Nuts & Volts - March 2006

⇒ FIGURE 1. The connection setup.

requested. For the hardware, the PIC16F876A fits the job perfectly.

The PICBasic Pro compiler offers two ways to implement serial communication: the SEROUT and SERIN commands, which are the software implementation of serial “ bit-bang-ing” communication, and the HSERIN and HSEROUT commands, which implement the hardware serial port on the PIC that runs in the background.

What I mean by background is the hardware serial port can send and receive serial data while a main loop of code is also running at the same time. I felt this was a little too advanced for the beginner, so I’ll use the SERIN/SEROUT here and save the hardware commands for another application.

standard PIC circuit connections with a 4 MHz resonator and MCLR pull-up resistor. The LEDs are connected to Port B and are driven on with a high signal from the individual Port B pins.

RS232 BASICS

PROJECT DESCRIPTION

The project I’ll present here may be simple, but it’s also a building block for bigger projects in the future. I’m going to control a bank of eight LEDs from the Hyperterminal terminal program found on most PCs running Windows. The serial cable will connect the PC’s serial port to an RS232 interface circuit built into the remote PIC/LED circuit. The interface circuit will adjust the signal voltage levels to what the PIC/PC need to see.

At some point, I’ll introduce this same RS232 interface circuitry to program the PIC in-circuit via bootloader mode, but for now, all programming will be done with the EZPIC serial port programmer.

Figure 1 shows what I’m trying to describe. The PC connects to the RS232 circuit through a straight-thru nine-pin cable. The RS232 circuit uses a standard MAX232 style chip with built-in charge pump to match the 0-5V PIC signal and -12V to +12V PC signal. The PIC16F876A has the

Serial communication can send any data you want as long as it comes in the form of bytes. It can take on many forms, and one is called the RS232 standard. In Figure 2, I show a typical RS232 signal format. The first pulse is low and that is called the start bit. After that, eight bits are sent, which is a byte of data. Following the data is a high bit, which is the stop bit or end-of-data indicator.

Every character a computer can show has a numerical code associated with it, and there are 256 different codes

(the maximum amount for eight bits). This just means that there are 256 different characters that can be sent as a byte inside a serial message. This ASCII code table can be found with a simple Google search.

This RS232 signal format I described is known as 8N1 or eight

⇒ FIGURE 2. RS232 signal format.

data bits, no parity check, and one stop bit. Later, we’ll set up the Hyperterminal program to receive 8N1 data at a speed of 9600 bits per second.

Since the PC and PIC have to communicate in the same language to understand each other, the width of the pulses is critical to determine if a pulse is actually multiple 1s or 0s, or a single bit. Timing is, therefore, very important to the success of communication.

Matching the bit rate between the PC and PIC confirms the PIC and PC are talking the same version of the language. It’s like trying to watch an American citizen speak with a British citizen. They both speak English but sometimes they need a little help with interpretation.