added --lower-rts and --lower-dtr command line options

see also https://github.com/npat-efault/picocom/pull/47

lowerrts.md

@@ -0,0 +1,81 @@
+Using uart handshake lines to control microcontroller reset signals in bare metal development
+=============================================================================================
+
+Introduction
+------------
+In bare metal software development, it's common practice to misuse uart handshake lines for e.g. controlling the reset signal of the target microcontroller.
+We're often using e.g. RTS to drive the microcontrollers reset input pin and e.g. DTR to switch between bootloader and normal operation mode.
+
+For example, if the microcontrollers reset input uses negative logic (low = reset, high = running), just connect the RTS ttl level to the microcontrollers reset input pin.
+
+Problem at open("port")
+-----------------------
+In both linux and osx, we observe, that the RTS handshake line will be driven active (positive voltage on RS232 signal level = logic low on ttl level implicitely within the `open()` call,
+even when the uart port is used without hardware handshake.
+
+This behaviour could not be avoided until patching the linux kernel driver (see also http://stackoverflow.com/a/21753723/2880699).
+
+Working around from user space Api
+----------------------------------
+
+All we can do in our terminal software is, to reset the RTS signal back to ttl high level after the `open()` call.
+But even when we do this quiet fast directly after the `open()` call, the RTS signal changes for a short time which might be enough to perform a reset of the microcontroller.
+
+With my picocom patch (`--lower-rts` command line option), I'd measured about 50µs-70µs on my linux machine and 250µs-450µs on my old Macbook Pro with running OSX (tested with Ftdi FT2232H).
+But note, that there is no guarantee for this times; because our PC is no realtime system, the OS might preempt our terminal application between toe `open()` and `ioctl()` calls.
+
+If this possibly microcontroller reset is disliked, we should work around against it by adding a bit more hardware between RTS and the microcontrollers reset input pin.
+Sometimes a simple low pass filter (resistor and capacitor) do this job.
+
+Using python3
+-------------
+Python's serial communication example tool `python3 -m serial.tools.miniterm` provides a command line option `--rts 0` which also lowers the RTS handline line after opening the port.
+On the same linux machine mentioned above, I measured a peak about duration of about 150µs-170µs.
+
+When using something like `python3 -c "import serial; ser = serial.Serial('/dev/ttyUSB0'); ser.setRTS(False); ... "`, I measured a peak about duration of about 180µs-260µs.
+
+Deeper analysis
+---------------
+Analyzed using:
+- Macbook Pro
+- Ft2232H based Usb serial adapter
+- Kubuntu 16.04 64 bit
+- picocom 2.3a, patched with `--lower-rts` command line option
+
+CRTSCTS=0x80000000. This bit is set in tty->termios->c_cflag when the uart is used with Rts/Cts handshake.
+When calling open() / ftdi_open(), I'd observed that this flag seems to be undefined. I found it either set or unset, depending on if the last session was with or without hardware handshake.
+So user space api open() calls the following functions of the `ftdi_sio` kernel driver module:
+    ftdi_open(c_cflag=000008BD)                    // note that CRTSCTS flag is undefined here
+        ftdi_set_termios(c_cflag=000008BD)         // called implicitely by ftdi_open
+    ftdi_dtr_rts(on=1, c_cflag=000008BD)           // called regardless if using hardware handshake or not
+        update_mctrl(set=0006, clear=0000)         // CRTSCTS flag is still undefined here
+
+Hacking the ftdi_sio kernel driver module
+-----------------------------------------
+
+see also http://stackoverflow.com/a/40811405/2880699
+
+    static void ftdi_dtr_rts(struct usb_serial_port *port, int on) {
+        ...
+        /* drop RTS and DTR */
+        if (on)
+            set_mctrl(port, TIOCM_DTR /*| TIOCM_RTS*/);    // <<-- HERE
+        else
+            clear_mctrl(port, TIOCM_DTR /*| TIOCM_RTS*/);  // <<-- and maybe even HERE
+    }
+
+Steps to perform on e.g. Kubuntu 16.04:
+$ sudo apt-get install build-essential    ;# etc.
+$ apt-get source linux-image-$(uname -r)  ;# of course, neets to have deb-src in /etc/apt/sources.list activated
+-> this creates a ~/linux-4.4.0 with about 760 MiB source code including linux-4.4.0/drivers/usb/serial/ftdi_sio.c
+$ cd ~/linux-4.4.0
+$ chmod +x debian/scripts/misc/splitconfig.pl
+$ chmod +x debian/scripts/config-check
+$ debian/rules genconfigs
+$ cp CONFIGS/amd64-config.flavour.generic .config
+and then after each change in drivers/usb/serial/ftdi_sio.c:
+$ make -C /lib/modules/$(uname -r)/build M=${PWD} drivers/usb/serial/ftdi_sio.ko
+$ sudo rmmod ftdi_sio.ko
+$ sudo insmod drivers/usb/serial/ftdi_sio.ko
+ergo:
+$ make -C /lib/modules/$(uname -r)/build M=${PWD} drivers/usb/serial/ftdi_sio.ko && sudo rmmod ftdi_sio.ko && sudo insmod drivers/usb/serial/ftdi_sio.ko

