89S51/52 ISP Programmer

Everyone in their daily life has been using microcontrollers knowingly or unknowingly embedded in machines like cars, TV, refrigerator, washing machine, etc. Many of you know the capabilities of microcontrollers and have worked with it. You should have programmed it using different programmers by manufacturers. Have you ever thought how does a programmer work? What is the basic principle behind it? Why do we have different programmers for different chips? In this series of tutorials we will try to get answers of such questions. We will do all the operations which are done by a programmer like read, write, erase etc. one by one. We will work with the 89S51 microcontroller belonging to the 8051 family. The same methodology can be used to program other members like 89S52 & 89S53 microcontrollers.

A programmer, also called as a burner, is a device which is used to feed a program in a microcontroller. It consists of a hardware and software. The software reads the hex file generated by the compiler and feeds it into the target microcontroller. For more details about the programmer, refer the tutorial on Microcontroller Programmer/Burner.

In this series of tutorials, we will perform every function a programmer performs one by one. We will not use any software but rather use two controllers. One is the target controller on which every operation will be performed and the second controller will be the master controller which will have the necessary codes to perform the different functions of a programmer. The objective of this exercise is to understand the concepts behind the working of a programmer and learn to design a programmer from scratch. If you are good at software designing you can design the software also for the programmer using any language.

Programming a microcontroller

In this particular article we will learn to erase the target microcontroller. The details about programming any microcontroller is given in datasheet. Datasheet discloses every minute detail about programming a controller. It tells about all the ways in which a programmer can be programmed, the circuit, the pins to be used, the signals and timing etc. There are generally two methods of programming a microcontroller

1. Parallel programming

2. Serial programming

In Parallel programming bits are transferred in multiple of 8 therefore one byte at a time. We have different address bus, data bus and control signal. Bus is collection of wires through which data is transmitted from one part to another. In this process we latch data on data bus, address on address bus and control signal are applied. As the control signals are applied data and address are accepted by the controller and data is stored in their corresponding position according to address. This method has become obsolete as the hardware needed for this method is quite complex and bulky.

In Serial programming single bit is transferred at a time using ISP protocol therefore Serial programmers also called ISP programmers. The hardware needed is very simple and small. There is no need to take out the target microcontroller from the circuit. ISP programming requires 6 pin of the target microcontroller (including Vcc and GND).

1. Vcc (Pin No. 40): Connected to +5V supply.

2. GND (Pin No. 20): Connected to Ground.

3. RESET (Pin No. 9): Made high (1). It tells the target controller that it is under the programming mode.

4. SCK (clock pin, Pin No. 8): This pin is used to supply clock to the target microcontroller.

5. MISO: Master In Slave Out (output pin, Pin No. 7): This is the output pin of the target microcontroller which sends the acknowledgement signals to the master controller.

6. MOSI: Master Out Slave In (input pin, Pin No. 6): This is the input pin of the target microcontroller. Every signal including data, address and control are received on this pin of the target microcontroller.

The circuit diagram for programming the target microcontroller using a master controller is shown below.

Fig. 1: Circuit Diagram of 8051 Programmer

Please note while programming an 89SXX series of microcontroller the pin number of the target microcontroller are fixed and not of the master microcontroller. It is not necessary to use the MOSI, MISO and SCK pin of the master controller. Any pin could be used for the purpose. However for the target controller it has to be MOSI, MISO and SCK only.

The first step is to dump any program in the target microcontroller using a programmer. Now the objective of this tutorial is to write a code in the master controller to erase the target microcontroller.

Instruction Sets

Instruction Sets (Byte pattern):

Fig. 2: Table listing 8051 instructions to perform different operations during the programming process

The above table shows the instruction sets used to perform different operations during the programming process. Every instruction is made up of 4 bytes. When a particular instruction is received by the target microcontroller, the corresponding operation is performed.

Every byte in an instruction set consists of eight bits. Every bit needs to be sent serially to the target controller. Let us first see as to how a single bit is sent to the target microcontroller in the programming mode.

The bit (data or address) to be sent is always received on the MOSI pin of the target microcontroller i.e., MOSI is the input pin of the target microcontroller. The MISO pin behaves as the output pin of the target microcontroller i.e., it sends the acknowledgement and other output signals from the target microcontroller to the master controller. It behaves as input to the master controller. The SCK pin is used to provide clock to the target controller from the master controller.

