AN2025 Freescale Semiconductor, Inc. General-Purpose Serial

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AN2025 Freescale Semiconductor, Inc. General-Purpose Serial

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Freescale Semiconductor, Inc.
General-Purpose Serial Communication Interface MMC2001 Using Motorola
Hans-Guenter Kremser WSSG Munich, Germany
Introduction
Even though many microcontrollers (MCU) have integrated UART (universal asynchronous receiver transmitter) board, some customers need another one. instance, part only UART, they need two. overcome this problem design software UART, also called (serial communication interface). This feasible general-purpose timer like MMC2001 does, with independent PWMs (pulse-width modulator). This application note describes software (SW) UART, written language. been developed MMC2001, Motorola MCU. Because program written high-level language, UART adapted easily other cores like PowerPC 68HCxx. This designed 8-bit data, start bit, stop bit, least significant (LSB) first protocol, which probably most common
PowerPC registered trademark International Business Machines Corporation.
Motorola, Inc., 2000
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Freescale Semiconductor, Inc. Application Note
configuration. baud rate (bits second) vary maximum 19,200 baud. protocol needs before establishing communication link. communicate only half-duplex mode. This means either transmit receive, cannot both functions simultaneously. more information, several Motorola application notes describe RS-232 protocol, including "HC05 Software-Driven Asynchronous Serial Communication Techniques Using MC68HC705J1A," document order number AN1240/D; "Software Implementation MISC Communication Protocol," number AN1667/D; "Software Routines with 16-Bit Timer Module," order number AN1818/D.
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Motorola Application Programming Interface (API)
Many development tools available Motorola's site www.mcu.motsps.com. Select Documentation. Also, there link device driver library, which used program described SCI. Using device driver library reduces development time provides: Documentation with example programs each function. Because documentation available electronically, paste used. Each peripheral "include" file. addresses structures registers defined these files. Sometimes makes sense these structures directly without using library function. library file containing functions file called libplib.a). Using CodeWarrior makes easy include this library. Click time folder project folder select "add file." under project button menu bar.
CodeWarrior registered trademark Metrowerks, Inc., wholly owned subsidiary Motorola, Inc.
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Application Note Concept
source code examples each peripheral with appropriate batch file compiling, linking under Diab Data/SDS (now WindRiver) environment. Also, tutorial program included.
Another advantage using this that address each register subregister doesn't need known. Even program status register (PSR) could accessed within source code using appropriate function. Because each function returns value that indicates successful completion, error handling used.
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effort ensure functions used this software better understood, this application note describes sets each register level. These device drivers also available MMC2003 (includes same functions MMC2001, well correlator (global positioning system) functionality), MMC2080 (MCU pagers supporting FLEX protocol), MMC2107, future MCUs.
Concept
During transmission reception, every sent received interrupt. advantage this concept quite obvious: allocation. transmits 9600 baud without using interrupts, would allocated about character (1/9600 times bits) only this task. While using interrupt approach, free other tasks. disadvantages this technique little jitter between bits while transmitting, and, during reception sampling time differ from bit. these mostly meaningless long stays specified drift range. Interrupt priorities different tasks system need assigned application program.
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Freescale Semiconductor, Inc. Application Note
Because MMC2001 based 32-bit RISC (reduced instruction computer) core, interrupt handling different compared other MCUs, like those M68HC05 Family, instance. This application note uses interrupt autovector. Each time interrupt occurs, this autovector called. program responsible determining interrupt source. described SCI, this done using "Set Interrupt Service Routine" functions provided library.
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user prefers program assembly, easy find interrupt source "Find First One" command (FF1 ISR) apply interrupt source register (ISR). returns offset position beginning most significant (MSB). because Motorola library used, single line programmed assembly. general, several steps necessary generate interrupt. They are: exception enable (EE) processor status register (PSR). either interrupt enable (IE) fast interrupt enable (FE), both, PSR. appropriate peripheral where generation interrupt desired (for instance, PWM5, number normal interrupt enable register (NIER), fast interrupt enable register (FIER), both. Finally, interrupt enabled peripheral's control register (for instance, number PWMCR5). channel used receiving channel used transmitting. Both PWMs used only maintain timing generate interrupt. associated hardware pins used mode. transmitter uses PWM5 output pin, which used generalpurpose input/output (/O). INT6 EDGE port used receiver.
