Laboratory Exercise 7 - Extended I/O & Parallel Processing

Transcription

1 Laboratory Exercise 7 - Extended I/O & Parallel Processing The purpose of this lab is to make an LED blink first by using the extended I/O function of the Microcontroller, and then by parallel processing function. Programmer Module The PIC16F877microcontroller does not have a USB support feature. To be able to program the chip, an external programmer is needed. The Microcontroller accomplishes the programming of PIC16 by implementing a Programmer Module controlled by PIC18F (will be referred to as PIC 18). The PIC18 comes with 28 pins and has a full speed USB 2.0 capability. It takes a hex file from the USB and burns onto the PIC16. Aside from its programming function, as a microcontroller, the PIC18 has many functions just like the PIC16. Excluding the pins used by programmer module, there are a total of 7 general purpose IO pins available, and 1 other input pin available. These are RA4, RB3, RB4, RB6, RB7, RC2, RC6, and RE3 (MCLR). If it is desired, the user can access those I/O pins by either controlling them from PIC16 (Extended I/O function), or directly by PIC18 (Parallel Processing). 3 Note this PIC18F2550 is not to be confused with the 40 pin PIC18F4620 mentioned in the course note as PIC18

2 Extended I/O The PIC16 communicates with PIC18 the same way that it does with DS1307 Real Time Clock chip. The PIC18 acts as a slave with PIC16 as its master through I2C communication. This effectively adds 7 to the main PIC I/O pin count. To control these 7 extra pins from the PIC16, I2C has to be initialized. An include file p2p_macro.inc contains the following macros which are designed to look and behave similar to regular I/O control operations in the PIC instruction set. For example, if it is desired to set PORTA 4, the user can simply write: bcf_porta_4 However, another set of corresponding codes has to be set up in the PIC18 to handle these I/O instructions. The code is included in a firmware called btldr_pgmr_i2c.hex (provided with the lab) and it comes loaded on the PIC18 by default. This firmware must be loaded into the programmer before the extended I/O set can be used. This firmware can be easily loaded to the PIC18 by using PICUSB. Instructions on how to load the firmware can be found in section 4.4 in the User Manuel under Download Firmware.

3 Exercise: Write a piece of code for PIC18 4 that sets and clears PORTB4 every half of a second by using the appropriate macros in p2p_macro.inc with PIC16. Next implement a circuit on a breadboard with an LED that takes a signal from pin RB3. Follow the subsequent steps to use the extended I/O pin set: 1. Add i2c_common.asm, rtc_macro.inc and p2p_macros.inc to the project (Files are included for the lab) 2. Add #include <p2p_macros.inc> in the code 3. Set TRISC3, and 4 for I2C, and call i2c_common_setup in the code 4. Set TRISB3 in PIC18 by calling macro bcf_trisb_3 5. Write a simply loop code that repeatedly sets and clears PORTB3 in PIC18: a_1 bsf_portb_3 call delay bcf_portb_3 call delay goto a_1 ;call macro to set PORTB3 ;call macro to clear PORTB3 Compile and load the HEX file into PIC16. Implement the above circuit on a breadboard. Make sure that the ground pin of PIC18 is properly connected with ground coming from the 5V power supply. Switch on the power supply, the LED should be blinking every half a second. 4 Note that the PIC configuration settings are different. Refer to section in the Course Notes. Also remember to select the right device in MPLAB when programming.

4 Parallel Processing The PIC18 comes loaded with a firmware btldr_pgmr_i2c.hex to be able to program PIC16 and to provide extended I/O pins through I2C. However, this firmware takes up all 32Kb of program memory which leaves the student no space to implement any user code on PIC18. Thusly, another version of the firmware btldr_pgmr.hex (also provided with the lab) excluded the I2C code, and therefore made available the memory address from 0x7D70 to 0x7FEF. User can make use of the free memory by appending their user code to the original firmware using the Append Hex to Firmware function on PICUSB 5. Note that the user code will be inserted into the firmware between location 0x7D70 and 0x7FEF. This means that the user code can be no longer than 640 instruction lines. Student can check this under Disassembly Listing window in MPLAB after compiling. Exercise: Write a piece of code for PIC18 that sets and clears PORTB4 every half of a second and append it to the original firmware. To demonstrate the true nature of parallel processing capability, we also load the RTC code onto PIC16. Start first by making sure that you code takes the following style: #include <p18f2550.inc> org 0x7D70 ;begin the user code at address 0x7D70 UserCode end The rest of the code should be similar to those of the exercise in extended I/O Once compiled, the user must export the custom code into the correct format in order to append to the firmware. This is done in MPLAB by the following steps: 1. Go to File >Export 2. A dialog opens with the title Export Hex File. Uncheck Configuration Bits, EEPROM Memory and User ID. 3. Make sure the Program Memory check box is checked. 4. Set the program memory to start at 0x7D70 and end at address 0x7FEF. Note: the end address must not exceed 0x7FEF or PICusb will reject the hex file. 5. Under the File Format tab, make sure the format selected is INHX32. 3 The user code which will only be run when the board is set to RUN mode

5 6. Click OK, and save the exported hex file. To insert user code into the existing programmer firmware, follow these steps: 1. Open PICusb and go to Tools > Append Hex to Firmware. 2. An Open File dialog pops up. Select the user code HEX file you would like to insert and click Open. 3. A Save File dialog pops up. Choose a file name and the directory for the new combined Hex file and click Save. To Load the modified firmware onto the PIC18F2550: 1. In the PICusb software, select Tools >Download Firmware. 2. A dialog opens. Select the modified (appended) hex file and click Open. 3. PICusb will automatically download the new firmware into the PIC18F2550 and then reset. Now by using the same circuit the user should be able to see the LED blink in the same fashion.