Bug fix release

Dependents:   AntiTheftGPS XbeeReceive XbeeSend Superball_Ball2 ... more

MODSERIAL is an easy to use library that extends Serial to add fully buffered input and output.

The features of MODSERIAL include:-

/media/uploads/mbedofficial/serial_interfaces.png

Connecting up the MODSERIAL module

The starting point for using MODSERIAL is the Mbed's own handbook for Serial library object. MODSERIAL inherits Serial and adds extensions for buffering. So getting started is easy. Follow the Mbed instructions for Serial to get setup. Here's a reproduction of Serial's simple code starter:-

1  #include "mbed.h"
2
3  Serial pc(USBTX, USBRX); // tx, rx
4 
5  int main() {
6      pc.printf("Hello World!");
7      while(1) {
8          pc.putc(pc.getc() + 1);
9      }
10 }

All we need to do to use MODSERIAL is to add a #include and alter one line thus:-

1  #include "mbed.h"
2  #include "MODSERIAL.h"
3  MODSERIAL pc(USBTX, USBRX); // tx, rx
4 
5  int main() {
6      pc.printf("Hello World!");
7      while(1) {
8          pc.putc(pc.getc() + 1);
9      }
10 }

As we can see, all we have done is add the header at line 2 and changed line 3 to specify the use of MODSERIAL in replacement for Serial. The default settings for MODSERIAL are that both the TX and RX buffers are assigned 256 bytes each of storage space. This storage space is acquired from the heap using malloc.

The default buffer assignment can be manipulated in three ways. First is the compile time setting which alters the default parameters used when creating a MODSERIAL object. This is done thus:-

1  #include "mbed.h"
2
3  #define MODSERIAL_DEFAULT_RX_BUFFER_SIZE 512
4  #define MODSERIAL_DEFAULT_TX_BUFFER_SIZE 1024 
5  #include "MODSERIAL.h"
6
7  MODSERIAL pc(USBTX, USBRX); // tx, rx
8  ...

By defining the two #defines before the #include "MODSERIAL.h" alters the defaults MODSERIAL uses to create it's buffers.

The second method is the run-time version. To get TX at 1024 and RX buffer at 512 as above during run-time initialisation, alter the constructor thus:-

1  #include "mbed.h"
2  #include "MODSERIAL.h"
3
4  // Make TX buffer 1024bytes and RX buffer use 512bytes.
5  MODSERIAL pc(USBTX, USBRX, 1024, 512); // tx, rx
6  ...

If you supply only one numeric value, as shown below, both TX and RX will have the same buffer sizes assigned to them:-

1  #include "mbed.h"
2  #include "MODSERIAL.h"
3
4  // Make both TX and RX use a 512byte buffer.
5  MODSERIAL pc(USBTX, USBRX, 512); // tx, rx
6  ...

The third method is reassigning a new buffer while the program is running. This allows the program to grow and shrink either buffer as required. However, there are caveats to do this as will be shown below.

First, expanding the buffer involves increasing the buffer size. This is fairly straight forward and is accomplished thus:-

1  #include "mbed.h"
2  #include "MODSERIAL.h"
3  MODSERIAL pc(USBTX, USBRX); // tx, rx
4 
5  int main() {
6
7      // Increase the TX buffer from the default 256bytes to 1024bytes.
8      if (pc.txBufferSetSize(1024) != MODSERIAL::Ok) {
9         error("Failed to allocate memory for new buffer");
10     }
11
12     pc.printf("Hello World!");
13     while(1) {
14         pc.putc(pc.getc() + 1);
15     }
16 }

As can be seen, growing the buffer is fairly straight forward. However, how it is done should be understood by the user. First, a new buffer allocation is made using malloc. Once acquired the current buffer is checked for contents. If the current buffer is not empty it is copied to the new buffer so the old buffer contents is maintained after resizing. The last step is then to free() the old memory buffer.

The buffer can also be contracted to a smaller length buffer. Here's the code:-

1  #include "mbed.h"
2  #include "MODSERIAL.h"
3  MODSERIAL pc(USBTX, USBRX); // tx, rx
4 
5  int main() {
6      int result;
7
8      // Decrease the TX buffer from the default 256bytes to 32bytes.
9      result = pc.txBufferSetSize(32);
10     if (result != MODSERIAL::Ok) {
11         switch(result) {
12             case MODSERIAL::BufferOversize: 
13                 error("Contents too big to fit into new allocation");
14                 break;
15             case MODSERIAL::NoMemory: 
16                 error("Not enough memory for new allocation");
17                 break;
18         }
19     }
11
12     pc.printf("Hello World!");
13     while(1) {
14         pc.putc(pc.getc() + 1);
15     }
16 }

Since buffer resizing involves the copying over of any existing old buffer contents the possibility exists that the current buffer contains more bytes than will fit into the new requested buffer. In these conditions the user must handle the return value of the resize functions. If the contents are of no concern then calling txBufferFlush() to empty of the contents before resizing.