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Application Note Concept
Implementation Brief
transmit data, function called. This function enables interrupt PWM5 starts timer. Each time period PWM5 reaches value, interrupt generated. This interrupt calls service routine which transmits data ready transmission until last last data word been sent. receive data, valid start detected edge port pin. This means falling edge logical middle start bit. soon this confirmed, PWM4 activated timer starts check values middle next eight data bits.
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Figure shows main program organized example possible implementation. Notice that receive function called because unknown when data arrives. main program ready reception during endless loop.
START MASK
DEFINE TRANSMIT VECTOR
INITIALIZATION
TRANSMIT ARRAY
LOOP FOREVER
Figure Main Program Implementation
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Freescale Semiconductor, Inc. Application Note Initialization
During initialization, following things have done: Transmit control register Setting division ratio, which ratio between processor clock frequency clock. adjusted three bits, coarse ratios possible. Enable interrupt configure port general-purpose output pin. MMC2001 provides independent PWMs (for instance, each programmed with different frequency duty cycle). transmitter makes PWM5, which starts address 0x10005028 indicates hexadecimal) with control register. pointers global assignments, right after #include section. Table describes configuration control register (PWMCR5).
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Table Control Register Setup
14-12 Doze mode (DOZE) interrupt request (PWMIRQ) Interrupt request enable (IRQEN) Load period width register (LOAD) data (DATA) Direction (DIR) Polarity (POL) Mode (MODE) Counter enable (COUNTEN) Reserved Disabling this channel doze mode Forcing interrupt, instance, debugging interrupt generated each time period completed Forcing period immediately this channel configured general-purpose (GPIO), value reflected this bit. used GPIO, sets output, input. Only valid when used mode; normal polarity; inverted polarity general-purpose mode; mode Setting this starts timer. Description Setting
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Application Note Initialization
Table Control Register Setup (Continued)
Description sets dividing ratio (32-MHz system clock MHz). reference clock generating baud rate. data rate 9600 baud, calculate 9600 833.33 right value program period register. Other configurations possible: division ratio (001) results fref. 9600 416.66 only thing consider that timing resolution decreases increasing pre-divider ratio. Setting
2-0: Clock select (CLKSEL)
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actual setting this register done using function PWM_A_SetRegister which three parameters: base address Actual register; each four 16-bit registers which control, period, width, timer register Data, which could bits wide, making possible write registers write statement
Transmit period width registers These 10-bit registers (upper bits read only) that fine tune PWM. Because only period needed generate interrupt, value width register doesn't matter long smaller than period value. already discussed, period value selected. Also, calculation done different way: reference clock equal period example, takes each time counter increased step. baud rate 9600, each needs 104.16 104.16 833. Transmitting bits (one start, eight data, stop bit) takes 1.04 registers with PWM_A_UpdateOutput() function. With this function, possible change LOAD control register write period width registers.
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Freescale Semiconductor, Inc. Application Note
Setting high, which idle condition This realized with macro because transmit must cleared often. macros defined: SCI_TX_ON, which sets logical high, SCI_TX_OFF, which clears pin. definitions these macros done main.h include file. this case, they directly programmed without using function, using acronyms like PWMCR PWM_A_DATA_MASK. switch high number PWM5 control register "binary OR'ed" with PWM_A_DATA_MASK. cleared "logical ANDing" register with inverted PWM_A_DATA_MASK. Receive control register Setting division ratio (the same transmit PWM) enabling interrupt counter reaches period value. other this register. associated GPIO input default. This will used because external interrupt will used receiver input pin. channel used receiver. period half value transmit period because user needs sample middle possible start bit. After this, period needs changed again transmit value. Setting vector base address supports many different interrupt sources, which have dedicated offsets. These offsets have added base address. function INTC_A_Init sets interrupt vector base address start internal address, which 0x30000000. offset 0x2C because fast interrupt autovector used. each time fast interrupt occurs, program fetches address 0x3000002C then vectors that address. interrupt handling assigned library function called INTC_A_SetISF, which checks interrupt source flagged assigns dedicated interrupt service routine name. This function name called when interrupt occurs.
