To verify that the Windows usbser.sys properly supports CTS signal, please ask on Microsoft Q&A forum, with tag “Windows Hardware Development”. Or use other Microsoft Windows support channels.
If you doubt the Windows usbser.sys behaviour, use Linux to test the STM32 firmware.
(I am not ST support, sorry for jumping in)
The USB CDC class spec document PSTN120.pdf version 1.2 (February 9, 2007) lacks definition of CTS bit in the UART state bitmap (6.5.4, Table 31 on page 33). But I've asked the Microsoft AI about the state bitmap and it produced the following:
// This example demonstrates how to update the CTS signal and send the SERIAL_STATE notification to the host
typedef union {
uint8_t val;
struct {
unsigned dcd :1;
unsigned dsr :1;
unsigned break_err :1;
unsigned ri :1;
unsigned frame_err :1;
unsigned parity_err :1;
unsigned overrun_err :1;
unsigned cts :1; // CTS signal bit
};
} UART_StateBitmap;
UART_StateBitmap uart_state;
// Function to send SERIAL_STATE notification
void send_serial_state_notification() {
uint8_t notification_packet[10] = {0xA1, 0x20, 0x00, 0x00, 0x00, 0x00, 0x02, 0x00, uart_state.val, 0x00};
USB_SendControlEndpoint(notification_packet, sizeof(notification_packet));
}
// Example of updating CTS signal
void update_cts_signal(bool cts_active) {
uart_state.cts = cts_active ? 1 : 0;
send_serial_state_notification();
}
The CTS bit is indeed documented in the USB spec, PSTN120.pdf as reserved.
