Using flash memory in MSP430

This article is about using the flash memory in MSP430. This will not teach you C programming, using MSP430 or flash memory basics. They are well documented on the internet. This article will help all of us who are struggling to use the flash memory.

I started off a project on MSP430G2553(Now on, I will refer it as micro). Soon I realised that I needed to store a large amount of data. I needed to store 4096 bytes. However, there was only 256 bytes of EEPROM space but 16KB flash memory. Initially, I was reluctant to use the flash but had no other option. So I started off. The trouble is that flash memory is not easy to handle. But not that difficult either. Reading is straight forward. There will be a pointer to the flash space and you just need to reference that pointer. However, the trouble starts with writing data.

The flash is arranged as segments A, B, C, D and other segments. A, B, C, D are 64 byte segments and the others are 512 byte segments. A in particular is locked as it contains information about calibration. When you wish to write to a segment, you need to erase it first. Even if you want to store one single byte, you need to erase the whole segment.

Let us first look at the flash erase function

// __DINT() is in IAR workbench
void flash_erase(int *addr)
  _DINT();                             // Disable interrupts. This is important, otherwise,
                                       // a flash operation in progress while interrupt may
                                       // crash the system.
  while(BUSY & FCTL3);                 // Check if Flash being used
  FCTL2 = FWKEY + FSSEL_1 + FN3;       // Clk = SMCLK/4
  FCTL1 = FWKEY + ERASE;               // Set Erase bit
  FCTL3 = FWKEY;                       // Clear Lock bit
  *addr = 0;                           // Dummy write to erase Flash segment
  while(BUSY & FCTL3);                 // Check if Flash being used
  FCTL1 = FWKEY;                       // Clear WRT bit
  FCTL3 = FWKEY + LOCK;                // Set LOCK bit

Initially, the interrupts need to be disabled. After that, the flash must be configured in erase mode. The information about the registers can be obtained from the datasheet. FWKEY is the password for the flash memory and should be included for every action. After configuring the registers, a dummy write must be done to the given address. This will erase the whole segment. Once the erase is complete, the memory should be locked and interrupts enabled again.
Once a segment is erased, data can be written to it. In the following example, I have used two global registers, data_buffer and data_dump which hold 64 16 bit integers each.

void flash_write(int *addr, char option)
  _DINT();                             // Disable interrupts(IAR workbench).
  int i = 0;
  FCTL2 = FWKEY + FSSEL_1 + FN0;       // Clk = SMCLK/4
  FCTL3 = FWKEY;                       // Clear Lock bit
  FCTL1 = FWKEY + WRT;                 // Set WRT bit for write operation
  if(option == WRITE_FROM_BUFFER)
    for (i=0; i<64; i++)
      *addr++ = data_buffer[i];         // copy value to flash
  else if(option == WRITE_FROM_DUMP)
    for (i=0; i<64; i++)
      *addr++ = data_dump[i];           // copy value to flash

  FCTL1 = FWKEY;                        // Clear WRT bit
  FCTL3 = FWKEY + LOCK;                 // Set LOCK bit

The code is self explanatory. Though I have used data buffers, there shouldn’t be any problem writing single data. Note that each segment is 512 bytes and hence it can hold 256 integers(each integer in msp430 is 2 bytes). Hence, once a segment is erased, we can perform 4 buffer writes. Also note that addresses 0xfe00-0xffff hold interrupt vectors. Hence don’t use the segment which will overwrite these. This will result in the microcontroller malfunctioning after restart since the interrupt vector will get erased and the microcontroller will not know where to start from.
With the above two functions, let us look at a simple example where we save 256 integers

void main(){
	// Assume all initiations are done.
	// flash_erase erases a segment
	// flash_write writes 64 values
	// get_value() is any function which returns some value
	// print() is a generic function which prints the value given as input
	int *addr = (int *)0x0E000	// Address of the flash memory segment starting
	int *addr_cpy;				// A copy of the address
	int buffer[64];				// data buffer
	int i, j;
	for(j=0; j<4; j++){
		for(i=0; i<64; i++){
			buffer[i] = get_value();	// Save a value.
		flash_write(addr + j*64, buffer, WRITE_FROM_BUFFER);	// Write the values to the flash
	// Now print all the values
	for(i=0; i<256; i++)

That ends our article on using flash memory. Please feel free to comment. I would be more than happy to learn from you.