picocom.1.md

@@ -270,7 +270,20 @@ Picocom accepts the following command-line options.
     be replaced before being echoed-back to the terminal, if
     local-echo is enabled). See
     **[INPUT, OUTPUT, AND ECHO MAPPING]**. (Defaul: **delbs,crcrlf**)
-    
+
+**--lower-rts**
+
+:   Lower the RTS control signal after opening the serial port (by
+    default RTS is raised after open implicitely by the OS). Only
+    supported when flow-control mode is not set to RTS/CTS, ignored
+    otherwise. Only supported in Linux.
+
+**--lower-dtr**
+
+:   Lower the DTR control signal after opening the serial port (by
+    default DTR is raised after open implicitely by the OS).
+    Only supported in Linux.
+
 **--help** | **-h**
 
 :   Print a short help message describing the command-line

picocom.c

@@ -190,6 +190,8 @@ struct {
 	int imap;
 	int omap;
 	int emap;
+	int lower_rts;
+	int lower_dtr;
 } opts = {
 	.port = "",
 	.baud = 9600,
@@ -208,7 +210,9 @@ struct {
 	.receive_cmd = "rz -vv -E",
 	.imap = M_I_DFL,
 	.omap = M_O_DFL,
-	.emap = M_E_DFL
+	.emap = M_E_DFL,
+	.lower_rts = 0,
+	.lower_dtr = 0
 };
 
 int sig_exit = 0;
@@ -1290,6 +1294,8 @@ show_usage(char *name)
 	printf("  --imap <map> (input mappings)\n");
 	printf("  --omap <map> (output mappings)\n");
 	printf("  --emap <map> (local-echo mappings)\n");
+	printf("  --lower-rts\n");
+	printf("  --lower-dtr\n");
 	printf("  --<h>elp\n");
 	printf("<map> is a comma-separated list of one or more of:\n");
 	printf("  crlf : map CR --> LF\n");
@@ -1331,6 +1337,8 @@ parse_args(int argc, char *argv[])
 		{"parity", required_argument, 0, 'y'},
 		{"databits", required_argument, 0, 'd'},
 		{"stopbits", required_argument, 0, 'p'},
+		{"lower-rts", no_argument, 0, 'R'},
+		{"lower-dtr", no_argument, 0, 'D'},
 		{"help", no_argument, 0, 'h'},
 		{0, 0, 0, 0}
 	};
@@ -1469,6 +1477,12 @@ parse_args(int argc, char *argv[])
 				break;
 			}
 			break;
+		case 'R':
+			opts.lower_rts = 1;
+			break;
+		case 'D':
+			opts.lower_dtr = 1;
+			break;
 		case 'h':
 			show_usage(argv[0]);
 			exit(EXIT_SUCCESS);
@@ -1569,6 +1583,18 @@ main(int argc, char *argv[])
 	if ( r < 0 )
 		fatal("failed to add device %s: %s", 
 			  opts.port, term_strerror(term_errno, errno));
+
+	if ( opts.lower_rts ) {
+		r = term_lower_rts(tty_fd);
+		if ( r < 0 )
+			fatal("failed to lower RTS of device %s: %s", opts.port, term_strerror(term_errno, errno));
+	}
+	if ( opts.lower_dtr ) {
+		r = term_lower_dtr(tty_fd);
+		if ( r < 0 )
+			fatal("failed to lower DTR of device %s: %s", opts.port, term_strerror(term_errno, errno));
+	}
+
 	r = term_apply(tty_fd, 0);
 	if ( r < 0 )
 		fatal("failed to config device %s: %s",