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SIM900 and ATMega88PA Serial Communication

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TinyReiny

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I am currently using a SIM900 and an ATMega88PA to conduct serial communication. I have ran into multiple problems, which are...

1) I changed the baud rate of the SIM900 by sending the AT command
"AT+IPR=115200"
and now I cannot change the baud rate back to 9600 because the characters that I type into the hyperterminal are not showing up as what they should be.

2) I have set up uart in code, but am confused as to how I can read back characters after I have sent data to the SIM900 from the ATMega88PA. For example, if I send the AT command
"AT" I should expect to receive "OK" back. But how exactly do I go about reading that data then printing it onto an LCD? My uart code is as follows...

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#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include "uart.h"
 
/*
 *  constants and macros
 */
 
/* size of RX/TX buffers */
#define UART_RX0_BUFFER_MASK ( UART_RX0_BUFFER_SIZE - 1)
#define UART_TX0_BUFFER_MASK ( UART_TX0_BUFFER_SIZE - 1)
 
#if ( UART_RX0_BUFFER_SIZE & UART_RX0_BUFFER_MASK )
    #error RX0 buffer size is not a power of 2
#endif
#if ( UART_TX0_BUFFER_SIZE & UART_TX0_BUFFER_MASK )
    #error TX0 buffer size is not a power of 2
#endif
 
#define UART0_RECEIVE_INTERRUPT   USART_RX_vect
#define UART0_TRANSMIT_INTERRUPT  USART_UDRE_vect
#define UART0_STATUS   UCSR0A
#define UART0_CONTROL  UCSR0B
#define UART0_DATA     UDR0
#define UART0_UDRIE    UDRIE0
 
static volatile uint8_t UART_TxBuf[UART_TX0_BUFFER_SIZE];
static volatile uint8_t UART_RxBuf[UART_RX0_BUFFER_SIZE];
static volatile uint8_t UART_TXHead;
static volatile uint8_t UART_TxTail;
static volatile uint8_t UART_RxHead;
static volatile uint8_t UART_RxTail;
static volatile uint8_t UART_LastRxError;
 
ISR(UART0_RECEIVE_INTERRUPT)
/*************************************************************************
Function: UART Receive Complete interrupt
Purpose:  called when the UART has received a character
**************************************************************************/
{
    uint16_t tmphead;
    uint8_t data;
    uint8_t usr;
    uint8_t lastRxError;
 
    /* read UART status register and UART data register */ 
    usr  = UART0_STATUS;
    data = UART0_DATA;
    
    lastRxError = (usr & (_BV(FE0)|_BV(DOR0)) );
        
    /* calculate buffer index */ 
    tmphead = ( UART_RxHead + 1) & UART_RX0_BUFFER_MASK;
    
    if ( tmphead == UART_RxTail ) {
        /* error: receive buffer overflow */
        lastRxError = UART_BUFFER_OVERFLOW >> 8;
    } else {
        /* store new index */
        UART_RxHead = tmphead;
        /* store received data in buffer */
        UART_RxBuf[tmphead] = data;
    }
    UART_LastRxError = lastRxError;   
}
 
 
ISR(UART0_TRANSMIT_INTERRUPT)
/*************************************************************************
Function: UART Data Register Empty interrupt
Purpose:  called when the UART is ready to transmit the next byte
**************************************************************************/
{
    uint16_t tmptail;
 
    if ( UART_TxHead != UART_TxTail) {
        /* calculate and store new buffer index */
        tmptail = (UART_TxTail + 1) & UART_TX0_BUFFER_MASK;
        UART_TxTail = tmptail;
        /* get one byte from buffer and write it to UART */
        UART0_DATA = UART_TxBuf[tmptail];  /* start transmission */
    } else {
        /* tx buffer empty, disable UDRE interrupt */
        UART0_CONTROL &= ~_BV(UART0_UDRIE);
    }
}
 
 
/*************************************************************************
Function: uart0_init()
Purpose:  initialize UART and set baudrate
Input:    baudrate using macro UART_BAUD_SELECT()
Returns:  none
**************************************************************************/
void uart0_init(uint16_t baudrate)
{
    UART_TxHead = 0;
    UART_TxTail = 0;
    UART_RxHead = 0;
    UART_RxTail = 0;
 
    /* Set baud rate */
    if ( baudrate & 0x8000 ) {
        UART0_STATUS = (1<<U2X0);  //Enable 2x speed
        baudrate &= ~0x8000;
    }
    UBRR0H = (uint8_t)(baudrate>>8);
    UBRR0L = (uint8_t) baudrate;
 
