Programming Tips

This wiki describes an example coding of the freertos_posix driver for the dsPic33 development board. Please refer to the actual coding used from here.

Memory

Memory Map for dsPIC33FJ256GP506

Table 11.1 Memory Location

Type	Start Address	End Address	Size
Flash	0x000000	0x0157FF	86K[1]
+--Flash: Reset Vector	0x000000	0x000003	4
+--Flash: Interrupt Vector Table	0x000004	0x0000FF	252
+--Flash: Alternate Vector Table	0x000104	0x0001FF	252
+--Flash: User Program	0x000200	0x0157FF	85.5K
Programming Executive	0x800000	0x800FFF	4K[1]
Config Registers	0xF80000	0xF80017	24
Device ID (0xE5)	0xFF0000	0xFF0003	4

[1] Each address is 16-bit wide. Every two addresses correspond to a 24-bit instruction. Each even address contains 2 valid bytes; each odd address contains 1 valid byte plus 1 phathom byte.

Data Location

Type	Description	Example
_XBSS(N) [1]	RAM Data in X-memory, aligned at N, no initilization	int _XBSS(32) xbuf[16];
_XDATA(N) [1]	RAM Data in X-memory, aligned at N, with initilization	int _XDATA(32) xbuf[] = {1, 2, 3, 4, 5};
_YBSS(N) [1]	RAM Data in Y-memory, aligned at N, no initilization	int _YBSS(32) ybuf[16];
_YDATA(N) [1]	RAM Data in Y-memory, aligned at N, with initilization	int _YDATA(32) ybuf[16] = {1, 2, 3, 4, 5};
__attribute__((space(const)))	Flash ROM data, constant, accessed by normal C statements, but 32K max.	int i __attribute__((space(const))) = 10;
__attribute__((space(prog)))	Flash ROM data, read/write by program space visibility window (psv)	int i __attribute__((space(prog)));
__attribute__((space(auto_psv)))	Flash ROM data, read by normal C statements, write by accessing psv	int i __attribute__((space(auto_psv)));
__attribute__((space(psv)))	Flash ROM data, read/write by (psv)	int i __attribute__((space(psv)));
_EEDATA(N) [1]	ROM Data in EEPROM, aligned at N, read/write with psv	int _EEDATA(2) table[]={0, 1, 2, 3, 5, 8};
_PERSISTENT	RAM Data, data remain after reset	int _PERSISTENT var1, var2;
_NEAR	RAM Data at near section	int _NEAR var1, var2;
__attribute__((__interrupt__))	Interrupt service rountine	void __attribute__((__interrupt__)) _INT0Interrupt(void);
_ISRFAST	Fast interrupt service rountine	void _ISRFAST _T0Interrupt(void);

1. N must be a power of two, with a minimum value of 2.

<asm/types.h>

• The following maps the basic data types:

 typedef unsigned char           __u8;
 typedef char                    __s8;
 typedef unsigned int            __u16;
 typedef int                     __s16;
 typedef unsigned long           __u32;
 typedef long                    __s32;
 typedef unsigned long long      __u64;
 typedef long long               __s64;
 
 //to be used in <time.h>
 typedef unsigned long           time_t;

• The following macros are the platform-dependent

 /** Interrupt Request */
 #define _IRQ                    __attribute__((__interrupt__))
 /** TRAP IRQ for saving program counter: declare __u16 StkAddrLo, StkAddrHi in trap.c (order matters) */
 #define _TRAP_IRQ               __attribute__((__interrupt__(__preprologue__( \
                                 "mov #_StkAddrHi,w1\n\tpop [w1--]\n\tpop [w1++]\n\tpush [w1--]\n\tpush [w1++]"))))
 /** IO Stub Functions are placed in .libc section so that the standard libraries can access these functions using short jumps. */
 #define _LIBC                   __attribute__((section(".libc")))
 /** FAST RAM */
 #define _DMA                    __attribute__((space(dma),aligned(256)))

Custom Linker Script to Maximize Space for Constant Data

• Constant data declared using keyword const will be stored in the .const section in the flash memory.
• Normally, during compilation, the linker will assign these data after the program code (.text section).
• Since .const is accessed by auto-psv function, to maximize the space for constant data (32kb), the .const section needs to be aligned at 0x80000 boundary.
• This requires the following change in linker script:

 __CONST_BASE = 0x8000;
 
 .text __CODE_BASE :
 {
 	*(.reset);
       *(.handle);
       *(.libc) *(.libm) *(.libdsp);  /* keep together in this order */
       *(.lib*);
       /* *(.text);		deleted to maximize space for const data */
 } >program
 
 .const __CONST_BASE :
 {
 	*(.const);
 } >program

• If your program is large, after this change in linker script, function calls may involve large jump in the memory map (>32kB). As a result, you may need to enable the large code and large memory model during compilation. In such case, use the following options in your build path:

   -mlarge-code -mlarge-data

• Meanwhile, functions that are defined in the standard C libraries, but are replaced with your own implementations (e.g. I/O stubs: open(), read(), write(), lseek(), ioctl() etc.) may have the following linker error:

   /usr/pic30-elf/lib//libc-elf.a(fflush.eo)(.libc+0x3c): In function '.LM11':
   : Link Error: relocation truncated to fit: PC RELATIVE BRANCH _write
   /usr/pic30-elf/lib//libc-elf.a(fclose.eo)(.libc+0x42): In function '.LM18':
   : Link Error: relocation truncated to fit: PC RELATIVE BRANCH _close 

• To resolve the problem, you need to place the functions in the .libc section rather than in the .text section, like this:

   int _LIBC open(const char *pathname, int flags){ ... }
   int _LIBC close(int fd){ ... }
   int _LIBC write(int fd, void* buf, int count) { ... }
   int _LIBC read(int fd, void* buf, int count) { ... }
   int _LIBC ioctl(int fd, int request, void* argp) { ... }
   int _LIBC lseek(int fd, int offset, int whence) { ... }

System Setup

Clock Speed

• System clock source can be provided by:

1. Primary oscillator (OSC1, OSC2)
2. Secondary oscillator (SOSCO and SOSCI) with 32kHz crystal
3. Internal Fast RC (FRC) oscillator at 7.37MHz (7372800Hz)
4. Low-Power RC (LPRC) oscillator (Watchdog Timer) at 512 kHz.

• These clock sources can be incorporated with interal Phase-locked-loop (PLL) x4, x8 or x16 to yield the osciallator frequrence F_OSC
• The system clock is divided by 4 to yield the internal instruction cycle clock, F_CY=F_OSC/4

System Clock

• Each timer is 16-bit (i.e. counting from 0 to 65535).
• Prescale is the ratio between timer counts and system clock counts. Prescales of 1:1, 1:8, 1:64 and 1:256 are available.
• Let required time for ticking be PERIOD.
• Number of instruction cycles during PERIOD = PERIOD*F_CY cycles
• Using a prescale of 1:x, the timer period count register = # of cycles/x
• e.g. PERIOD = 10ms; # of cycles = 10ms*40MHz = 400000 cycles; Using 1:8 Prescale, register setting = 400000/8 = 50000

  void
  prvSetupTimerInterrupt (void)
  {
    T1CON = 0;
    TMR1 = 0;
    PR1 = 50000;
    //============================================================
    IPC0bits.T1IP = configKERNEL_INTERRUPT_PRIORITY;
    IFS0bits.T1IF = 0;
    IEC0bits.T1IE = 1;
    //============================================================
    T1CONbits.TCKPS0 = 1;
    T1CONbits.TCKPS1 = 0;
    T1CONbits.TON = 1; 
  }
  //********************************************************************
  void _IRQ 
  _T1Interrupt (void)
  {
    IFS0bits.T1IF = 0;
    vTaskIncrementTick();
    portYIELD();
  }

<asm/system.h>

• Registers are involved in Interrupts includes:

1. Interrupt Flag Status (IFS0-IFS2) registers
2. Interrupt Enable Control (IEC0-IEC2) registers
3. Interrupt Priority Control (IPC0-IPC10) registers
4. Interrupt Priority Level (IPL) register
5. Global Interrupt Control (INTCON1, INTCON2) registers
6. Interrupt vector (INTTREG) register

• User may assign priority level 0-7 to a specific interrupt using IPC. Setting priority to 0 disable a specific interrupt. Level 7 interrupt has the highest priority.
• Current priority level is stored in bit<7:5> of Status Register (SR). Setting Interrupt Priority Level (IPL) to 7 disables all interrupts (except traps).
• sti() and cli() can be defined to enable and disable global interrupts for time critical functions:

 #define IPL              ( 0x00e0 )
 #define cli()            SR |= IPL    //Set IPL to 7
 #define sti()            SR &= ~IPL   //Set IPL to 0

POSIX System Call and Drivers

• POSIX System calls (open(), close(), read(), write(), lseek()) are used to access hardware devices related to data stream.
• The file descriptor return by open() for these devices are statically assigned at compile time.