Image registration using python

As per wikipedia, ‘Image registration is the process of transforming different sets of data into one coordinate system’. Simply put, image registration is comparing images with a base image and quantifying the changes. For example when a video is streamed, we would like to find out whether the camera is changing its position, and if so, how do you correct it.

We will look at a snippet written in python which compares two images and returns a homography matrix which relates both the images.

A homography matrix is a 3×3 matrix which decides the transformation between two set of coordinate systems.Mathematically, if A is a set of coordinates in one space and B is a set of corresponding coordinates in another space, then

A = HB

Wher H is our homography matrix.

To calculate our homography matrix,we need a set of corresponding points in the two spaces. For this, SIFT(Scale Invariant Feature Transform) is used. SIFT generates a set of ‘interest’ points which can be used for tracking an image.

Once we get the corresponding points, we can calculate the homography matrix. But we need a good algorithm which will pick the necessary points and discard the outliers. To implement this, we use the RANSAC algorithm. RANSAC(RANdom SAmple Consensus) algorithm picks a minimum number of points from a given set and estimates the model. It then takes into account more points from the set which closely fit the model and then calculates the error in the estimated model. This is done for a predefined number of times to get a good model. This model ensures that only inliers are included and outliers are not taken into account.

Using SIFT implementation in python and calculation of homography matrix in python, we apply a RANSAC algorithm to find the homography matrix and change the first image accordingly so that it matches the orientation of the second image.

You can find the source code here:

Python image registration

For further study:
1. Image registration
2. Scale Invariant Feature Transform

Note: SIFT is a patented technique by David Lowe. It is protected by the following patents:
Method and apparatus for identifying scale invariant features in an image and use of same for locating an object in an image
David G. Lowe, US Patent 6,711,293 (March 23, 2004). Provisional application filed March 8, 1999. Asignee: The University of British Columbia

SPICE Backend

SPICE stands for Simulating Program with Integrated Circuit Emphasis. Essentially a program for simulation of electrical circuits. We will look at a very small spice backend, written in C. The project was part of CAD Lab assignment for electrical engineering students.
Since there will be many equations, i have compiled the document into a PDF using LYX, the document processor. But i will briefly explain about the project.

Circuit simulation, is an easy task, till there are only few nodes. However, as the nodes and elements increase, the complexity increases exponentially. To ease our task and solve the repetitive process of solving equations, we write a simple program.

Our job, as the end user is very simple:
1. Write a net list, containing all the information about the circuit. Please note that, you need to draw the circuit on paper, label the nodes and write the net list accordingly.
2. Invoke the program.

Simple isnt it?

Now let us look from the programmer’s perspective:
1. Read the netlist. Convert all the data into numericals and node names. Scan for all errors
2. Create the circuit as a set of structure elements.
3. Create a matrix to solve the simultaneous linear equations
4. Write the output to file

The first step is a little difficult among all of them. Parsing a file in C with numericals, especially with muultipliers like nano, pico, micro etc is not trivial.

Once done, we use linked lists to store the information about the elements, the nodes to which it is attached, the values, the dependencies if any etc. Once this is done, we traverse the linked list again to fit all the values into a neat matrix. Solving this matrix will give back all the node voltages.

Download the following file to read about the spice backend.


A few things i wanted to add:

1. The elements as plugins to the program, so that it is possible to add new elements

2. Better matrix inversion algorithms

3. A GUI frontend

In case you want the code, please drop a comment with your mail ID, so that i can mail it to you.

