CCS News

USB Made Easy with the Compiler and USB Project Wizard!

Friday 10 February, 2023

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 PICR MCU application.

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.

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.

#include <16F1459.h>
#use delay(internal=48MHz,USB_FULL,ACT=USB)

#define USB_CON_SENSE_PIN PIN_A5 // Connected to USB +5V


#define USB_CONFIG_VID 0x2405 // CCS VID
#define USB_CONFIG_PID 0x8001

#define USB_STRING(x) (sizeof(_UNICODE(x))+2), \
//Microsoft Defined for US-English

char const USB_STRING_DESC[]={
USB_ENGLISH_STRING, //string 0 - language
USB_STRING("CCS"), //string 1 - manufacturer
USB_STRING("My-PIC-Device") //string 2 - product

#include <usb_cdc.h>

USB Wizard
The wizard allows the use of one of three communication protocols:

Communication Device Class, or CDC, creates a virtual COM port on a PC. On Windows PCs, this is the legacy COM ports (such as COM1) which can be opened with legacy terminal software. The CCS CDC library allows the use of printf(), putc() and getc() to communicate to the host PC using the virtual COM port.

Human Interface Devices, or HID, is a simple protocol for things like Mice and Keyboards. This wizard allows the user to create a HID project using a vendor specific HID descriptor report. CCS also provides a PC program written in VB.NET to communicate with the PIC in this manner. CCS also provides HID keyboard and HID mouse examples for the PIC.

Bulk transfers is a method of reading/writing directly to the buffer endpoint. Using this method does require drivers for your operating system, but since XP Microsoft has been shipping WinUSB which is compatible with this mode. CCS does provide a VB.NET demo application to show how to use this driver to communicate with the PIC.

CCS IDE Project Wizard includes USB. The IDE requires a minimum operating system of Windows 95(or later) or Linux.