Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 123)

AN93
Rev. 1.3 123
The system should then flush the receive buffer 100 ms after the escape command has been removed, send AT,
and wait for OK. This ensures that the modem is in command mode because OK is caused by the AT command
and not by the escape command.
5.9.1. +++ Escape
The +++ escape is enabled by default and is controlled by U70 [13] (TES). There are equal guard time periods
before (leading) and after (trailing) the +++ set by the register S12, during which there must be no interface (UART,
SPI or parallel) activity. If this inactivity criterion is met, the ISOmodem escapes to the command mode at the end
of the S12 time period following the +++. Any activity in the host interface during either the leading or trailing time
period causes the ISOmodem to ignore the escape request and remain in data mode. Timing for this escape
sequence is illustrated in Figure 27.
Figure 27. +++ Escape Timing
5.9.2. “9th Bit” Escape
The “9th Bit” escape mode feature is enabled by sending the AT\B6 command through autobaud, which detects a
9th bit space as “9th bit” escape mode. If this escape method is selected, a 1 detected on the ninth bit in a data
word returns the modem to the command mode. The 9th bit is ignored when the modem is in the command mode.
Timing for this escape sequence is illustrated in Figure 28.
Figure 28. “9th Bit” Escape Timing
5.9.3. “Escape Pin” Escape
The “escape pin is controlled by U70 [15] (HES). This bit is 0 by default, which disables the escape pin, ESC. If
HES is set to a 1, a high level on the ESC pin causes the modem to transition to the on-line command mode. The
ESC pin status is polled by the processor, and there is a latency before OK is received and the modem is in
command mode. The escape pin must be kept active until OK is received. In parallel or SPI interface mode, the
function of the escape pin is replaced by bit 2 in Hardware Interface Register 1, described in "2.2.4.2. Hardware
Interface Register 1" on page 25. Setting that bit high causes the modem to escape to the command mode.
+++
Leading Guard
Tim e
Trailing Guard
Tim e
Guard Tim e = S12 (20 m sec units)
Default Guard Tim e S12 = 50 (1.0 sec)
Guard Tim e Range = 10–255 (0.2–5.1 sec)
UART Tim ing for M odem Transm it Path (9N1 M ode with 9th Bit Escape)
D0 D1 D2 D3 D4 D5 D6 ESC D7 Stop Start
CTS
TX
t
RTS t
CTH
9-Bit Data
Mode