AN1274-HC08 SCI Operation with Various Input Clocks

Order this document by AN1274/D © Motorola, Inc., 1996 AN1274/D AN1274 HC08 SCI Operation with Various Input Clocks By Rick Cramer CSIC MCU Product Engineering Austin, Texas INTRODUCTION This application note describes the operation of the serial communications interface (SCI) as it relates to the MC68HC708XL36 (XL36) with clock generation module A (CGMA). Specifically, the information here provides an analysis of the effects of the input clock on the SCI baud rate. SCI communication in various hardware applications is also examined, as well as code segments and listings. GENERAL SCI INFORMATION What is SCI? SCI is a defined standard for transmitting and receiving data. Serial refers to the method used to transfer data. This method uses one wire to transfer a data byte serially, that is, one bit at a time. Data also can be transferred by a parallel method. A parallel data transfer uses eight wires to send eight bits of a byte of data simultaneously. Figure 1 shows the serial hardware configuration, and Figure 2 shows the parallel. Figure 1. Serial Hardware Configuration MCU 1 D0, D1, D2, D3 ... ... D3, D2, D1, D0 MCU 2 F re e sc a le S e m ic o n d u c to r, I For More Information On This Product, Go to: www.freescale.com n c .. . rxzb30 ForwardLine AN1274 2 Figure 2. Parallel Hardware Configuration Serial communication has an advantage over parallel data transfer because serial uses fewer connections which results in a cheaper manufacturing cost. The drawback of using serial, however, is that this type of communication generally makes a slower transfer than parallel. The SCI is used in most applications to communicate with other microcontroller units (MCU) over a 2-wire system. Using two wires allows full duplex operation. Essentially, both MCUs can transmit information to each other simultaneously. Figure 1 shows full duplex operation. Baud Rate Baud rate is the rate in which the data is transferred. Baud rate also conveys the capacity of a data channel and is expressed most commonly as bits per second (BPS). Although not exactly a correct definition, BPS is often used synonymously with baud, and, for the purpose of this document, they are considered the same. For example, if an SCI system is set to run at 1200 baud, 1200 bits of data can be transferred in one second. This is a measure of speed and capacity. The bit period, which is the length of time required to transfer one bit of data, is calculated by dividing one second by the baud rate. Therefore, the bit rate for a 1200-baud system is 8.334 x 10-4s, or 834 µs. Input Clock The input clock is a square wave that determines the chip speed. Ideally, the input clock could be set to any frequency, and the chip could run as fast as it is driven. Realistically, however, the MCU can run only within specific limits. Consult the appropriate technical data sheet for the device in use to determine its maximum and minimum input clock frequencies. Typically for the XL36, the input clock frequencies range from 4 MHz to 8 MHz. The input clock in the XL36 using CGMA is referred to as CGMXCLK. MCU 1 D0 D1 D2 D3 D4 D5 D6 D7 CLOCK MCU 2 F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 3 How does the input clock affect the baud rate? The input clock is related directly to the baud rate. This relationship is defined by the baud rate equation in Figure 3. Figure 3. Baud Rate Equation In this equation, PD is the prescaler divisor and BD is the baud rate divisor. PD and BD are variables that allow the user to change the baud rate; however, the baud rate continues to be directly related to GCMXCLK. So when the CGMXCLK frequency is increased, the baud rate also increases. Why are two communicating devices required to have the same baud rate? When using the SCI system, no clock signal is sent with the data. A clock signal normally is used to tell the receiving device when the bits of data should be read. Therefore, since no clock is sent with the SCI data, the receiver must determine when each data bit is valid to read. This is done with critical timing. The receiver marks the time when the first bit starts and then reads each following bit at the time they are supposed to be valid at the receiver. If the receiver and transmitter are attempting to communicate at different baud rates, the