Canny Edge Detection

Edge detection algorithms are not a part of the trivial programs. They comprise of an advanced thinking algorithm and implementation. There are various types of Edge detection algorithms:

1. Laplace : This edge detection Gives the second order differentiation of an image. A very crude one

2. Prewitt: This edge detection is a composition of two edge maps. One is the original image differentiated along the horizontal axis and the other differentiated along the vertical axis. Adding these two as two perpendicular vectors will give the final edge map.

3. Sobel: Similar to Prewitt, but the differentiation technique is different.

4. Canny: The most complex of all the edge detection algorithms. It is considered as the most efficient one.

We will now look at the process of the canny edge detection

Canny edge detection is a four step process.

1. Initially, a gaussian blur is applied to clear any speckles and free the image of noise.

2. Now a Sobel or Prewitt operator is applied to get two edge maps. Horizontal edge map gradx and vertical edge map grady. The edge map is a combination of these two edge maps. Also, The direction of gradient at each point, i.e, the angle is calculated by

theta = arctan(grady/gradx)

Now, since we are interested in only major edges, we will remove edges below a certain threshold.

3. A non-maximum suppression algorithm is applied now. The concept is very simple. For a given pixel, there are 8 neighboring pixels. Hence, there are 4 possible directions, 0 degrees,45 degrees, 90 degrees and 135 degrees.

The angles are hence quantized into these 4 angles(We will later look how we do it in python). Now consider a pixel in the image. Let us assume that the angle of gradient at that point is 0 degrees. This implies that the direction of edge should be perpendicular to it,i.e running along the 90 degrees line. Hence we will check for the left and the right neighbouring gradient values. If the gradient at the present pixel is greater than its left and right neighbours, then it is considered an edge, else it is discarded. This process gives single pixel thick edges.

4. Now a hysteresis based edge scanning is done. It is known that major important edges will be along curves, not as isolated points. A two threshold scanning is done. The high threshold, Th will decide the starting of the edge and the Tl will decide the ending of the edge. Once this process is done, we get a binary map of the edges.

Let us see how it is implemented in Python using scipy. The steps are numbered in the same order as above.

Few points first:

P1.Why should an image be differentiated to get its image? Since, the grayscale value of the image changes much rapidly near the edges than other places. When the gray scale value is almost constant , the differentiation at that pixel will not yield any sharp edges.

P2. We will be using a term called convolution. I will briefly present it, as a tool, not as a concept that is taught for electrical engineers.

Consider a 3×3 matrix | 0 1 0|

| 1 1 1|

|0 1 0|

We know that our image is actually a huge matrix, with each value showing the grayscale intensity. Convlution is running the 3×3 matrix across this image. What does that mean? It means that, The above matrix is centered at every pixel in the image. Now, the 3×3 matrix’s values are multiplied with the correspoding points in the image and added. This result replaces the current value of the pixel. Hence if we want a differentiation matrix ( we will call the multiplying 3×3 matrix as kernel from now), we have:

|-1 0 1|

|-1 0 1|

|-1 0 1|

This is called a Prewitt operator.

Now let us go ahead and do it in python.

1. The code depends on scipy for array operations. First, the image is loaded as a grayscale image array. A gaussian kernel with a chosen standard deviation and kernel size is applied to the image to remove any noise.

2. A Prewitt operator is applied to create two images, horizontally differentiated and vertically differentiated. Let us call them gradx and grady respectively. The gradient map is given by grad = squareroot(gradx^2+grady^2), and the angle of gradient at each point is arctan(grady/gradx). In the code, arctan2 function is used to find angle, since arctan2 also takes care of the signs of the numerator and denominator. arctan2 output runs from -180 to 180. We add 180 further to make the angles go from 0 to 360.

3. Now we quantize the angles to help the non maximum suppression. The division is as follows:

a.0-22.5 or 157.5-202.5 or 337.5-360 => 0 degrees

b.22.5-67.5 or 202.5-247.5 =>45 degrees

c.67.5-112.5 or 247.5 to 292.5 =>90 degrees

d.112.5-157.5 or 292.5-337.5 => 135 degrees.

