Nuts & Volts - September 2006

INTERRUPTS IN ASSEMBLY

Because accuracy is important in a real time clock application, this is a great opportunity to introduce assembly language interrupts inserted into a Basic language program. The problem with higher level languages is the final assembly language code doesn’t easily clarify how many internal clock cycles each high level instruction takes. This program needs to respond quickly to the Timer 1 overflow so, in cases where you need to react quickly and know exactly how many clock cycles occurs, assembly language is the answer. The Microchip PICs make this even easier because each assembly language command takes only one internal clock cycle (except for branches and GOTOs).

To keep this clock accurate, I will use a small section of assembly language inserted into a PICBasic Pro program to handle the Timer1 overflow and interrupt. This way, very few clock cycles are used to handle the overflow interrupt.

At this point, you might be thinking, I just learned how to program PICs in Basic, why rush into learning assembly?

In my opinion, Basic better prepares you for assembly than any other method. Basic allows you to get a program working inside a PIC along with learning how the programmer works, how to wire the resonator or crystal, how to set up the configuration, and how to establish register and RAM (variables). It even forces you to read the data sheets a little bit. Writing assembly code after all that is just learning a few new commands that work on the bit level just like the

HIGH and LOW Basic commands do.

SOFTWARE LISTING

The software is written for PICBasic Pro. I wanted to show how to mix assembly language and BASIC to achieve an accurate real time clock. To get a quick response to a Timer1 overflow, I chose to use assembly language interrupts. This may seem a bit complex if you don’t know assembly language, but it’s really not that difficult to understand. Let’s start at the beginning to explain this software.

I start by defining the fact I’m using a bootloader to program the PIC and I’ll run that PIC at 20 MHz. Later on we’ll set up the 32.768 kHz crystal running Timer1.

DEFINE LOADER_USED 1
DEFINE OSC 20

‘ Needed for bootloader redirect
‘ 20 MHz resonator

Next, all the DEFINES for the LCD are established. I’m using PortB bits 4-7 for the data communication, bit 3 for the E line, and bit 0 for the RS line. The R/W pin is just grounded on the LCD for “write-only” operation. One of the key setups here is the DEFINE LCD_DATAUS 100. I found the default of 50 microseconds did not work well with my LCD. I had to slow down the data transmission. This is why I like to define all of the LCD setup rather than rely on the defaults.

PROJECT DETAILS

For this project, I’m going to drive a 2 x 16 LCD module to display the time generated by the Timer1 overflow. The software of the main loop will handle converting the one-second overflow into a meaningful clock display of hours : minutes : seconds. The hardware involved is not much different than my original article that started this series back in January. We do most of it in software.

The completed project is shown in Figure 1. I built it on a breadboard again, using one of my original prototype Ultimate OEM modules, but a stand-alone PIC is a simple alternative. The schematic in Figure 2 shows my setup. The key component is the 32.768 kHz crystal between the C0 and C1 pins of the PIC. It also has two 22 pf capacitors from the crystal leads to ground. The rest of the circuit drives a 2 x 16 LCD. (I’ve covered driving before.)