UART

• Serves as the default communication channel for STDIN, STDOUT and STDERR.
• Implementation of this driver allows transparent operation of printf() in standard C library.

I²C

• A number of I2C devices can be added using this driver (e.g. I2C DAC, I2C EEPROM, etc)
• Two lines are devoted for the serial communication. SCL for clock, SDA for data.
• Standard communication speed includes

1. Standard speed mode: 100kHz
2. Fast speed mode: 400kHz
3. High speed mode: 3.4MHz

• Pull-up resistors are required for both SCL and SDA. Minimum pull-up resistance is given by:

    Pull-up resistor (min) = (Vdd-0.4)/0.003  ......  [See section 21.8 in Family reference manual]

• 2.2Kohm is typical for standard speed mode.
• After initiating a start/stop/restart bit, add a small delay (e.g. no operation) before polling the corresponding control bit (hardware controlled).
• After sending a byte and receiving an acknowledgment from the slave device, ensure to change to idle state.

ADC

• 12-bit ADC: (Max 18 Channels)
• ADC uses DMA to buffer the adc data.
• A maximum of 500kps of sampling rate when using auto sampling mode.

Simple PWM (Output Compare Module)

• The PWM module consists of 8 channels using the output compare module of dsPic.
• These channels are locate at pin 46 (OC1), 49 (OC2), 50 (OC3), 51 (OC4), 52 (OC5), 53 (OC6), 54 (OC7), 55 (OC8). These pins are shared with port D.
• The range of PWM freqeuencies obtainable is 2Hz to 15MHz (See Figure 6.3). Suggested range of operation is 2Hz to 120kHz. The relationship between resolution r and PWM frequency f_PWM is given by:

        fPWM = fCY/(Prescale*10rlog(2))

Relationship of Resolution and PWM Frequency

Resolution (bit)	Prescale=1	Prescale=8	Prescale=64	Prescale=256
1	15,000,000	1,875,000	234,375	58,594
2	7,500,000	937,500	117,188	29,297
3	3,750,000	468,750	58,594	14,648
4	1,875,000	234,375	29,297	7,324
5	937,500	117,188	14,648	3,662
6	468,750	58,594	7,324	1,831
7	234,375	29,297	3,662	916
8	117,188	14,648	1,831	458
9	58,594	7,324	916	229
10	29,297	3,662	458	114
11	14,648	1,831	229	57
12	7,324	916	114	29
13	3,662	458	57	14
14	1,831	229	29	7
15	916	114	14	4
16	458	57	7	2

Flash-emulated EEPROM

• Using built-in functions __builtin_tblpage(), __builtin_tbloffset() to set special-purpose registers to access flash memory
• Using assembly code to read and write flash memory.

DSP Library

• Not POSIX compliant
• Library functions in <dsp.h> include the following categories:

1. Vector
2. Window
3. Matrix
4. Filtering
5. Transform
6. Control

Data Types

• Signed Fractional Value (1.15 data format)
  • Inputs and outputs of the dsp functions adopt 1.15 data format, which consumes 16 bits to represent values between -1 to 1-2^-15 inclusive.
  • Bit<15> is a signed bit, positive = 0, negative = 1.
  • Bit<14:0> are the exponent bits e.
  • Positive value = 1 - 2^-15*(32768 - e)
  • Negative value = 0 - 2^-15*(32768 - e)
• 40-bit Accumulator operations (9.31 data format)
  • The dsp functions use the 40 bits accumalators during arithmatic calculations.
  • Bit<39:31> are signed bits, positive = 0x000, negative = 0x1FF.
  • Bit<30:0> are exponent bits.
• IEEE Floating Point Values
  • Fractional values can be converted to Floating point values using: fo = Fract2Float(fr); for fr = [-1, 1-2^-15]
  • Floating point values can be converted to Fractional values using: fr = Float2Fract(fo); or fr = Q15(fo); for fo = [-1, 1-2^-15]
  • Float2Fract() is same as Q15(), except having saturation control. When +ve >= 1, answer = 2¹⁵-1 = 32767 (0x7FFF). When -ve < -1, answer = -2¹⁵ = -32767 (0x8000)