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1 Introduction

This application note describes how to implement a USB to multiple VCOMs'

functions on K32L2 series FRDM boards (FRDM-K32L2B3/FRDM-K32L2A).

The function of USB to VCOM can be implemented by using the common AT

commands in the subclass of the abstract control model in the CDC class

specified by the USB protocol. A USB device can support one or more VCOMs,

the number of VCOM depends mainly on the number of endpoints (EP)

supported by USB device. The Full-speed (FS) USB device controller for

K32L2 series MCU supports 16 bidirectional endpoints and supports up to 15

VCOMs. Based on the SDK code

(dev_composite_cdc_vcom_cdc_vcom_lite_bm), this application note

implements the function of a USB to 15 VCOMs and the development tool is

MCUXpresso IDE.

2 USB descriptor configuration

The USB descriptor is equivalent to the business card of the USB device, and

describes all the attributes and configurable information of the USB device,

such as the class, interface information, and endpoint information. If the

descriptor of the device is obtained, the type, purpose, and the parameters of

the communication of the device, etc. are known, and the USB host can

configure it so that both parties of the communication work with the same

parameters.

The function of a USB to multiple VCOMs can be implemented by using the USB Composite class. The Composite class is a

special USB class that can implement multiple different functions in a USB device. Such as a device can implement “Mouse +

Keyboard” function, or “VCOM + Keyboard” function. In fact, the USB Composite class can implement almost any combination

of USB functions, and it is not just a combination of two functions, it can be three or more, so you can use the composite class

to implement the functions of two CDCs or multiple CDCs.

A CDC class device consists of two subclass interfaces: a CDC class interface and a data class interface. A CDC device can

contain zero or more data interfaces.

• The CDC class interface uses a standard interface descriptor that requires a control endpoint and an optional interrupt IN

endpoint.

• The data class interface requires a Bulk IN and a Bulk OUT type of endpoint. But since the 16 endpoints on the K32L2B3/

K32L2A are bidirectional endpoints, an endpoint can implement both input and output functions, ie the functions of Bulk IN

and Bulk OUT can be implemented by one endpoint.

In order to support more VCOM, the interrupt IN endpoint in CDC class interface can be removed, so that in addition to a common

control endpoint only one bidirectional endpoint is needed to implement a VCOM. That is, except that EP0 is used as the control

endpoint, the other 15 endpoints can implement 15 VCOMs and this application note implement such a feature. The descriptor

structure of the composite class containing 15 CDC subclasses used in this application note is shown in Figure 1.

Contents

1 Introduction............................................ 1

2 USB descriptor configuration................. 1

3 Endpoint usage......................................2

4 Endpoint Buffer Configuration................4

4.1 Buffer Descriptor Table...........4

4.2 Endpoint Buffer....................... 6

5 Software workflow chart.........................8

5.1 USB interrupt service

function flowchart.................. 11

5.2 USB device request.............. 13

6 Key Steps............................................ 15

6.1 How to expand the number

of supported VCOMs.............15

6.2 How to remove the

Interrupt IN endpoint in the

CDC interface........................16

6.3 Code optimization................. 18

7 Function Test....................................... 23

8 Conclusion........................................... 24

9 References.......................................... 24

AN12597

USB to Multi VCOM on K32L2 Series MCU

Rev. 0 — November 2019

Application Note

Summary of content (25 pages)

