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AR-B1570 User’s Guide
11
2.5 SERIAL PORT
The ACEs (Asynchronous Communication Elements ACE1 to ACE4) are used to convert parallel data to a serial
format on the transmit side and convert serial data to parallel on the receiver side. The serial format, in order of
transmission and reception, is a start bit, followed by five to eight data bits, a parity bit (if programmed) and one,
1.5 (in a five-bit format only) or two stop bits(in a 6,7, or 8-bit format). The ACEs are capable of handling divisors of
1 to 65535, and produce a 16x clock for driving the internal transmitter logic.
Provisions are also included to use this 16x clock to drive the receiver logic. Also included in the ACE a completed
MODEM control capability, and a processor interrupt system that may be software tailored to the computing time
required to handle the communications link.
The following table is a summary of each ACE accessible register
DLAB Port Address Register
Receiver buffer (read) 0 base + 0
Transmitter holding register (write)
0 base + 1 Interrupt enable
X base + 2 Interrupt identification (read only)
X base + 3 Line control
X base + 4 MODEM control
X base + 5 Line status
X base + 6 MODEM status
X base + 7 Scratched register
1 base + 0 Divisor latch (least significant byte)
1 base + 1 Divisor latch (most significant byte)
ACE Accessible Registers
(1) Receiver Buffer Register (RBR)
Bit 0-7: Received data byte (Read Only)
(2) Transmitter Holding Register (THR)
Bit 0-7: Transmitter holding data byte (Write Only)
(3) Interrupt Enable Register (IER)
Bit 0: Enable Received Data Available Interrupt (ERBFI)
Bit 1: Enable Transmitter Holding Empty Interrupt (ETBEI)
Bit 2: Enable Receiver Line Status Interrupt (ELSI)
Bit 3: Enable MODEM Status Interrupt (EDSSI)
Bit 4: Must be 0
Bit 5: Must be 0
Bit 6: Must be 0
Bit 7: Must be 0
(4) Interrupt Identification Register (IIR)
Bit 0: “0” if Interrupt Pending
Bit 1: Interrupt ID Bit 0
Bit 2: Interrupt ID Bit 1
Bit 3: Must be 0
Bit 4: Must be 0
Bit 5: Must be 0
Bit 6: Must be 0
Bit 7: Must be 0
(5) Line Control Register (LCR)
Bit 0: Word Length Select Bit 0 (WLS0)
Bit 1: Word Length Select Bit 1 (WLS1)