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Microcontroller-Based Serial Data Transfer to PC

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Microcontroller-Based Serial Data Transfer To PC

T. Shankar And S. Thangamani

Communication from or to a computer has become almost essential in our day-to-day lives. Though we can execute many programs within a computer itself, it will not be of much use without outside communication.

Author’s prototype

The basic elements of a communication are:

1. A sender (source) that creates the message to be transmitted
2. A medium that carries the message
3. A receiver (sink) that receives the message

There are two types of communications— parallel and serial. Parallel data transfer can be made through an 8-bit data line for faster communication.

Serial communication is often used to control or receive data from a microcontroller. It is a form of input/output (I/O) in which the bits of a byte being transferred appear one after the other in a timed sequence on a single wire. Serial data transfer is made through a serial link between the micrcontroller and the PC using RS-232C.


Fig.1 show the waveform on a single conductor to transmit a byte (say, 0x41) serially. The upper waveform is the TTL-level waveform at the transmit pin of the microcontroller (AT89S52). The lower waveform is the same waveform converted by the RS-232 driver (MAX232) into RS-232C level. The  to assure error-free transmission over greater distances than would be possible with TTL levels.

Fig.1. Serial Waveform

Each byte is preceded by a start bit and followed by a stop bit. The start and stop bits are used to synchronise the serial receivers. The data byte is always transmitted with least-significant bit (LSB) first. For error checking, it is possible to include a parity bit as well, just prior to the stop bit.

The bits are transmitted at specific time intervals determined by the baud rate of the serial signal. The baud rate is the reciprocal of the time to send one bit. Error-free serial communication requires the baud rate, number of data bits, number of stop bits, and presence or absence of a parity bit to be the same at the transmitter and receiver ends.

Circuit description

Fig.2 shows the circuit for microcontroller- based serial data transfer to a PC. It comprises microcontroller AT89S52, RS-232 driver MAX232, regulator 7805 and a few discrete components.The microcontroller detects the switch pressed and the message is transmitted to the computer through the RS-232 driver.

Fig.2:Circuit for microcontroller-based serial
data transfer to a PC

The AT89S52 is a low-power, high performance CMOS 8-bit microcontroller. It has 8 kB of Flash, 256 bytes of RAM, 32 I/O lines, a watchdog timer, two data pointers, three 16-bit timers/counters, a six-vector two level interrupt architecture, a full-duplex serial port, and on chip oscillator and clock circuitry. In addition, the AT89S52 is designed with static logic for operation down to zero frequency and supports two software-selectable power-saving modes. The idle mode stops the CPU while allowing the RAM, timers/counters, serial port and interrupt system to continue functioning. The power - down mode saves the RAM contents but freezes the oscillator, disabling all other chip functions until the next interrupt or hardware reset.

Port pins P2.0 through P2.7 are pulled high by resistor network RNW1 and also connected to switches S1 through S8, respectively. An 11.0592MHz crystal is used for providing the basic clock frequency. All I/O pins are reset to ‘1’ as soon as RST pin goes high. Holding RST pin high for two machine cycles while the oscillator is running, resets the device. Power-on reset is derived from resistor R2 and capacitor C6. Switch S9 is used for manual reset.

The microcontroller provides a transmit channel and a receive channel for serial communication. The transmit data pin (TXD) is specified at port pin P3.1, and the receive data pin (RXD) is at port pin P3.0. RXD and TXD pins of the microcontroller are connected  to TOUT (pin 7) and RIN (pin 8) of the MAX232, respectively. TIN (pin 10) and ROUT (pin 9) of IC2 are connected to pins 2 and 3 of the COM port connector, respectively. The signals provided on these pins are TTL-level and must be boosted and inverted through a MAX232 converter to comply with the RS-232 standard.

The MAX232 has two internal charge-pumps that convert +5V into ±10V (unloaded) for RS-232 driver operation. The first converter uses capacitor C7 to double the +5V input to +10V on capacitor C9 at pin 2. The second converter uses capacitor C8 to invert +10V to -10V on capacitor C10 at pin 6.

All the serial communication is controlled through special function register SCON. This register contains mode-selection bits, ninth data bit for transmit and receive (TB8 and RB8), and serial-port interrupt bits (TI and RI). Serial communication requires standard baud rate. Timer 1 is configured in the auto-reload mode to generate the baud rate. The baud rate is determined as:

Baud rate = (2SMOD × Oscillator frequency)/( 32×12×(256 - TH1))

To derive the power supply, the 230V AC mains is stepped down by transformer X1 to deliver the secondary output of 9V, 300 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and then regulated by IC 7805 (IC3). Capacitor C2 bypasses the ripples present in the regulated 5V power supply. LED1 acts as the poweron indicator and resistor R1 limits the current through LED1.

Construction and testing

An actual-size, single-side PCB for microcontroller-based serial data transfer circuit is shown in Fig.3(View as PDF) and its component layout in Fig.4(View as PDF). Assemble the circuit on the PCB to minimise assemble the components and double- check for any overlooked error. Connect the assembled circuit to the COM port available at the back side of a computer. The information stored in the microcontroller is sent to the PC through the COM port using the HyperTerminal program.

Fig.5: HyPerterminal window
for name and icon

To open the HyperTerminal program, go to Start → Programs → Accessories → Communications → HyperTerminal. You will see a window as shown in Fig. 5. Type in the desired name and click ‘ok.’ Select a COM port (refer Fig.6) while ignoring the other options and click ‘ok.’

Fig.6: HyPerterminal window for COM port selection

Now select the baud rate as ‘9600,’ data bits as ‘8,’ parity as ‘none,’ stop bits as ‘1’ and flow control as ‘none,’ and click ‘ok’ (refer Fig.7).

Fig.7: HyPerterminal window for selection of baud
rate, data bits, parity, stop bits and flow control

When you press switches S1 through S8, the microcontroller sends the stored message and you will see the message appear on the screen as shown in Fig.8.

Fig.8: Message on HyPerterminal screen


The software for this project is given at the end of this article. It is written in ‘C’ language and compiled using Keil μ Vision3 compiler. The finally obtained ‘.hex’ file is downloaded to the microcontroller using a suitable programmer. The source program is well commented and easy to understand.

In the software, first enable timer and serial interrupts. Then configure the timer by mode and reload value to give the correct speed (9600 bits per second). Next, make the transmit interrupt 1 (TI) move the data to SCON. SCON will give the necessary UART signal and your data will be sent serially.

EFY note. The source code of this article is included in the link give below:



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