73K222AU
Single-Chip Modem
with UART
MODEM CONTROL REGISTER (MCR)
STNDLN:
0
ADDRESS:
UA2 - UA0 = 100
1
UA3 - UA0 = 0100
UART SECTION
The MCR register controls the interface with the modem. Bits D1 and D0 are also available as read only bits in
the UART Control Register in the Modem Registers. In single-port mode, bits D1 and D0 are available inverted at
the RTS and DTR pins.
BIT NO.
NAME
CONDITION DESCRIPTION
D0
DTR
D1
RTS
1
This bit controls the Data Terminal Ready (DTR) output.
When bit 0 is set to a logic 1, the DTR output is forced to
a logic 0. When bit 0 is reset to a logic 0, the DTR
output is forced to a logic 1.
1
This bit controls the Request to Send (RTS) output.
When bit 1 is set to a logic 1, the RTS output is forced to
a logic 0. When bit 1 is reset to a logic 0, the RTS
output is forced to a logic 1.
D2
µPRST*
(OUT1 in 16C450)
1
In single-port mode inactive unless loop = 1, then
functions as below (D4). In dual-port mode the µPRST
pin is the logical OR of this bit and the RESET pin.
D3
Enable Interrupt
(OUT2 in 16C450)
0
Sets INTRPT pin to high impedance if STNDLN = 1.
1
INTRPT output enabled.
D4
LOOP
1
This bit provides a local loopback feature for diagnostic
testing of the UART portion of the 73K222AU. When bit
D4 is set to logic 1, the following occurs:
1. TXD is forced to mark, RXD is ignored.
2. The output of the Transmitter is looped to the
Receiver.
3. The four modem control inputs to the UART (CTS,
DSR, DCD, and RI) are ignored and the UART
signals RTS, DTR, Enable Interrupt, and µPRST are
forced inactive.
4. The UART signals RTS, DTR, Enable Interrupt, and
µPRST are internally connected to the four control
signals CTS, DSR, DCD and RI respectively. Note
that the Modem Status Register Interrupts are now
controlled by the lower four bits of the Modem
Control Register. The interrupts are still controlled
by the Interrupt Enable Register.
D5 - D7
0
These bits are permanently set to logic 0.
* Note: The µPRST bit has an upgraded function which was not included in the initial definition of the 73K222AU.
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