Baud Rate = 20.48 MHz N = 2048 Baud Rate × DL × 2CD × 16 × 2 For example, for a baud rate of 19,200 bps, with CD = 0, DL = 33 and LHSVAL0 = 2133, then N = 21 and the baud rate is 19,197 bps. If DL = 32 and N = 85 is used, the baud rate is then 19,203 bps. 20.48 MHz N ⎞ DL × 2CD × 16 × 2 × ⎛⎜ M + ⎟ 2048 ⎠ ⎝ Rev. 0 | Page 2 of 4 AN-891 LIN BAUD RATE CALCULATIONS EXAMPLE C CODE When programming in C, the preceding equations can be simply written by using the << and >> shift commands. // DL = LHSVAL0 >> CD_Bits + 6 iDL = LHSVAL0 >> (( POWCON & 0x7)+6); // writing DL as 2^iDL_Power iDL_Power = 0; iDL_temp = iDL; while(iDL >> (iDL_Power +1 )) { iDL_Power++; } // Configuration of the fractional divider: // M = 1 // N = LHSVAL0 × 2 ^ (5 – (iDL_Power + CD)) - 2048 iDL_temp = iDL_Power + (POWCON & 0x7); if (iDL_temp > 5) { iDL_temp = (LHSVAL0 >> (iDL_temp -5)) - 2048; } else { iDL_temp= (LHSVAL0 << (5 - iDL_temp)) - 2048; } COMDIV2 = 0x8800 + iDL_temp; COMCON0 = 0x080; // Setting DLAB // Setting DIV0 and DIV1 to DL calculated COMDIV0 = (1<< iDL_Power) & 0xff; COMDIV1 = (1<< iDL_Power) & 0xff00; COMCON0 = 0x03; COMIEN0 = 0x1; // Setting DLAB // Enable RX interrupt Rev. 0 | Page 3 of 4 AN-891 NOTES ©2006 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. AN06488-0-11/06(0) Rev. 0 | Page 4 of 4

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