PAGE 1
AN12597 USB to Multi VCOM on K32L2 Series MCU Rev. 0 — November 2019 1 Introduction This application note describes how to implement a USB to multiple VCOMs' functions on K32L2 series FRDM boards (FRDM-K32L2B3/FRDM-K32L2A). The function of USB to VCOM can be implemented by using the common AT commands in the subclass of the abstract control model in the CDC class specified by the USB protocol.
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NXP Semiconductors Endpoint usage Figure 1. USB descriptor structure block diagram In Figure 1, the interface association descriptor associates a CDC interface with a data interface to describe a VCOM function. For details on the USB descriptor, refer to Chapter 9.6, Standard USB Descriptor Definitions, USB Specification 2.0 and SDK code. NOTE If the interrupt endpoint of the CDC interface is removed, the serial port status cannot be sent and received.
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NXP Semiconductors Endpoint usage Table 1.
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NXP Semiconductors Endpoint Buffer Configuration Figure 2. Endpoint configuration 4 Endpoint Buffer Configuration Each endpoint needs a buffer to store the received data or data to be sent. This chapter describes the configuration of the USB endpoint buffer. 4.1 Buffer Descriptor Table To manage USB endpoint communications, USBFS implements a Buffer Descriptor Table (BDT) in system memory，as shown in Figure 3 .
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NXP Semiconductors Endpoint Buffer Configuration Table 2. Buffer descriptor format (continued) TOK_PID[ 3] TOK_PID[ TOK_PID[ TOK_PID[ 2] 1] 0] Buffer Address(32-Bits) The address of the EP Buffer is stored in the Buffer Address field. BDT is stored in a 512 byte aligned space and the location of the BDT can be determined by BDT Page registers: BDT Page register 1, BDT Page register 2 and BDT Page register 3.
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NXP Semiconductors Endpoint Buffer Configuration Figure 3. Buffer descriptor table Software should manage buffers for USBFS by updating the BDT when needed. This allows USBFS to manage data transmission and reception, while the microprocessor performs communication overhead processing and other function-dependent applications. 4.2 Endpoint Buffer The buffer of the control endpoint is not fixed. When receiving the Setup data, the EP0 buffer is s_UsbDeviceKhciState.
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NXP Semiconductors Endpoint Buffer Configuration Table 6.
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NXP Semiconductors Software workflow chart s_currRecvBuf[USB_DEVICE_CONFIG_CDC_ACM][DATA_BUFF_SIZE]; USB_DMA_NONINIT_DATA_ALIGN(USB_DATA_ALIGN_SIZE) static uint8_t s_currSendBuf[USB_DEVICE_CONFIG_CDC_ACM][DATA_BUFF_SIZE]; The value of the macro DATA_BUFF_SIZE is 64.
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NXP Semiconductors Software workflow chart Figure 5. Correspondence between source code and USB protocol stack architecture The basic workflow of the device stack depends on callback functions and function calls. The callback functions notify all state changes and data requests of the device stack to application. There are two types of callback functions in the device stack: • Device callback function - It notifies the state of device stack to the application.
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NXP Semiconductors Software workflow chart Figure 6. Callback function processing When the USB host recognizes that a USB device is plugged into the USB interface, it starts an enumeration process. The essence of USB enumeration is the process in which the USB host obtains the parameter information of the USB device and configures the configurable parameters. The USB enumeration process is done in the USB interrupt service function.
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NXP Semiconductors Software workflow chart 5.1 USB interrupt service function flowchart Figure 7. Main flowchart of the USB interrupt service function When a USB interrupt occurs, the interrupt service function determines which type of interrupt: Token done interrupt, Reset interrupt, Endpoint stalled interrupt, then go to the corresponding operation. 5.1.1 Reset interrupt process If the USB interrupt is a reset interrupt, the program executes the flow shown in Figure 8.
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NXP Semiconductors Software workflow chart Figure 8. USB device reset process The main thing to do in the reset interrupt is to set the USB device to the default state and initialize the control IN pipe and OUT pipe. 5.1.2 Token done interrupt processing If it is a Token done interrupt, the program executes the process shown in Figure 9. Application Note USB to Multi VCOM on K32L2 Series MCU, Rev.