    /* Enable USART receiver and transmitter and receive complete interrupt */
    UART0_CONTROL = _BV(RXCIE0)|(1<<RXEN0)|(1<<TXEN0);
 
    /* Set frame format: asynchronous, 8data, no parity, 1stop bit */
//#ifdef URSEL0
    //UCSR0C = (1<<URSEL0)|(3<<UCSZ00);
//#else
    UCSR0C = (3<<UCSZ00);
 
} /* uart0_init */
 
 
/*************************************************************************
Function: uart0_getc()
Purpose:  return byte from ringbuffer
Returns:  lower byte:  received byte from ringbuffer
          higher byte: last receive error
**************************************************************************/
uint16_t uart0_getc(void)
{
    uint16_t tmptail;
    uint8_t data;
 
    if ( UART_RxHead == UART_RxTail ) {
        return UART_NO_DATA;   /* no data available */
    }
 
    /* calculate /store buffer index */
    tmptail = (UART_RxTail + 1) & UART_RX0_BUFFER_MASK;
    UART_RxTail = tmptail;
 
    /* get data from receive buffer */
    data = UART_RxBuf[tmptail];
 
    return (UART_LastRxError << 8) + data;
 
} /* uart0_getc */
 
/*************************************************************************
Function: uart0_peek()
Purpose:  Returns the next byte (character) of incoming UART data without
          removing it from the ring buffer. That is, successive calls to
          uartN_peek() will return the same character, as will the next
          call to uartN_getc()
Returns:  lower byte:  next byte in ring buffer
          higher byte: last receive error
**************************************************************************/
uint16_t uart0_peek(void)
{
    uint16_t tmptail;
    uint8_t data;
 
    if ( UART_RxHead == UART_RxTail ) {
        return UART_NO_DATA;   /* no data available */
    }
 
    tmptail = (UART_RxTail + 1) & UART_RX0_BUFFER_MASK;
 
    /* get data from receive buffer */
    data = UART_RxBuf[tmptail];
 
    return (UART_LastRxError << 8) + data;
 
} /* uart0_peek */
 
/*************************************************************************
Function: uart0_putc()
Purpose:  write byte to ringbuffer for transmitting via UART
Input:    byte to be transmitted
Returns:  none
**************************************************************************/
void uart0_putc(uint8_t data)
{
    uint16_t tmphead;
 
    tmphead  = (UART_TxHead + 1) & UART_TX0_BUFFER_MASK;
 
    while ( tmphead == UART_TxTail ) {
        ;/* wait for free space in buffer */
    }
 
    UART_TxBuf[tmphead] = data;
    UART_TxHead = tmphead;
 
    /* enable UDRE interrupt */
    UART0_CONTROL    |= _BV(UART0_UDRIE);
 
} /* uart0_putc */
 
 
/*************************************************************************
Function: uart0_puts()
Purpose:  transmit string to UART
Input:    string to be transmitted
Returns:  none
**************************************************************************/
void uart0_puts(const char *s )
{
    while (*s) {
        uart0_putc(*s++);
    }
 
} /* uart0_puts */
 
 
/*************************************************************************
Function: uart0_puts_p()
Purpose:  transmit string from program memory to UART
Input:    program memory string to be transmitted
Returns:  none
**************************************************************************/
void uart0_puts_p(const char *progmem_s )
{
    register char c;
 
    while ( (c = pgm_read_byte(progmem_s++)) ) {
        uart0_putc(c);
    }
 
} /* uart0_puts_p */
 
 
 
/*************************************************************************
Function: uart0_available()
Purpose:  Determine the number of bytes waiting in the receive buffer
Input:    None
Returns:  Integer number of bytes in the receive buffer
**************************************************************************/
uint16_t uart0_available(void)
{
    return (UART_RX0_BUFFER_SIZE + UART_RxHead - UART_RxTail) & UART_RX0_BUFFER_MASK;
} /* uart0_available */
 
/*************************************************************************
Function: uart0_flush()
Purpose:  Flush bytes waiting the receive buffer.  Acutally ignores them.
Input:    None
Returns:  None
**************************************************************************/
void uart0_flush(void)
{
    UART_RxHead = UART_RxTail;
} /* uart0_flush */


3) My SIM900 module is on a KUNS Design board. I am having trouble finding documentation on this. If you know where I can find the datasheet please let me know.

Someone, anyone, please help me. This is a school project and worth a large chunk of my grade.
 
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