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Application Note Transmitter
Three interrupt service routines have assigned: sci_tx() transmitting Will called when transmit PWM5 generates interrupt sci_rx() receive Will called when external interrupt occurs INT6 sci_receive() receiving Will called when receive PWM4 generates interrupt
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Finally, interrupts have enabled. This also done with library function INTC_A_IntEnable(). sets above mentioned three bits (PWM4, PWM5, INT6) fast interrupt enable register (FIER) enables bits processor's program status register (PSR). external interrupt pin, INT6, needs configured input falling edge sensitive because this could start valid RS-232 protocol. this case, function called. Instead, realized manually following edge port pointer edge port assignment register (EPPAR) masking with appropriate mask, which INT6 external EPPAR_EPPA6_FALLING_EDGE_MASK.
Transmitter
transmitting part split into functions. They are: function that transmit array length function that puts received data into global array does interrupt-driven transmission real transmission handled function sci_tx, which called each time PWM5 matches value period register. This will case every Some functions Motorola's device driver library allow passing variables interrupt functions. This could have been used, using global variables this application note solves problem passing variables interrupt routines. state machine strong word counting variable) implemented differentiate
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Freescale Semiconductor, Inc. Application Note
among start bit, eight data bits, stop bit. transmission stops stopping associated soon last (MSB) last byte transmit array reached. flowchart Figure shows overview transmitter used transmitter; used receiver).
START
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TRANSMIT FLAG
ASSIGN PASSED VARIABLES FROM MAIN PROGRAM GLOBAL VARIABLES
START TRANSMIT TIMER GENERATE INTERRUPT
Figure Transmit Function Used Main Program transmit function calls transmit function. flowchart transmit function shown Figure
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Application Note Transmitter
START CAUSED TIMER INTERRUPT
START
SWITCH TRANSMIT
DATA
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MASKING CHARACTER TRANSMIT ARRAY
SWITCH TRANSMIT
HIGH
SWITCH TRANSMIT
SHIFT MASK LEFT
BEEN SENT
CHANGE STATUS STOP
STOP
SWITCH TRANSMIT REINITIALIZE MASK CHANGE STATUS START CHANGE STATUS DATA
ARRAY
STOP TIMER CLEAR LENGTH COUNTER ALLOW RECEIVING AGAIN
RETURN
Figure Transmit Function, Called Transmit Timer Interrupt
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Freescale Semiconductor, Inc. Application Note Receiver
receiver slightly more difficult, since time that message will received unknown. best solution external interrupt pin. will fact that received message always starts with negative transition followed logic (start bit) configure interrupt negative transition detection.
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what just spike? This problem solved timer which starts negative slope looks middle start still logic unlikely another spike will occur exactly this time. this point, flag indicating valid data reception could set. timer changed period look data value middle each bit. Another (PWM4) interrupt used look eight bits data. receiving buffer filled long valid data detected external interrupt pin. Figure shows receiver realized.
START CAUSED NEGATIVE SLOPE EXTERNAL INTERRUPT
TRANSMITTER STILL ACTIVE
START RECEIVE TIMER
STOP RECEIVE TIMER
RETURN
Figure Receive Function, Called External Interrupt
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Application Note Receiver
receive function calls reception function, shown Figure
START CAUSED TIMER INTERRUPT
START
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DATA
NEXT BITS FROM EXTERNAL INTERRUPT FILL FIRST CHARACTER RECEIVE BUFFER FROM STOP
CHANGE SAMPLE RATE FROM RATE RATE
STOP RECEIVE TIMER; INCREMENT LENGTH COUNTER RECEIVE BUFFER; CHANGE SAMPLE RATE 1/(Tbit/2) AGAIN
CHANGE STATUS START
CHANGE STATUS STOP
CHANGE STATUS DATA
RETURN
Figure Reception Function, Called Receive Timer Interrupt
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Freescale Semiconductor, Inc. Application Note Summary
This application note describes realize half-duplex software UART, completely written high-level language uses Motorola device driver library which accessible Motorola site. bits transmitted received interrupt functions. This application note demonstrates good concept implementing these routines operating system callable application.