Having quantized the angles, the previously discussed non maximum suppression algorithm is run to get a single pixel thick edges of the image.

4. The final step is hysteresis edge detection. For our test image, a Th of 50 and Tl of 5 is used. This implies that, an edge is started if the pixel gray scale value is greater than 50 and is terminated when the grayscale value is less than 5. The edges are given white colour and the non edges are left black. The following is the final image obtained.

I have also included the Python code to help you understand better. Please note that you will need python along with scipy to use the module. Also, Python Image Library can be optionally installed to view the image.

Module for Canny edge detection
Requirements: 1.scipy.(numpy is also mandatory, but it is assumed to be
                      installed with scipy)
              2. Python Image Library(only for viewing the final image.)
Author: Vishwanath
    import Image
except ImportError:
    print 'PIL not found. You cannot view the image'
import os

from scipy import *
from scipy.ndimage import *
from scipy.signal import convolve2d as conv

class Canny:
        Create instances of this class to apply the Canny edge
        detection algorithm to an image.

        input: imagename(string),sigma for gaussian blur
        optional args: thresHigh,thresLow

        output: numpy ndarray.

        P.S: use canny.grad to access the image array        

        1. Large images take a lot of time to process, Not yet optimised
        2. thresHigh will decide the number of edges to be detected. It
           does not affect the length of the edges being detected
        3. thresLow will decide the lenght of hte edges, will not affect
           the number of edges that will be detected.

        usage example:
        >>>canny = Canny('image.jpg',1.4,50,10)
        >>>im = canny.grad
    def __init__(self,imname,sigma,thresHigh = 50,thresLow = 10):
        self.imin = imread(imname,flatten = True)

        # Create the gauss kernel for blurring the input image
        # It will be convolved with the image
        gausskernel = self.gaussFilter(sigma,5)
        # fx is the filter for vertical gradient
        # fy is the filter for horizontal gradient
        # Please not the vertical direction is positive X
        fx = self.createFilter([1, 1, 1,
                                0, 0, 0,
        fy = self.createFilter([-1,0,1,

        imout = conv(self.imin,gausskernel)[1:-1,1:-1]
        gradx = conv(imout,fx)[1:-1,1:-1]
        grady = conv(imout,fy)[1:-1,1:-1]

        # Net gradient is the square root of sum of square of the horizontal
        # and vertical gradients

        grad = hypot(gradx,grady)
        theta = arctan2(grady,gradx)
        theta = 180 + (180/pi)*theta
        # Only significant magnitudes are considered. All others are removed
        x,y = where(grad < 10)
        theta[x,y] = 0
        grad[x,y] = 0

        # The angles are quantized. This is the first step in non-maximum
        # supression. Since, any pixel will have only 4 approach directions.
        x0,y0 = where(((theta<22.5)+(theta>157.5)*(theta<202.5)
                       +(theta>337.5)) == True)
        x45,y45 = where( ((theta>22.5)*(theta<67.5)
                          +(theta>202.5)*(theta<247.5)) == True)
        x90,y90 = where( ((theta>67.5)*(theta<112.5)
                          +(theta>247.5)*(theta<292.5)) == True)
        x135,y135 = where( ((theta>112.5)*(theta<157.5)
                            +(theta>292.5)*(theta<337.5)) == True)

        self.theta = theta
        Image.fromarray(self.theta).convert('L').save('Angle map.jpg')
        self.theta[x0,y0] = 0
        self.theta[x45,y45] = 45
        self.theta[x90,y90] = 90
        self.theta[x135,y135] = 135
        x,y = self.theta.shape        
        temp ='RGB',(y,x),(255,255,255))
        for i in range(x):
            for j in range(y):
                if self.theta[i,j] == 0:
                elif self.theta[i,j] == 45:
                elif self.theta[i,j] == 90:
                elif self.theta[i,j] == 45:
        self.grad = grad.copy()
        x,y = self.grad.shape