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NXP Semiconductors Software workflow chart Figure 9. Token done interrupt processing The endpoint callback function includes two types: control endpoint callback function and non-control endpoint callback function. If the current endpoint is a control endpoint, the control endpoint callback function is called; if the current endpoint is a non-control endpoint, the non-control endpoint callback function is called.
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NXP Semiconductors Software workflow chart (Device Request) to the device. The device request is specified by the Setup data of length 8 bytes, and the direction is always from the USB host to the USB device. Commonly used standard requests are shown in Table 7. Table 7. Common Standard requests Standard request Direction SetAddress OUT SetConfiguration OUT GetDescriptor(Device/Configuration/String) IN CDC class devices also support some class requests in addition to the standard requests.
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NXP Semiconductors Key Steps Table 10. Line Coding Structure (continued) 1: Odd 4: Space 2: Even 6 bDataBits 1 Data bits (5,6,7,8 or 16) SetLineCoding is an output class request, which corresponds to GetLineCoding. The USB host uses this command to set the serial port property of the device, and its data structure is the same as GetLineCoding. After the USB host completes these standard requests and class requests, the enumeration process ends, and the USB host will recognize multiple VCOMs.
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NXP Semiconductors Key Steps #define HS_CDC_VCOM_INTERRUPT_IN_INTERVAL_3 (0x07) #define FS_CDC_VCOM_INTERRUPT_IN_INTERVAL_3 (0x08) #define #define #define #define HS_CDC_VCOM_BULK_IN_PACKET_SIZE_3 (512) FS_CDC_VCOM_BULK_IN_PACKET_SIZE_3 (64) HS_CDC_VCOM_BULK_OUT_PACKET_SIZE_3 (512) FS_CDC_VCOM_BULK_OUT_PACKET_SIZE_3 (64) 3. Modify the usb_device_descriptor.
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NXP Semiconductors Key Steps #define #define #define #define #endif USB_CDC_VCOM_CIC_INTERRUPT_IN_ENDPOINT_12 USB_CDC_VCOM_CIC_INTERRUPT_IN_ENDPOINT_13 USB_CDC_VCOM_CIC_INTERRUPT_IN_ENDPOINT_14 USB_CDC_VCOM_CIC_INTERRUPT_IN_ENDPOINT_15 (0) (0) (0) (0) 3. Modify the usb_device_descriptor.
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NXP Semiconductors Key Steps g_CdcVcomDicInterfaceIndex g_CdcVcomCicInterfaceIndex, g_CdcVcomBulkInPacketSize, g_CdcVcomBulkOutPacketSize array. NOTE These arrays are only used to simplify the initialization of each endpoint in the USB_DeviceCdcVcomSetConfigure() function. Modify the initialization process for the interrupt endpoint in each CDC class in the USB_DeviceCdcVcomSetConfigure() function.
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NXP Semiconductors Key Steps 6.3.1 Use loop instead of list For example, the USB_DeviceCdcVcomConfigureEndpointStatus () function in the virtual_com.
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NXP Semiconductors Key Steps } return error; } usb_status_t USB_DeviceCdcVcomConfigureEndpointStatus(usb_device_handle handle, uint8_t ep, uint8_t status) { usb_status_t error = kStatus_USB_Error; uint8_t i; if (status) { for(i = 0; i < USB_DEVICE_CONFIG_CDC_ACM; i++) { if ((g_CdcVcomDicBulkInEndpoint[i] == (ep & USB_ENDPOINT_NUMBER_MASK)) && (ep & USB_DESCRIPTOR_ENDPOINT_ADDRESS_DIRECTION_MASK)) { error = USB_DeviceStallEndpoint(handle, ep); } else if ((g_CdcVcomDicBulkOutEndpoint[i] == (ep & USB_ENDPOINT
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NXP Semiconductors Key Steps /* Packet size.
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NXP Semiconductors Key Steps } } 6.3.4.2 Assign endpoint number dynamically void USB_CdcVcomEndpointInit(void) { uint8_t i; for(i = 0; i < USB_DEVICE_CONFIG_CDC_ACM; i++) { #if USB_CDC_CIC_INTERRUPT_IN_ENDPOINT_ENABLE g_CdcVcomCicInterruptInEndpoint[i] = 1 + i*2; g_CdcVcomDicBulkInEndpoint[i] = 2 + i*2; g_CdcVcomDicBulkOutEndpoint[i] = 2 + i*2; #else g_CdcVcomDicBulkInEndpoint[i] = 1 + i*1; g_CdcVcomDicBulkOutEndpoint[i] = 1 + i*1; #endif } } 6.3.4.
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NXP Semiconductors Function Test p += p[3] p[4] p += USB_DESCRIPTOR_LENGTH_CDC_ABSTRACT; = g_CdcVcomCicInterfaceIndex[i]; = g_CdcVcomDicInterfaceIndex[i]; USB_DESCRIPTOR_LENGTH_CDC_UNION_FUNC; #if USB_CDC_CIC_INTERRUPT_IN_ENDPOINT_ENABLE p[2] = g_CdcVcomCicInterruptInEndpoint[i] | (USB_IN <<7); p += USB_DESCRIPTOR_LENGTH_CDC_CIC_INTERRUPT_ENDPOINT; #endif /* data interface descriptor */ p[2] = g_CdcVcomDicInterfaceIndex[i]; /* bulk in endpoint descriptor */ p += USB_DESCRIPTOR_LENGTH_INTERFACE; p[2] = g_C
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NXP Semiconductors Conclusion Figure 11. USB host recognition result When not using the Interrupt IN endpoint in the CIC interface, a USB device can support up to 15 VCOMs; when using an Interrupt IN endpoint, a USB device can support up to 7 VCOMs. NOTE If the window system comes with a CDC driver, users do not need to manually install the driver; if the CDC driver is not installed on the PC, users need to manually drive the driver. For the driver installation, refer to the attached readme.pdf document.
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