PC87108AVJE NSC [National Semiconductor], PC87108AVJE Datasheet - Page 8

• Manufacturer Part Number: PC87108AVJE
• Description: Advanced UART and Infrared Controller
• Manufacturer: NSC [National Semiconductor]

2.0 Functional Description
(Continued)
The clock for both transmit and receive channels is provided by an internal baud generator that divides its input clock by any di-
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visor value from 1 to 2
− 1. The output clock frequency of the baud generator must be programmed to be sixteen times the baud
rate value. The baud generator input clock is derived from a 24 MHz clock through a programmable prescaler. The prescaler
value is determined by the PRESL bits in the EXCR2 register. Its default value is 13. This allows all the standard baud rates, up
to 115.2 kbaud to be obtained. Smaller prescaler values will allow baud rates up to 921.6 kbaud (standard) and 1.5 Mbaud (non
standard).
Before operation can begin, both the communications format and baud rate must be programmed by the software. The commu-
nications format is programmed by loading a control byte into the LCR register, while the baud rate is selected by loading an ap-
propriate value into the baud generator divisor register. The software can read the status of the device at any time during opera-
tion. The status information includes FULL/EMPTY state for both transmit and receive channels, and any other condition detected
on the received data stream, like a parity error, framing error, data overrun, or break event.
DS012549-15
FIGURE 3. Serial Data Stream Format
2.3 SHARP-IR MODE
This mode supports bi-directional data communication with a remote device using infrared radiation as the transmission medium.
Sharp-IR uses Digital Amplitude Shift Keying (DASK) and allows serial communication at baud rates up to 38.4 kbaud. The format
of the serial data is similar to the UART data format. Each data word is sent serially beginning with a zero value start bit, an op-
tional parity bit, and ending with at least one stop bit with a binary value of one. A zero is signalled by sending a 500 kHz con-
tinuous pulse train of infrared radiation. A one is signaled by the absence of any infrared signal. The PC87108A can perform the
modulation and demodulation operations internally, or it can rely on the external optical module to perform them.
Operation in Sharp-IR is similar to the operation in UART mode. The main difference being that data transfer operations are nor-
mally performed in half duplex fashion, and the modem control and status signals are not used. Selection of the Sharp-IR mode
is controlled by the MDSL bits in the MCR register when the device is in extended mode, or by the IR__SL bits in the IRCR1 reg-
ister when the device is in non-extended mode. This prevents legacy software, running in non-extended mode, from spuriously
switching the device to UART mode, when the software writes to the MCR register.
2.4 IrDA 1.0 SIR MODE
This is the first operational mode that has been defined by the IrDA committee and, similarly to Sharp-IR, it also supports
bi-directional data communication with a remote device using infrared radiation as the transmission medium. IrDA 1.0 SIR allows
serial communication at baud rates up to 115.2 kbaud. The format of the serial data is similar to the UART data format. Each data
word is sent serially beginning with a zero value start bit, followed by 8 data bits, and ending with at least one stop bit with a binary
value of one. A zero is signaled by sending a single infrared pulse. A one is signaled by not sending any pulse. The width of each
pulse can be either 1.6 µs or 3/16ths of a single bit time. (1.6 µs equals 3/16ths of a bit time at 115.2 kbaud). This way, each word
begins with a pulse for the start bit.
Operation in IrDA 1.0 SIR is similar to the operation in UART mode. The main difference being that data transfer operations are
normally performed in half duplex fashion, and the modem control and status signals are not used. Selection of the IrDA 1.0 SIR
mode is controlled by the MDSL bits in the MCR register when the device is in extended mode, or by the IR__SL bits in the IRCR1
register when the device is in non-extended mode. This prevents legacy software, running in non-extended mode, from spuriously
switching the device to UART mode, when the software writes to the MCR register.
2.5 IrDA 1.1 MIR AND FIR MODES
The PC87108A supports both IrDA 1.1 MIR and FIR modes, with data rates of 576 kbps, 1.152 Mbps and 4.0 Mbps. Details on
the frame format, encoding schemes, CRC sequences, etc. are provided in the appropriate IrDA documents. The MIR transmitter
front-end section performs bit stuffing on the outbound data stream and places the Start and Stop flags at the beginning and end
of MIR frames. The MIR receiver front-end section removes flags and “de-stuffs” the inbound bit stream, and checks for abort con-
ditions.
The FIR transmitter front-end section adds the Preamble as well as Start and Stop flags to each frame and encodes the transmit
data into a 4PPM (Four Pulse Position Modulation) data stream. The FIR receiver front-end section strips the Preamble and flags
from the inbound data stream and decodes the 4PPM data while also checking for coding violations.
Both MIR and FIR front-ends also automatically append CRC sequences to transmitted frames and check for CRC errors on re-
ceived frames.
2.5.1 High Speed Infrared Transmit Operation
When the transmitter is empty, if either the CPU or the DMA controller writes data into the TX__FIFO, transmission of a frame will
begin. Frame transmission can be normally completed by using one of the following methods:
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