        for i in range(x):
            for j in range(y):
                if self.theta[i,j] == 0:
                    test = self.nms_check(grad,i,j,1,0,-1,0)
                    if not test:
                        self.grad[i,j] = 0

                elif self.theta[i,j] == 45:
                    test = self.nms_check(grad,i,j,1,-1,-1,1)
                    if not test:
                        self.grad[i,j] = 0

                elif self.theta[i,j] == 90:
                    test = self.nms_check(grad,i,j,0,1,0,-1)
                    if not test:
                        self.grad[i,j] = 0
                elif self.theta[i,j] == 135:
                    test = self.nms_check(grad,i,j,1,1,-1,-1)
                    if not test:
                        self.grad[i,j] = 0
        init_point = self.stop(self.grad, thresHigh)
        # Hysteresis tracking. Since we know that significant edges are
        # continuous contours, we will exploit the same.
        # thresHigh is used to track the starting point of edges and
        # thresLow is used to track the whole edge till end of the edge.
        while (init_point != -1):
            print 'next segment at',init_point
            self.grad[init_point[0],init_point[1]] = -1
            p2 = init_point
            p1 = init_point
            p0 = init_point
            p0 = self.nextNbd(self.grad,p0,p1,p2,thresLow)
            while (p0 != -1):
                #print p0
                p2 = p1
                p1 = p0
                self.grad[p0[0],p0[1]] = -1
                p0 = self.nextNbd(self.grad,p0,p1,p2,thresLow)
            init_point = self.stop(self.grad,thresHigh)

        # Finally, convert the image into a binary image
        x,y = where(self.grad == -1)
        self.grad[:,:] = 0
        self.grad[x,y] = 255

    def createFilter(self,rawfilter):
            This method is used to create an NxN matrix to be used as a filter,
            given a N*N list
        order = pow(len(rawfilter),0.5)
        order = int(order)
        filt_array = array(rawfilter)
        outfilter = filt_array.reshape((order,order))
        return outfilter
    def gaussFilter(self,sigma,window = 3):
            This method is used to create a gaussian kernel to be used
            for the blurring purpose. inputs are sigma and the window size
        kernel = zeros((window,window))
        c0 = window // 2

        for x in range(window):
            for y in range(window):
                r = hypot((x-c0),(y-c0))
                val = (1.0/2*pi*sigma*sigma)*exp(-(r*r)/(2*sigma*sigma))
                kernel[x,y] = val
        return kernel / kernel.sum()

    def nms_check(self,grad,i,j,x1,y1,x2,y2):
            Method for non maximum supression check. A gradient point is an
            edge only if the gradient magnitude and the slope agree

            for example, consider a horizontal edge. if the angle of gradient
            is 0 degress, it is an edge point only if the value of gradient
            at that point is greater than its top and bottom neighbours.
            if (grad[i,j] > grad[i+x1,j+y1]) and (grad[i,j] > grad[i+x2,j+y2]):
                return 1
                return 0
        except IndexError:
            return -1
    def stop(self,im,thres):
            This method is used to find the starting point of an edge.
        X,Y = where(im > thres)
            y = Y.min()
            return -1
        X = X.tolist()
        Y = Y.tolist()
        index = Y.index(y)
        x = X[index]
        return [x,y]
    def nextNbd(self,im,p0,p1,p2,thres):
            This method is used to return the next point on the edge.
        kit = [-1,0,1]
        X,Y = im.shape
        for i in kit:
            for j in kit:
                if (i+j) == 0:
                x = p0[0]+i
                y = p0[1]+j
                if (x<0) or (y<0) or (x>=X) or (y>=Y):
                if ([x,y] == p1) or ([x,y] == p2):
                if (im[x,y] > thres): #and (im[i,j] < 256):
                    return [x,y]
        return -1
# End of module Canny