1. Programming Embedded Systems in C++
Day 4: C++ for Embedded - Case Study No. 1   
22 May 2014 Colin Walls, Mentor Graphics

2. [ # ]
This is what a question slide will look like. Please answer in the chat.
Also, questions to me start with ?

3. Agenda
Monday: C++ for Embedded Programming Tuesday: C to C++ Migration Strategy Wednesday: C++ and a Real Time Operating System Thursday: Case study #1 Friday: Case Study #2

4. [ 1 ]
How big is your embedded software development team? Is it growing?

5. Encapsulation of Expertise
Software teams getting larger
Team members each have own expertise
Use objects to contain that knowledge
can be used by others without understanding
distribution and re-use simplified
2 case studies
non-volatile RAM
write-only port

6. Using NVRAM Retains data after the power has been removed
Often used for configuration parameters
Integrity checking required
Facility for initialization
Need transparent access

7. Case Study: NVRAM Issues: Access time may be limited
software must control timing
Memory may have limited lifetime
software must even out the “wear”
May have block access
software must manage buffering

8. Software Requirements
Declaration of NVRAM variables [objects]
Means to set the value
Means to retrieve the value
Must be done in a natural fashion

9. Code using NVRAM void main(void) { nvramchar x(2, 0x80000, 16);
nvramchar y(3, 0x80000, 16);
char z=99;
x = 3;
y = z;
z = x + y; }

10. nvramchar class characteristics
Constructor with 3 parameters:
NVRAM base address
size
offset
Overloaded = operator
to set the object’s value
Overloaded char operator
to retrieve object’s value

11. Assumptions in Case Study
No special timing needs
could easily be added later
Last 7 bytes are for “housekeeping”:
checksum byte
exclusive-OR of all the data bytes
signature string
“NVRAM”
NULL terminated
Re-initialization to all zeros is OK

12. NVRAM Architecture
Address
Offset
Data
Size
Checksum
Signature

13. nvramchar class class nvramchar { char* base; int size; char* data;
int checksum(void);
int sigcheck(void);
void init(void);
public:
nvramchar(int offset, unsigned baseaddress, int totalsize);
operator =(char);
operator char(void); };

14. [ 2 ]
What return type does a constructor or destructor have?

15. nvramchar Constructor
nvramchar::nvramchar(int offset, unsigned baseaddress, int totalsize) {
base = (char *)baseaddress;
size = totalsize;
data = base + offset;
if (!sigcheck() || checksum() != base[size-7])
init(); }

16. checksum() Function int nvramchar::checksum() { int check=0;
for (int i=0; i<size-7; ++i)
check ^= base[i];
return check; }

17. sigcheck() Function int nvramchar::sigcheck() {
if (strcmp(&base[size-6], "NVRAM"))
return false;
else
return true; }

18. [ 3 ]
why not:
return !strcmp(&base[size-6], "NVRAM");

19. sigcheck() Function int nvramchar::sigcheck() {
if (strcmp(&base[size-6], "NVRAM"))
return false;
else
return true; }

20. init() Function void nvramchar::init() { for (int i=0; i<size; i++)
base[i] = 0;
strcpy(&base[size-6], "NVRAM"); }

21. Operator Functions nvramchar::operator =(char value) { *data = value;
base[size-7] = checksum(); }
nvramchar::operator char()
return *data;

22. Enhancements: Memory Timing
Add private member functions:
nvramwrite()
nvramread()
Exclusive access to actual memory addresses
Functionality similar or operator functions
Timing algorithm added

23. Enhancement: Reentrancy
nvramchar::nvramchar(int offset, unsigned baseaddress, int totalsize) {
base = (char *)baseaddress;
size = totalsize;
data = base + offset;
lock();
if (!sigcheck() || checksum() != base[size-7])
init();
unlock(); }

24. Enhancement: Reentrancy
nvramchar::operator =(char value) {
lock();
*data = value;
base[size-7] = checksum();
unlock(); }

25. Enhancement: Single Size/Location Spec.
Make base address and size member variables static
Add single parameter constructor:
nvramchar::nvramchar(int offset) {
data = base + offset; }
but no guarantee that other constructor has been run first

26. Enhancement: Single Size/Location Spec.
Could eliminate 3 parameter constructor entirely
Initialize statics:
char* nvramchar::base = (char*)0x80000;
int nvramchar::size = 16;
Or use inheritance
base class for whole NVRAM
derived class for individual variables
Or use default parameters in constructor

27. Other Enhancements Validity flag for each NVRAM element
Flash RAM implementation
Public member function to clear NVRAM

28. Agenda
Monday: C++ for Embedded Programming Tuesday: C to C++ Migration Strategy Wednesday: C++ and a Real Time Operating System Thursday: Case study #1 Friday: Case Study #2

29. [ 4 ]
Are peripheral devices generally “software friendly”?

30. Question review ...

31. [ Review - 2 ]
What return type does a constructor or destructor have?

32. sigcheck() Function int nvramchar::sigcheck() {
if (strcmp(&base[size-6], "NVRAM"))
return false;
else
return true; }

33. [ Review - 3 ]
why not:
return !strcmp(&base[size-6], "NVRAM");

34. Colin Walls