MODSERIAL Interrupts

Users of Serial will be familar with the fact that you can attach functions or methods to TxIrq or RxIrq. This attachment of callbacks allows users to have Interrupt Service Routines (ISR) for both the TX and RX channel of the Uart. MODSERIAL uses both of these callbacks to maintain it's buffers and so are not available to users. However, MODSERIAL does contain five potential callbacks the user can use. These are:-

  • TxIrq - This callback is used to inform the user's program that a character was transferred from the TX buffer to the Uart's TX THR FIFO.
  • RxIrq - This callback is used to inform the user's program that a character was transferred from the Uart's RX FIFO RBR to the RX buffer.
  • RxOvIrq - This callback is used to inform the user's program that a character in the Uart's RX FIFO RBR failed to transfer to the RX buffer because the RX buffer was full. The failed byte is availble via xxGetLastChar() methods.
  • TxOvIrq - As RX overflow above
  • TxEmpty - This callback is made when the last byte in the TX buffer is transferred to the Uart's TX THR FIFO. It informs the user's program that the TX buffer has become empty. However, it does not mean transmission is complete. See the example1.cpp example for more information.

Delineating "packets"

Many devices send information on RS232 interfaces in distinct "packets". As an example of this is NMEA information sent by many GPS modules. Each NMEA sentence is delineated by a '\n' newline character. Each sentence can be of vary length depending upon the information being sent, however, all are seperated by a '\n' newline. Detecting this if very simple with MODSERIAL. Here's an example:-

#include "mbed.h"
#include "MODSERIAL.h"

// Connect the TX of the GPS module to p10 RX input
MODSERIAL gps(NC, p10);

bool newline_detected = false;

// Called everytime a new character goes into
// the RX buffer. Test that character for \n
// Note, rxGetLastChar() gets the last char that
// we received but it does NOT remove it from
// the RX buffer.
void rxCallback(MODSERIAL_IRQ_INFO *q) {
    MODSERIAL *serial = q->serial;
    if ( serial->rxGetLastChar() == '\n') {
    	newline_detected = true;
    }
}

int main() {
    gps.baud(9600);
    gps.attach(&rxCallback, MODSERIAL::RxIrq);

    // Wait here until we detect the \n going into the buffer.
    while (! newline_detected ) ;    
    
    // When we get here the RX buffer now contains a NMEA sentence.
    // ...

}

Note, the txGetLastChar() and rxGetLastChar() methods only return the last character but they do not remove that character from the associated buffer.

If this is your first time using MODSERIAL or would just like to test it out then see the example.cpp that comes with the library.



Committer:
AjK
Date:
Tue Jan 08 18:01:03 2013 +0000
Revision:
25:ae0408ebdd68
Parent:
18:21ef26402365
See ChangeLog.c

Who changed what in which revision?