receiver will misread the incoming data, and errors will occur. BASIC SCI OPERATION WITH EQUAL INPUT CLOCK FREQUENCIES The SCI operates easily under ideal conditions, such as these: • Using two identical HC08 microcontrollers • Operating both microcontrollers at the same input clock frequency • Placing both microcontrollers adjacent to each other • Allowing enough time in the application for a complete SCI transfer • Accepting the default values of SCI parameters • Operating the circuit at room temperature An important point to realize is that the simplest way to use HC08 microcontrollers in unison is to leave most of the registers at their default values. Since all XL36 microcontrollers have the same default value, assuming that they will operate together is a fair assumption. Seven registers are associated with the SCI operation on the XL36, but only three are needed. Figure 4 illustrates the process of using the SCI to transmit data. baud rate = CGMXCLK 64 x PD x BD F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 4 Figure 4. SCI Transmitter Operation The flowchart in Figure 4 illustrates the basic operation of how the SCI transmits data. However, some further explanations and a software code example are necessary. Appendix A: Basic SCI Transmitter Code shows the implementation of the basic SCI operation. This code is mostly academic. For the code to be of any actual value, the transmitted code must be received. Several variables are defined at the beginning of the program. Notice that most variables are not used in the program code but are included to provide a complete header of SCI code for later use. BASIC SCI TRANSMITTER ENABLE SCI ENABLE TRANSMITTER CLEAR SCI TRANSMITTER EMPTY BIT WRITE DATA TO DATA REGISTER F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 5 Connecting Two HC08s Figure 5 shows how to cross-couple two MMDS08 emulators. Note that these signals are cross-coupled. Essentially, TxD is connected to RxD on MMDS08 2. Figure 5. Cross-Coupled MMDS08 Diagram Once TxD and RxD are cross-coupled, software then can be written on the other MMDS08 to receive the data being transmitted. The flowchart in Figure 6 illustrates the basic operation of the SCI as it receives data. However, further explanation and a software code example are necessary. Appendix B: Basic SCI Receiver Code shows the implementation of the basic SCI receiver. This code is also mostly academic, since it is seldom that only one character will be received. Additional code is necessary to demonstrate the flow of data between two HC08 microcontrollers. Several variables are defined at the beginning of the program. Most of them are not actually used in the program code but are included to provide a complete header of SCI code for later use. MMDS08 1 TOP COVER 68HC708XL36 TOP BOARD CONNECTORS MMDS08 2 TOP COVER 68HC708XL36 TOP BOARD CONNECTORS PTE2/TxD PTE1/RxD PTE2/TxD PTE1/RxD OPENING OPENING F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 6 Figure 6. Basic SCI Receiver Sending packets of information from one device to another is often necessary. For the purpose of this application note, a packet is defined as a string of characters of data, which is variable in length and is terminated with, but does not contain, the carriage return character (decimal value 13). For example, the phrase, ’This is a test,’ as shown here, is a packet: 546869732069 DATA: fcb 'This is a test' 0D fcb 13 To better understand the SCI transmitter and receiver, see Figure 7 for an example of how to send packets of data between two microcontrollers. By using the circuit diagram in Figure 5, new software is developed to provide a smoother flow between the two chips.The Ping-Pong got its name from the back and forth nature of its operation. One device sends a packet to the other. The receiving device then verifies that it has received the correct information and sends a packet of information back to the original sender. This creates a Ping-Pong effect such that when the data is viewed with an oscilloscope, the data actually bounces from one device to the other and then back again. Appendix C: SCI Ping-Pong Code is the software used to illustrate the Ping-Pong algorithm. BASIC SCI RECEIVER ENABLE SCI ENABLE RECEIVER RECEIVED A CHARACTER? READ DATA FROM THE DATA REGISTER YES NO