Servo - March 2010

Accessing the VEX Microcontroller I/O Ports

Now that we know that the standard VEX microcontroller uses two PIC18F8520 microcontrollers, the question is which of the external controller I/O pins correspond to the internal PIC18F8520 pins. To answer this question, refer to Table 3. IFI uses different names (aliases) for the ports than those specified in the PIC18F8520 datasheet. This can be a source of confusion since the PIC18F8520 port registers PORTA through PORTH are mapped to the VEX 16 digital and analog I/O, eight PWM ports, and five interrupts. Table 3 is the Rosetta stone for being able to carry out advanced VEX microcontroller experiments by allowing you to use the internal PIC peripherals and registers directly in your application. Another resource available from Microchip is their application notes specific to the PIC18F8520. These notes provide details on using the PIC timers, PWM, and PIC peripherals including the USART, I2C, SPI, and interrupts. You may also want to learn the PIC assembly language which gives you access to almost all the registers and peripherals, and also allows you to write very fast code needed for Interrupt Service Routines (ISRs). Assembly language routines can also be called from C (mixed mode programming).

IFI aliases for the direction register are defined as follows: • Pin directions IO1 to IO16.

• Inputs as rc_dig_in01 to rc_dig_in16.

• Outputs as rc_dig_out01 to rc_dig_out16.

The code to turn the LEDs on or off is leds.c which is available in the downloads for this article at www.servomagazine.com. It was developed using MPLAB and the PIC18 C compiler. It configures the VEX controller’s digital ports to outputs and sets each LED segment to the desired state. In the code, you see an infinite while loop and an inner loop that toggles the LEDs in binary from 0 to 255. Once compiled, the leds.hex file is generated that is used to program the VEX controller. The leds.hex file is provided as well so that it can be used without having to compile it.

You’ll need programming cable to download the necessary firmware.

Programming the VEX Controller

To program the microcontroller for semi-autonomous or fully autonomous operation, there are a few programming tool options which include Easy C 2.0, Easy C Professional, ROBOT C, and PIC18 C with MPLAB. The firmware code generated by these compilers can be downloaded using the IFI bootloader or the loader application that comes with each product. Firmware files have the extension filename.hex; they are not easy to read since they are in hexadecimal format.

VEX PIC18F8520

PIN DIRECTION INPUT OUTPUT

1

TRISAbits.TRISA0 PORTAbits.RA0 LATAbits.LATA0

2

TRISAbits.TRISA1 PORTAbits.RA1 LATAbits.LATA1

3

TRISAbits.TRISA2 PORTAbits.RA2 LATAbits.LATA2

4

TRISAbits.TRISA3 PORTAbits.RA3 LATAbits.LATA3

5

TRISAbits.TRISA5 PORTAbits.RA5 LATAbits.LATA5

6

TRISFbits.TRISF0 PORTFbits.RF0

LATFbits.LATF0

7

TRISFbits.TRISF1 PORTFbits.RF1

LATFbits.LATF1

8

TRISFbits.TRISF2 PORTFbits.RF2

LATFbits.LATF2

9

TRISFbits.TRISF3 PORTFbits.RF3

LATFbits.LATF3

10

TRISFbits.TRISF4 PORTFbits.RF4

LATFbits.LATF4

11

TRISFbits.TRISF5 PORTFbits.RF5

LATFbits.LATF5

12

TRISFbits.TRISF6 PORTFbits.RF5

LATFbits.LATF6

13 TRISHbits.TRISH4 PORTHbits.RH4

LATHbits.LATH4

14 TRISHbits.TRISH5 PORTHbits.RH5 LATHbits.LATH5

15 TRISHbits.TRISH6 PORTHbits.RH6

LATHbits.LATH6

16 TRISHbits.TRISH7 PORTHbits.RH7

LATHbits.LATH7

TABLE 2. This table is the Rosetta stone for being able to carry out advanced VEX microcontroller experiments since it shows the mapping of the PIC18F8520 ports to the controller’s digital input/output pins located on the block on top of the VEX controller. The firmware defines aliases for each of the PIC18F4520 port registers in the ifi_aliases.h file.

The next step is to download the provided led.hex application into the controller and run it. Start by copying it to your laptop or PC hard disk and place it in a folder.

Once the led.hex file has been downloaded to the VEX; the LEDs should start flashing in a pattern.

That’s a Wrap

This experiment — while relatively simple — can be used in VEX-based projects and provide status indicators using discrete LEDs, display voltage levels and motor speed using the LED BAR, and even display eight to 10-bit numeric quantities in binary. I hope you’ve gained insight in how the VEX digital and analog outputs are mapped.