Bailey O.H.Embedded systems.Desktop integration.2005

clock and data lines can be attached to many different devices. The slave ID defines which device is to be awakened to be given commands to or receive data from. If you are asking for the difference between the SPI and I2C communications, here is the answer: Both devices can be attached to multiple slaves. In SPI communications one line is allocated to keeping the attention of the device being addressed. So in SPI we can use a common data and clock line, but each device must have its own attention line. If you have many devices you will either need additional components to select the active device or use a processor pin for each device. In I2C we replace that additional attention line with a device ID, eliminating the need for the additional components or processor pins.

The following program is written in HI-TECH C and performs the same functions as Listing 9-5.

Listing 9-6

/***************************************************************************** htI2C - HI-TECH I2C interface for dsPIC. Copyright 2004, Oliver H. Bailey

This program implements the I2C interface using the HI-TECH dsPICC compiler.

*****************************************************************************/

// Alternate method of setting configuration bits. __CONFIG(FOSC, POSC & XTPLL4 & CLKSWDIS & FSCMDIS); __CONFIG(FWDT, WDTDIS);

__CONFIG(FBORPOR, PWRT16 & MCLREN & BORDIS); __CONFIG(FGS, GCPU & GWRU);

__CONFIG(FCOMM, PGEMU);

char stmsg[]={"Hello From Embedded Systems\n\r\0"}; // Test String

Chapter 9

I2C_SIDL = 0; I2C_SCLREL = 0; I2C_IPMIEN = 0; I2C_A10M = 0; I2C_DISSLW = 1; I2C_SMEN = 0; I2C_GCEN = 0; I2C_STREN = 0; I2C_ACKDT = 1; I2C_ACKEN = 0; I2C_RCEN = 0; I2C_PEN = 0; I2C_RSEN = 0; I2C_SEN = 0;

//Continue in Idle Mode

//Hold SCL Clock Low

//IPMI Mode Disabled

//7 Bit Slave Address

//Disable Slew Rate

//Disable SMBus Input

//Disable General Call Address

//Disable SCL Clock Stretch

//Send NACK During Acknowledgement

//Disable Acknowledge Sequence

//Disable Receive Sequence

//Disable Stop Condition

//Disable Repeated Start Condition

//Disable Start Condition

The differences between how C30 and dsPICC implement the built-in dsPIC functions come down to the amount of “wrapper code” implemented by the vendor. The function calls we’ve written in C30 read or set the registers for us, whereas the dsPICC product gives us direct access, which allows us to write our own “wrapper” code. I like to use both techniques, depending on the amount of available program space available. These days I tend to use the direct access method when working with parts that have smaller program memeory areas.

The Dallas 1-Wire Interface

Our final communications method to implement on the dsPIC is the Dallas 1-Wire protocol. Now you may think that we have quite a chore ahead of us in writing the Dallas 1-Wire handlers for the PIC. Fortunately, that is not the case. There are several sources of 1-Wire interface routines written both in Assembler and C. If you want to understand how the 1-Wire and other communications interfaces work at the assembly level, then I would suggest reading Serial Communications by Square 1. That book provides an in-depth view of how many communications interfaces are implemented in Assembler for various PIC processors. Sadly, the dsPIC isn’t covered since it is a new device.

If you already understand the 1-Wire protocol (if you’ve read Part I of this book, you should) and want to find a starting point for C code implementation, I would recommend the 1-Wire Public Domain Kit from Dallas Semiconductor at www.dalsemi.com. This kit comes with source code for various platforms and drivers for several types of interfaces as well. This is a very comprehensive kit and will take some time to completely understand.

If you just want a starting point without a lot of extra files and documention, I suggest the 1-Wire interface available from www.microchipc.com. This site contains many useful C language routines including I2C and the Dallas 1-Wire. These routines are

Chapter 9

We will be using the PIC 16C745 low-speed USB interface chip. In addition to being a USB interface, this microcontroller also has other I/O capabilities. So we’re using the USB port on the chip and a 4-Wire TTL serial interface from the dsPIC. This is a 28-pin device with remaining capability. In this chapter we are using an inexpensive Ethernet controller chip with built-in TCP/IP capabilities. The Ethernet controller also has a 4-Wire TTL serial interface. The PIC 16C745 is a 28-pin device and the 765 is a 40-pin device. This means both devices have more than enough capacity to handle the Ethernet transceiver in addition to the dsPIC. Now you may be asking why we don’t eliminate the dsPIC altogether; that is a reasonable question. The dsPIC is a faster processor with more memory and program space. If we limit the use of the 16C745 to being a communications processor, we will be able to avoid running out of program space and timing issues. Remember that only one communications interface is active at a time.

The PIC 16C745 USB Chip

This chip is a 28-pin EPROMor ROM-based chip that contains a low-speed USB interface with two endpoints. Now this isn’t the fastest chip in the world, but for our needs it will work just fine. Since we’ve decided to add the Ethernet interface to this chip we need to break down our development efforts because they will cover two separate areas. First, we need to develop our USB to host interface. This task will entail the development of our embedded protocol and associated host routines.

To simplify the Windows development effort, we’ll make the 16C745 appear as an HID (human interface device) and use HIDmaker to develop the host application code for Windows and the USB communications piece for the PIC. Next, we will develop the serial I/O required for the 16C745 to communicate with the Ethernet controller. This will also require code to set up the IP address and other configurable items related to Ethernet

Chapter 9

We need to take a look at the pinout of the 16C745 device to see which pins are best used for our needs. We need three TTL serial ports and one USB port. One of the TTL ports will be connected to the dsPIC and in use all the time. The remaining two ports will be our RS-232 and Ethernet host interfaces. The USB port will also be a host interface option.

The Microchip USB Developer Kits

Currently, Microchip has three development kits for USB. The first is a new product designed to support the full-speed devices. The second kit is an inexpensive PIC programmer with a break-away prototype area that uses the 16C745 as the host interface. The PICKit 1 uses the 16C745 to drive a charge circuit and program lower end chips. In the following photo the 16C745 is the long chip on the left.

Chapter 9