F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 7 Notice that there is much more flow control in Ping-Pong than in the previous two program examples. This software can be classified as the first program described in this application note that can be used in practice. Notice, too, the many similarities between the transmitting and receiving MCUs. Also notice that the variable baud located in Appendix C: SCI Ping-Pong Code is set to $00. This value is updated in later program examples to explain how to change that baud rate. The default reset value of the SCI baud rate register is $00, which means that setting the value to $00 is the same as not setting it at all after a reset. Figure 7. Ping-Pong Flowchart PING-PONG INITIALIZE TRANSMITTER TRANSMITTER READY? SEND NEXT DATA BYTE IS BYTE A $0D ? TRANSMISSION COMPLETE? TURN OFF TRANSMITTER YES YES YES NO TO RECEIVE RECEIVE INITALIZE RECEIVER RECEIVER READY? GET DATA BYTE IS BYTE A $0D ? TURN OFF RECEIVER YES YES NO TO PING-PONG STORE DATA IN RAM NO NO NO F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 8 SCI OPERATION WITH UNEQUAL INPUT CLOCK FREQUENCIES When using unequal input clock frequencies, adjust the baud rate so that both devices operate at the same rate. As shown in Figure 3, there is a direct relationship between the input clock and the baud rate. The two divisors, PD and BD, are variables that allow the user to change the baud rate. The baud rate remains directly related to GCMXCLK so that when the input clock frequency increases, the baud rate also increases. NOTENOTE When using the XL36 with CGMA, the phase lock loop (PLL) settings have no effect on the baud rate, even though changing the settings within the PLL cause the MCU internal clock to operate at a different frequency. This happens because the clock for the SCI is taken from the input clock and not the PLL output. Figure 8 shows how to calculate the baud rate using the equation shown in Figure 3. Figure 8. Baud Rate Calculation Example 1 baud rate = CGMXCLK 64 x PD x BD If using a 4.9152-MHz clock source and PD and BD are set to one, the equation becomes: baud rate = 4915200 Hz 64 x 1 x 1 baud rate = 76,800 F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 9 Figure 9 shows that using the same settings but a different clock frequency produces a different baud rate. Figure 9. Baud Rate Calculation Example 2 If these two previous example MCUs were configured similarly to those in Figure 5, communication between them would not occur, because they operate at different baud rates. Appendix D: SCI Baud Rate Chart shows these two input clock frequencies. To allow these two example MCUs to communicate, pick a baud rate that is common between both sets. For example, choose 600 baud, which is common to both sets. For an 8-MHz input clock, use PD = 13 and BD = 16. For a 4.9152-MHz input clock, use PD = 4 and BD = 32. See Figure 10 for more information. Figure 10. Baud Rate Calculation Example 3 baud rate = CGMXCLK 64 x PD x BD If using a 8.000-MHz clock source and PD and BD are set to one, the equation becomes: baud rate = 8000000 Hz 64 x 1 x 1 baud rate = 125,000 baud rate = CGMXCLK 64 x PD x BD If using an 8.000-MHz clock source, PD = 13, and BD = 16, the equation becomes: baud rate = 8000000 Hz 64 x 13 x 16 baud rate = 600.96 baud rate = CGMXCLK 64 x PD x BD If using a 4.915-MHz clock source, PD = 4, and BD = 32, the equation becomes: baud rate = 4915200 Hz 64 x 4 x 32 baud rate = 600.00 F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 10 Although the baud rates are not exactly equal, they are close enough to work properly. Baud rates should be within 3.4% of each other to operate properly. See Figure 11 for more information. Figure 11. Baud Rate Deviation Equation If one MCU operates at 600 baud, the other may operate at as much as 2.04 baud out of frequency, which is 602.04 baud. So as shown in Figure 10, the baud rate of 600.96 is well within the 3.4% deviation limit and, therefore, will work properly in this implementation. SCI OPERATION WITH COMMONLY USED BAUD RATES Baud rates are used widely when communicating with modems. Modems are devices that convert binary data into audible frequencies that can be sent across a normal telephone line. Modems also can receive these frequencies and convert them back into digital data. The SCI format is in exactly the same