The USART features the Modem mode, which enables control of the signals DTR (Data Terminal Ready), DSR (Data Set Ready), RTS (Request to Send), CTS (Clear to Send), DCD (Data Carrier Detect), and RI (Ring Indicator). While operating in Modem mode, the USART behaves as a DTE (Data Terminal Equipment) as it drives DTR and RTS and can detect level change on DSR, DCD, CTS, and RI.
Modem mode is enabled by writing the value 0x3 to US_MR.USART_MODE. While operating in Modem mode, the USART behaves as though in Asynchronous mode and all the parameter configurations are available.
The following table provides the correspondence of the USART signals with modem connection standards.
| USART Pin | V24 | CCITT | Direction |
|---|---|---|---|
| TXD | 2 | 103 | From terminal to modem |
| RTS | 4 | 105 | From terminal to modem |
| DTR | 20 | 108.2 | From terminal to modem |
| RXD | 3 | 104 | From modem to terminal |
| CTS | 5 | 106 | From terminal to modem |
| DSR | 6 | 107 | From terminal to modem |
| DCD | 8 | 109 | From terminal to modem |
| RI | 22 | 125 | From terminal to modem |
The control of the DTR output pin is performed by writing a ‘1’ to US_CR.DTRDIS and US_CR.DTREN. The disable command forces the corresponding pin to its inactive level, i.e., high. The enable command forces the corresponding pin to its active level, i.e., low. The RTS output pin is automatically controlled in this mode.
The level changes are detected on the RI, DSR, DCD and CTS pins. If an input change is detected, the bits RIIC, DSRIC, DCDIC and CTSIC in US_CSR are set and can trigger an interrupt. The status is automatically cleared when US_CSR is read. Furthermore, the CTS automatically disables the transmitter when it is detected at its inactive state. If a character is being transmitted when the CTS rises, the character transmission is completed before the transmitter is actually disabled.