UserRevisionLine numberNew contents of line
AjK 18:21ef26402365 1 /*
AjK 18:21ef26402365 2 Copyright (c) 2011 Andy Kirkham
AjK 18:21ef26402365 3
AjK 18:21ef26402365 4 Permission is hereby granted, free of charge, to any person obtaining a copy
AjK 18:21ef26402365 5 of this software and associated documentation files (the "Software"), to deal
AjK 18:21ef26402365 6 in the Software without restriction, including without limitation the rights
AjK 18:21ef26402365 7 to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
AjK 18:21ef26402365 8 copies of the Software, and to permit persons to whom the Software is
AjK 18:21ef26402365 9 furnished to do so, subject to the following conditions:
AjK 18:21ef26402365 10
AjK 18:21ef26402365 11 The above copyright notice and this permission notice shall be included in
AjK 18:21ef26402365 12 all copies or substantial portions of the Software.
AjK 18:21ef26402365 13
AjK 18:21ef26402365 14 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
AjK 18:21ef26402365 15 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
AjK 18:21ef26402365 16 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AjK 18:21ef26402365 17 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
AjK 18:21ef26402365 18 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
AjK 18:21ef26402365 19 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
AjK 18:21ef26402365 20 THE SOFTWARE.
AjK 18:21ef26402365 21
AjK 18:21ef26402365 22 @file example3b.cpp
AjK 18:21ef26402365 23 @purpose Demos a simple filter.
AjK 18:21ef26402365 24 @version see ChangeLog.c
AjK 18:21ef26402365 25 @author Andy Kirkham
AjK 18:21ef26402365 26 */
AjK 18:21ef26402365 27
AjK 18:21ef26402365 28 /*
AjK 18:21ef26402365 29 This example shows how to use the new callback system. In the old system
AjK 18:21ef26402365 30 Mbed's FunctionPointer[1] type was used to store abd make calls to callbacks.
AjK 18:21ef26402365 31 However, that limits the callback function prototype to void func(void);
AjK 18:21ef26402365 32 which means we cannot pass parameters.
AjK 18:21ef26402365 33
AjK 18:21ef26402365 34 This latest version of MODSERIAL now uses its own callback object. This allows
AjK 18:21ef26402365 35 the passing of a pointer to a class that holds information about the MODSERIAL
AjK 18:21ef26402365 36 object making the callback. As of version 1.18 one critcal piece of information
AjK 18:21ef26402365 37 is passed, a pointer to the MODSERIAL object. This allows callbacks to use the
AjK 18:21ef26402365 38 MODSERIAL functions and data.
AjK 18:21ef26402365 39
AjK 18:21ef26402365 40 Additionally, since MODSERIAL and the callback parameter class MODSERIAL_IRQ_INFO
AjK 18:21ef26402365 41 are friends, MODSERIAL_IRQ_INFO can access the protected functions of MODSERIAL.
AjK 18:21ef26402365 42 This is used to ensure functions that can only be called during a callback
AjK 18:21ef26402365 43 can be invoked from a callback.
AjK 18:21ef26402365 44
AjK 18:21ef26402365 45 [1] http://mbed.org/projects/libraries/svn/mbed/trunk/FunctionPointer.h
AjK 18:21ef26402365 46 */
AjK 18:21ef26402365 47
AjK 18:21ef26402365 48
AjK 18:21ef26402365 49 #ifdef COMPILE_EXAMPLE3_CODE_MODSERIAL
AjK 18:21ef26402365 50
AjK 18:21ef26402365 51 #include "mbed.h"
AjK 18:21ef26402365 52 #include "MODSERIAL.h"
AjK 18:21ef26402365 53
AjK 18:21ef26402365 54 void rxCallback(MODSERIAL_IRQ_INFO *info) {
AjK 18:21ef26402365 55
AjK 18:21ef26402365 56 // Get the pointer to our MODSERIAL object that invoked this callback.
AjK 18:21ef26402365 57 MODSERIAL *pc = info->serial;
AjK 18:21ef26402365 58
AjK 18:21ef26402365 59 // info->serial points at the MODSERIAL instance so we can use it to call
AjK 18:21ef26402365 60 // any of the public MODSERIAL functions that are normally available. So
AjK 18:21ef26402365 61 // there's now no need to use the global version (pc in our case) inside
AjK 18:21ef26402365 62 // callback functions.
AjK 18:21ef26402365 63 char c = pc->rxGetLastChar(); // Where local pc variable is a pointer to the global MODSERIAL pc object.
AjK 18:21ef26402365 64
AjK 18:21ef26402365 65 // The following is rather daft but demos the point.
AjK 18:21ef26402365 66 // Don't allow the letter "A" go into the RX buffer.
AjK 18:21ef26402365 67 // Basically acts as a filter to remove the letter "A"
AjK 18:21ef26402365 68 // if it goes into the RX buffer.
AjK 18:21ef26402365 69 if (c == 'A') {
AjK 18:21ef26402365 70 // Note, we call the MODSERIAL_IRQ_INFO::rxDiscardLastChar() public function which
AjK 18:21ef26402365 71 // is permitted access to the protected version of MODSERIAL::rxDiscardLastChar()
AjK 18:21ef26402365 72 // within MODSERIAL (because they are friends). This ensures rxDiscardLastChar()
AjK 18:21ef26402365 73 // can only be called within an rxCallback function.
AjK 18:21ef26402365 74 info->rxDiscardLastChar();
AjK 18:21ef26402365 75 }
AjK 18:21ef26402365 76 }
AjK 18:21ef26402365 77
AjK 18:21ef26402365 78 #endif