Placa para aquisição de dados USB

Esta placa de aquisição de dados para ligar ao porto USB possui oito saídas digitais, oito entradas digitais, duas saídas analógicas com uma resolução de 10 bits e oito entradas analógicas de 10 bits, para tensões que variam entre 0 e 5 V. O núcleo do sistema é o microcontrolador PIC18F4550 da Microchip, programado em linguagem C. O circuito é montado numa placa de circuito impresso muito compacta e não requer nenhuma fonte de alimentação externa.

Hardware

In the circuit diagram, the USB+ and USB– pins on K2 should be swapped. The PCB track layout shown in the article should also be corrected. PCBs supplied through the Elektor Shop have the correct USB signal routing and are not affected.

Although different values are indicated in the circuit diagram and the parts list, both 27 Ω and 33 Ω resistors may be used in positions R3 and R4.

Software

Contrary to what some readers have written there is no bug in the firmware CUSTOM_TAD. If it is compiled with MPLAB C18 C Compiler v3.02 as described in the magazine, in the section ‘PIC Firmware’, it compiles perfectly (6 warnings appear, without importance) and the .hex file is generated without problems.

However, if the firmware is compiled with a later version of the C18 Compiler, for instance, with MPLAB C18 C Compiler v3.14 (recent version), some errors appear that inhibit compilation. To be able to compile successfully, the following modifications must be made:

1. File main.c, line 52

      Text: #pragma config FCMEM = OFF

      Should be: #pragma config FCMEN = OFF

2. File user.c, line 76

      Text: OpenPWM2(0xFF);

      Should be: TRISCbits.TRISC1=0; CCP2CON=0b00001111;

3. File user.c, line 79

      Text: OpenADC(ADC_FOSC_64 & ADC_RIGHT_JUST & ADC_6_TAD,ADC_CH0 & ADC_INT_OFF & ADC_VREFPLUS_VDD & ADC_VREFMINUS_VSS,7);

      Should be: OpenADC(ADC_FOSC_64 & ADC_RIGHT_JUST & ADC_6_TAD,ADC_CH0 & ADC_INT_OFF & ADC_REF_VDD_VSS,7);

These tree changes are indispensable. After making the changes the firmware is compiled perfectly (6 warnings appear, without importance) and the .hex file is generated correctly.

To be able to execute the software for the PC it is necessary that Microsoft .NET Framework is installed (free downloaded from Microsoft via Windows Update).