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functions main program located file main.c, main.h file contains local definitions. other header files which main.c file includes provided device driver library. linker command file called linker.cmd handles memory organization. this file, RAM, ROM, vectors, stack addresses defined. CodeWarrior from Metrowerks provides file. code been tested with Motorola MMC2001 evaluation board (MMCEVB1200PV) Motorola EBDI (enhanced background debug interface) Metrowerks CodeWarrior environment.
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Application Note Source Code
Source Code
Serial Communication Interface filename: main.c PWM5 with PWM5 used PWM4 with INT6 used
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#include #include #include #include #include #include
"plibdefs.h" "plib.h" "pwm_a.h" "intc_a.h" "edgeport_a.h" "main.h"
global initializations (provided API) pPWM_A_t tx_pwmptr (pPWM_A_t) _PWS_PWM5; pPWM_A_t rx_pwmptr (pPWM_A_t) _PWS_PWM4; static intTbl_t funcs; interrupt signal service tables pINTC_A_t intctlr (pINTC_A_t) _PWS_INTCTLR; pEdgePort_A_t edgeportptr (pEdgePort_A_t) _PWS_EdgePort;
Function name: sci_tx Purpose: this function called every time PWM5 timer generates interrupt (every 104us) transmits data global array tx_buffer[] bit. Input: global array tx_buffer Output: PWM5 GPIO used transmit void sci_tx (void) tx_pwmptr->PWMCR PWM_A_IRQ_MASK; clear interrupt request switch (tx_state) case SCI_TX_OFF; start tx_state=1; break; case (tx_buffer[j] mask)// first SCI_TX_ON; else SCI_TX_OFF; mask if(!mask) tx_state=2; last data (MSB) been sent break; case SCI_TX_ON; stop mask=MASK_INIT; tx_state=0; j++; tx_length) array, stop transmission PWM_A_Stop (tx_pwmptr); j=0; //clear length counter next message transmit=FALSE; allow receiving again
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Freescale Semiconductor, Inc. Application Note
Freescale Semiconductor, Inc.
Function name: sci_rx Purpose: this function called interrupt transmitting Input: none Output: none void sci_rx(void) INT6 edgeportptr->EPFR EPFR_EPF6_MASK; (!transmit) PWM_A_Start (rx_pwmptr); else PWM_A_Stop (rx_pwmptr);
INT6 pin. allows reception only while
function called first falling edge clear flag!! half duplex!!
Function name: sci_receive Purpose: this function called every time PWM4 timer generates interrupt (every 104us) valid falling edge been detected INT6 rx_buffer[] filled bit. Input: external INT6 Output: global array rx_buffer void sci_receive (void) intctlr->FIER ~INTSRC_PWM5_MASK; disable interrupts rx_pwmptr->PWMCR PWM_A_IRQ_MASK; clear interrupt request ((edgeportptr->EPDR EPDR_EPD6_MASK) sample middle start rx_pwmptr->PWMPR 833; valid start bit;change sample rate 9600 baud (k==0) k++; //skip start else (k<9) rx_buffer[rx_length] >>=1; first time nothing happens, then shift rx_buffer[rx_length] (edgeportptr->EPDR EPDR_EPD6_MASK)<<1; INT6 k++; else (edgeportptr->EPDR EPDR_EPD6_MASK) PWM_A_Stop (rx_pwmptr); stop received k=0; rx_length++; intctlr->FIER INTSRC_INT6_MASK;// enable INT6 interrupts rx_pwmptr->PWMPR 416; change sample rate Tbit/2 again
Function name: sci_init Purpose: this function called main program setup baudrate receiver transmitter, enable appropriate interrupts, configure GPIO pins, etc. Input: none Output: none void sci_init(void) void *VBA _PWS_OnChipRamBase; //start internal PWM_A_SetRegister initialize divider
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Application Note Source Code
tx_pwmptr, PWM_A_PWMCR_SWITCH, tx_pwmptr->PWMCR PWM_A_IRQEN_MASK PWM_A_DATA_MASK PWM_A_DIR_MASK PWM_A_DIV_4 PWM_A_UpdateOutput tx_pwmptr, FALSE, 833,
channel force period period: 32MHz clock (clock divider) 9604 baud ton/toff doesn't matter interrupt only dependent period