Fig. 3: Signal Diagram of MOSI, SCK and MISO pins while flashing 8051 Microcontroller

Serial Programming Characteristics, TA= – 40^OC to 85^OC, VCC = 4.0V – 5.5V

Symbol	Parameter	Min	Typ	Max	Units
1 / tCLCL	Oscillator Frequency	3		33	MHz
tCLCL	Oscillator Period	30			ns
tSHSL	SCKPulseWidth High	8tCLCL			ns
tSLSH	SCKPulseWidth Low	8tCLCL			ns
tOVSH	MOSISetup toSCK High	tCLCL			ns
tSHOX	MOSIHold after SCKHigh	2tCLCL			ns
tSLIV	SCKLow to MISO Valid	10	16	32	ns
tERASE	Chip Erase Instruction Cycle Time			500	ms
tSWC	Serial Byte Write Cycle Time			64tCLCL+ 400	µs

The above diagram shows how a single bit of data is sent and received from the target microcontroller. The following is an algorithm to send a bit from the master to the target microcontroller.

1. Make SCK low.

2. Load the bit to be sent on the MOSI pin.

3. Make SCK high. When SCK goes from low to high the bit is received by the target microcontroller.

4. The SCK should remain high for the duration mentioned in the figure above.

Codes for various operatiions

The code to send a bit from the master to the target microcontroller.

sck=0;
bit=byte/128;
byte=byte<<1;
mosi=bit;
sck=1;
delay(1);
_nop_();
sck=0;
delay(1);
_nop_();

In order to send the next bit the same process is repeated.

Whenever the target microcontroller receives a bit during the programming mode, it acknowledges the receipt of the bit by sending an acknowledgement bit on the MISO pin, which should be received by the master controller. The algorithm for receiving the bit by the master is:

1. Make SCK high.

2. Make the input pin of the master controller active to take input from the MISO pin.

3. Make SCK low.

If you carefully look at the timing diagram the transmission and reception of a single bit can be completed in a single clock cycle. The timing for the high and low pulse of different signals (MOSI, MISO and SCK) has been given in the table above.

The code to receive a single bit is as follows:

sck=1;
delay(1);
_nop_();
data=data<<1;
if(miso==1)
	data++; 
sck=0;
delay(1);
_nop_();

Codes Contd..

The following is the code to send and receive a single bit to and from the target controller:

sck=0;
bit = byte/128;
byte=byte<<1;
mosi=bit;
sck=1;
delay(1);
_nop_();
data=data<<1;
if(miso==1)
	data++; 
sck=0;
delay(1);
_nop_();

Now let us see how an entire byte comprising of eight bits is sent and received serially to and from the target microcontroller.

Fig. 4: Signal Diagram showing bytes transferred through MOSI, SCK and MISO pins while flashing 8051 Microcontroller

The above timing diagram shows how a byte is sent and received serially. A careful observation shows that when the transmission and reception of a single bit is repeated 8 times a byte will be transferred and received from the target microcontroller. The important point to be noted is the most significant bit (MSB) of the byte in sent and received first.

The algorithm for transmission and reception of a single byte using the serial ISP mode is as follows.

1. Extract the MSB from the data to be sent by dividing by 128. In this case the data to be sent is stored in the variable “byte”.

2. Left shift the byte to be sent by 1 so that the next significant bit becomes the MSB.

3. Load the bit on the MOSI pin of target microcontroller.

4. Make SCK 1.

5. Provide a delay of 30 ms.

6. The acknowledgement data coming from the target microcontroller is received in the variable “data”.

7. Left shift the variable “data” by 1 so that the MSB which is going to be received from the target microcontroller can be received.

8. Check the MISO pin of target microcontroller. If it is “1” increment the data variable by 1.

9. Make SCK 0.

10. Provide a delay of 30 ms.

11. Repeat the steps 1-10 eight times to send and receive 1 byte.

The code for the sending and receiving a single byte is as follows:

sck=0;
for(i=0;i<8;i++)
{
	bit = byte/128;
	byte=byte<<1;
	mosi=bit;
	sck=1;
	delay(1);
	_nop_();
	data=data<<1;
	if(miso==1)
		data++; 
	sck=0;
	delay(1);
	_nop_();
}