diff --git a/doc/openocd.texi b/doc/openocd.texi
index 73f53c212..6aa1c49ed 100644
--- a/doc/openocd.texi
+++ b/doc/openocd.texi
@@ -2583,10 +2583,23 @@ and initially asserted reset signals.
 @end deffn
 
 @deffn {Command} {ftdi oscan1_mode} on|off
-Enable or disable OSCAN1 mode. This mode is intended for use with an adapter,
+Enable or disable OScan1 mode. This mode is intended for use with an adapter,
 such as the ARM-JTAG-SWD by Olimex, that sits in between the FTDI chip and the
-target. The adapter uses the normal JTAG signals to control TCKC and TMSC
-(bidirectional) signals used in 2-wire cJTAG.
+target. The cJTAG prococol is composed of two wires: TCKC (clock) and TMSC (data).
+TMSC is a bidirectional signal which is time-multiplexed alternating TDI, TMS and
+TDO. The multiplexing is achieved by a tri-state buffer which puts TMSC in Hi-Z
+when the device is supposed to take the control of the line (TDO phase).
+
+The ARM-JTAG-SWD adapter uses standard TRST and TMS signals to control TMSC
+direction. TRST is used by the adapter as selector for the multiplexers which set
+the JTAG probe in 2-wire mode. Whatever signal is used for this purpose, it must
+be defined with the name JTAG_SEL using @command{ftdi layout_signal}. JTAG_SEL is
+set to 0 during OScan1 initialization.
+
+Some JTAG probes like the Digilent JTAG-HS2, support cJTAG by using a
+separate pin to control when TMS is driven onto TMSC. You can use such
+probes by defining the signal TMSC_EN using
+@command{ftdi layout_signal TMSC_EN -data <mask>}.
 @end deffn
 
 @deffn {Command} {ftdi layout_signal} name [@option{-data}|@option{-ndata} data_mask] [@option{-input}|@option{-ninput} input_mask] [@option{-oe}|@option{-noe} oe_mask] [@option{-alias}|@option{-nalias} name]