SCI_TX_ON; idle line PWM_A_SetRegister initialize divider rx_pwmptr, PWM_A_PWMCR_SWITCH, rx_pwmptr->PWMCR PWM_A_IRQEN_MASK PWM_A_UpdateOutput rx_pwmptr, channel FALSE, force period 416, timer Tbit/2 sampling middle start ton/toff doesn't matter interrupt only dependent period INTC_A_Init intctlr, VBA, Vector Base Address &funcs Function Table Addr INTC_A_SetISF intctlr, INTC Device Handle INTSRC_PWM5_BITNO, PWM5 Interrupt Source INTSRC_PWM5_MASK, PWM5 Interrupt Mask (ddErr_t(*)(void void *))sci_tx,// interrupt service function address NULL, Dummy Parameter NULL Dummy Parameter INTC_A_SetISF intctlr, INTSRC_INT6_BITNO, INT6 Interrupt Source INTSRC_INT6_MASK, INT6 Interrupt Mask (ddErr_t(*)(void void *))sci_rx, interrupt service function address NULL, Dummy Parameter NULL Dummy Parameter INTC_A_SetISF intctlr, INTC Device Handle INTSRC_PWM4_BITNO, PWM4 Interrupt Source INTSRC_PWM4_MASK, PWM4 Interrupt Mask (ddErr_t(*)(void void *))sci_receive,/*interrupt service function address NULL, Dummy Parameter NULL Dummy Parameter INTC_A_IntEnable intctlr, INTSRC_PWM4_MASK INTSRC_PWM5_MASK INTSRC_INT6_MASK, TRUE, Fast Interrupt TRUE Enable
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Freescale Semiconductor, Inc. Application Note
edgeportptr->EPPAR INT6 falling edge sensitive, input
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Function name: sci_send Purpose: this function called from main program Input: data send length transmit array Output: global array tx_buffer global variable tx_length void sci_send(u1 *buffer_ptr, buffer_length) transmit=TRUE; tx_length buffer_length; save buffer length tx_buffer buffer_ptr; fill tx_buffer with transmit data PWM_A_Start (tx_pwmptr);
example main program demonstration receiving function could tested follows: connect e.g. serial port (e.g. COM1) INT6 MCORE evaluation board start main program into endless loop send data from port using copy filename.txt COM1 (where filename.txt contains data ASCII), global array rx_buffer will contain data
void main (void) p[]={0x81,0xab}; sci_init(); sci_send(p,2); while main.h
initialize send-array call initialize routine call send function pass sendarray endless loop, waiting receive data
Local defines #define SCI_TX_OFF tx_pwmptr->PWMCR ~PWM_A_DATA_MASK DATA 0:led #define SCI_TX_ON tx_pwmptr->PWMCR PWM_A_DATA_MASK DATA 1:led #define MASK_INIT 0x01 first #define RX_BUFFER_LENGTH global static static static static static static static variables *tx_buffer; rx_buffer[RX_BUFFER_LENGTH]; tx_length; rx_length; tx_state=0; mask=MASK_INIT; j=0,k=0;
length buffer length buffer state machine sci_tx mask byte index byte: sci_buffer, buffer
bool transmit=FALSE;
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Application Note Source Code
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Freescale Semiconductor, Inc. Application Note
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Motorola reserves right make changes without further notice products herein. Motorola makes warranty, representation guarantee regarding suitability products particular purpose, does Motorola assume liability arising application product circuit, specifically disclaims liability, including without limitation consequential incidental damages. "Typical" parameters which provided Motorola data sheets and/or specifications vary different applications actual performance vary over time. operating parameters, including "Typicals" must validated each customer application customer's technical experts. Motorola does convey license under patent rights rights others. Motorola products designed, intended, authorized components systems intended surgical implant into body, other applications intended support sustain life, other application which failure Motorola product could create situation where personal injury death occur. Should Buyer purchase Motorola products such unintended unauthorized application, Buyer shall indemnify hold Motorola officers, employees, subsidiaries, affiliates, distributors harmless against claims, costs, damages, expenses, reasonable attorney fees arising directly indirectly, claim personal injury death associated with such unintended unauthorized use, even such claim alleges that Motorola negligent regarding design manufacture part. Motorola registered trademarks Motorola, Inc. Motorola, Inc. Equal Opportunity/Affirmative Action Employer.
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AN2025 Freescale Semiconductor, Inc. General-Purpose Serial

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