This data acquisition card for connection to the USB has eight digital outputs, eight digital inputs, two 10-bit analogue outputs and eight 10-bit analogue inputs for voltage swings of 0 to 5 V. The system’s core is a Microchip USB-savvy microcontroller type PIC18F4550 programmed in C. The circuit is built on a compact PCB and requires no external power supply.
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).
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