Specifications

AN93
Rev. 0.9 101
.
Figure 19. UART Serial Interface
3.4.4. Parallel Interface
(24-Pin TSSOP Only)
The parallel interface is intended for applications where
a serial interface is not available. The parallel interface
has an 8-bit data bus and a single address bit. The
parallel interface is selected by forcing AOUT/INT
(Si2493/57/34/15/04 Pin 15) to a logic 0 (low) through
an external pulldown resistor ≤ 10 kΩ. 27 MHz
operation is possible in parallel mode. See Table 24 on
page 57 for details. Several pins on the Si2457 change
function when the parallel interface mode is selected. In
parallel mode, the modem must be configured for a DTE
Interface or 8N1 only. The host processor must
calculate parity for MSB. The modem sends bits as
received by the host and does not calculate parity. Refer
to “AN60: Si2493/57/34/15/04 Parallel Interface
Software” for detailed parallel interface applications
information*.
*Note: The parallel port has been modified in Si2456
Revision H and Si2457 Revision B and later to allow
interrupt-driven operation and remove the requirement
of using CTS and RTS for flow control (see “AN60:
Si2456/33/14 Parallel Interface Software”). Updates
that may affect existing host software written for the
Si2456 family with revisions before Revision H or the
Si2457 family Revision A are:
1. It is possible to clear the RXF bit by writing “0” in this
bit position of parallel register 1. It is recommended
that this bit always be written with “1” unless
intentionally clearing the RXF bit to remove an RXF
interrupt.
2. An inactivity timer controlled by register U6F will
assert an interrupt if data is available in the RX FIFO
for U6F milliseconds (default 255). This is important to
note when upgrading a hardware design from the
Si2456 family to the Si2457 family. A small change to
existing host software may be necessary.
Table 71 shows the function of the affected pins in the
serial and parallel interface modes.
11 Bits
to Data Bus
CONTROL
RX Shift
Register
TX Shift
Register
TX FIFO
MUX
TXD
(10)
CTS
(11)
RTS
(8)
RXD
(9)
INT
(16)
RX FIFO