|
The CCS C Compiler from the beginning has made it easy to communicate over an RS232 like port. Many of our users have used this extensively, not only for communicating with serial devices but also for diagnostics and debugging. Now, PC's and many other devices have replaced their RS232 ports with USB ports. The USB hardware and software is much more complex than RS232. This has discouraged many from migrating over or they have used a crutch like an external chip to do the USB magic (like FTDI). This article shows how to easily add a USB port to your PIC® MCU application.
HARDWARE
Microchip has a number of chips with built in USB. For example, the PIC16F1459 ($1.38/100) or PIC18F14K50 ($1.79/100). Here is an example schematic: (note: pin numbers apply to PDIP, SOIC, and SSOP packages)
Shown here is a USBmicro style connector. The more common type B connector is the same pinout except there is no pin 5 and 4 is the ground. The D+ and D- pins are for the data and sometimes there will be a 27 ohm series resistor and/or zener protection diodes on those pins. When using a peripheral device, pin 1 will supply 5 volts (up to a half amp). The board can be powered from that 5 volts; however, in this schematic it is to simply detect if the USB cable is plugged in. Although users can do that from the data lines, it is easier to detect the 5 volts like this. Sometimes users will put series coils on the 5 volts and ground to reduce noise.
The Vusb on this chip simply needs a cap for an internal voltage regulator. Some chips have a dedicated Vbus pin for the 5 volts detect. Careful with the pin names, as they are not consistent between chips.
HOST SOFTWARE
The USB bus has a number of protocols that can be used, each with many options and configurations. HID (human Input device) is used for keyboards and mice and is easy to use on any OS because the drivers are built in. There is a data limit however, (like 8 bytes per ms) that makes it impractical for many applications. CDC is a protocol designed to emulate an RS232 port. The Windows drivers will create a virtual COM port for a CDC USB device so the PC application can use COM1... just like an RS232 port.
When a Windows 10 sees a CDC device, it automatically installs a driver for it. For older versions of Windows, users need a short .inf file to describe their device and include the device VID (vendor ID) and PID (product ID). Examples .inf files are in the CCS C Compiler examples directory. Every USB device is supposed to have a unique VID/PID and serial number. If users have two of the same devices plugged in, the VID/PID will match but they are differentiated by serial number. To get a VID users need to register with the USB standards organization.
USB is point to point and one point is a host and the other a device. Hub's can also be involved but that is beyond our concern for this article. The host initiates all activity. For example, a PC is a host and it will poll every device about once a millisecond and transfer any data needing transferring. When the device is first plugged into a port, there is a handshaking that takes place that involves the device identifying itself along with the protocol it will use and various parameters. For example, how much current it expects to draw off the bus. This handshake is called enumeration. In Windows users know it happened by the beep.
Some PIC24 parts have USB hardware that can also be a host. For example, this might be used to read a USB flash drive.
PIC® MCU SOFTWARE
The CCS C Compiler has several example programs for CDC as well as the required drivers. The following are the key declarations you need for a USB CDC program:
#include <16F1459.h>
#use delay(internal=48MHz,USB_FULL,ACT=USB)
#define USB_CON_SENSE_PIN PIN_A5 // Connected to USB +5V
#define USB_STRINGS_OVERWRITTEN
#define USB_CONFIG_VID 0x2405 // CCS VID
#define USB_CONFIG_PID 0x8001
#define USB_DESC_STRING_TYPE 3
#define USB_STRING(x) (sizeof(_UNICODE(x))+2), \
USB_DESC_STRING_TYPE,_UNICODE(x)
#define USB_ENGLISH_STRING 4,USB_DESC_STRING_
TYPE,0x09,0x04
//Microsoft Defined for US-English
Incoming and outgoing data is buffered and transfer is controlled by the host. The program typically only needs to know if enumerated. To manage all this the USB task needs to be periodically called to do housekeeping. The following is an example main program that simply sends an ADC reading to the host every second:
void main(void) {
setup_adc(ADC_CLOCK_INTERNAL);
setup_adc_ports(sAN10); // B4
set_adc_channel(10);
usb_init();
while(TRUE)
{
if(usb_enumerated())
To see the data start up a terminal program on the PC and select the COM port. In the CCS PCW IDE use TOOLS > Serial Port Monitor. The CDC driver has many more options for more complex programs, however the above function calls plus usb_cdc_getc() not shown here will be all most users need.
Summary
That is all there is to it. Find a suitable chip, add a USB connector and slip some of the above into the program. For high speed transfers there is a USB bulk mode, however the driver interface is more complex and there is no standard. Several companies have free generic USB bulk drivers that can be used, however the customer will need to install those on the target PC. The CDC protocol can transfer data at the RS232 equivalent of 250K baud, twice the speed of RS232 and it may be faster than many PIC's can produce the data.
Like us on Facebook. Follow us on Twitter.
About CCS:
CCS is a leading worldwide supplier of embedded software development tools that enable companies to develop premium products based on Microchip PIC® MCU and dsPIC® DSC devices. Complete proven tool chains from CCS include a code optimizing C compiler, application specific hardware platforms and software development kits. CCS' products accelerate development of energy saving industrial automation, wireless and wired communication, automotive, medical device and consumer product applications. Established in 1992, CCS is a Microchip Premier 3rd Party Partner. For more information, please visit https://www.ccsinfo.com.
PIC® MCU, MPLAB® IDE, MPLAB® ICD2, MPLAB® ICD3 and dsPIC® are registered trademarks of Microchip Technology Inc. in the U.S. and other countries.
|