format as that used by most modems. The only difference is the voltage levels. The SCI operates at the voltage supplied to the VDD pin and the voltage supplied at the VSS pin (commonly +5 V and 0 V, respectively). Since standard modems (or the RS-232 standard) require the voltage be +12 V and –12 V, circuitry is necessary to convert the levels of the MCU to these standard RS-232 levels. The MC1488 and MC1489 shift voltage levels to meet the RS-232 standard. Figure 12 shows how to connect these chips to a modem. Figure 12. Modem Circuit baud rate deviation = 600 x 0.034 = 2.04 baud rate = baud rate + baud rate deviation = 600 = 602.04 + 2.04 MCU MODEM 1/4 MC1488 +12 V –12 V +5 V 1/4 MC1489 GND TxD RxD F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 11 Typically, the baud rate between the modem and the MCU is higher than the baud rate between the modem and another modem. This is because modern modems have become so advanced that they automatically can sense the maximum baud rate of the second modem. As long as the link between the MCU and modem is as high as possible, the modem will, in most cases, find the highest baud rate. Over the years of modem development, several baud rates have been chosen. These are 300, 1200, 2400, 4800, 9600, 14400, 19200, 28800, 38400, and 57600. To communicate with a modem, choose one of these baud rates. For instance, to communicate with a 1200-baud modem using the MC1488 and MC1489 chips as shown in Figure 12, choose 1200 from the baud rate chart. See Appendix D: SCI Baud Rate Chart. For a clock input frequency of 8 MHz, choose PD = 13 and BD = 8. This gives a baud rate of 1201.92. Using the baud rate deviation equation in Figure 13, calculate that the maximum deviation of the baud rate at 1200 baud is 4.08 as shown. Figure 13. Baud Rate Deviation Equation This allows a baud rate between 1204.08 and 1195.92. Since the chosen baud rate is 1201.92, it is within tolerable limits. baud rate deviation = 1200 x 0.034 = 4.08 baud rate = baud rate + baud rate deviation = 1200 = 1204.08 + 4.08 F re e sc a le S e m ic o n d u c to r, I Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com n c .. . AN1274 12 Appendix A: Basic SCI Transmitter Code ********************************************************************* ** Basic SCI Transmitter program ** ** By Rick Cramer ** ** 25 November 1995 ** **-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-** ** ** ** This program illustrates the basic operation of transmitting ** ** data using the SCI on the 68HC708XL36 using CGMA. ** ********************************************************************* *-------------------------------------------------------------------* DATA EQU $55 ; Data to transmit *-------------------------------------------------------------------* SCC1 EQU $13 ; SCI Control Register 1 LOOPS EQU 7 ; Bit 7: Loop Mode Select Bit ENSCI EQU 6 ; Bit 6: ENable SCI Bit TXINV EQU 5 ; Bit 5: Transmit Inversion Bit M EQU 4 ; Bit 4: Mode (Character Length) Bit WAKE EQU 3 ; Bit 3: Wake-up Condition Bit ILTY EQU 2 ; Bit 2: Idle Line Type Bit PEN EQU 1 ; Bit 1: Parity Enable Bit PTY EQU 0 ; Bit 0: Parity Bit *-------------------------------------------------------------------* SCC2 EQU $14 ; SCI Control Register 2 SCTIE EQU 7 ; Bit 7: SCI transmit Interrupt Enable Bit TCIE EQU 6 ; Bit 6: Transmission Complete Interrupt Enable ; Bit SCRIE EQU 5 ; Bit 5: SCI Receive Interrupt Enable Bit ILIE EQU 4 ; Bit 4: Idle Line Interrupt Enable Bit TE EQU 3 ; Bit 3: Transmitter Enable Bit RE EQU 2 ; Bit 2: Receiver Enable Bit RWU EQU 1 ; Bit 1: Receiver Wake Up Bit SBK EQU 0 ; Bit 0: Send Break Bit *-------------------------------------------------------------------* SCC3 EQU $15 ; SCI Control Register 3 R8 EQU 7 ; Bit 7: Received Bit 8 T8 EQU 6 ; Bit 6: Transmitted Bit 8 DMARE EQU 5 ; Bit 5: DMA Receive Enable Bit DMATE EQU 4 ; Bit 4: DMA Transfer Enable Bit ORIE EQU 3 ; Bit 3: Receiver Overrun Interrupt Enable Bit NEIE EQU 2 ; Bit 2: Receiver Noise Error Interrupt Enable ; Bit FEIF EQU 1 ; Bit 1: Receiver Framing Error Interrupt Enable ; Bit PEIE EQU 0 ; Bit 0: Receiver Parity Error Interrupt Enable ; Bit *-------------------------------------------------------------------* SCS1 EQU $16 ; SCI Status Register 1 SCTE EQU 7 ; Bit 7: SCI Transmitter Empty Bit TC EQU 6 ; Bit 6: Transmission Complete Bit SCR