
DS26518 8-Port T1/E1/J1 Transceiver 
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9. 
FUNCTIONAL DESCRIPTION 
9.1 
Processor Interface 
Microprocessor control of the DS26518 is accomplished through the 28 hardware pins of the microprocessor port. 
The 8-bit parallel data bus can be configured for Intel or Motorola modes of operation with the bus type select 
(BTS) pin. When the BTS pin is a logic 0, bus timing is in Intel mode, as shown in 
Figure 13-2
 and 
Figure 13-3
. 
When the BTS pin is a logic 1, bus timing is in Motorola mode, as shown in 
Figure 13-4
 and 
Figure 13-5
. The 
address space is mapped through the use of 13 address lines, A[12:0]. Multiplexed mode is not supported on the 
processor interface. 
The chip-select bar (
CSB
) pin must be brought to a logic-low level to gain read and write access to the 
microprocessor port. With Intel timing selected, the read-data bar (
RDB
) and write-read bar (
WRB
) pins are used to 
indicate read and write operations and latch data through the interface. With Motorola timing selected, the read-
write bar (
RWB
) pin is used to indicate read and write operations while the data-strobe bar (
DSB
) pin is used to 
latch data through the interface. 
The interrupt output pin (
INTB
) is an open-drain output that asserts a logic-low level upon a number of software 
maskable interrupt conditions. This pin is normally connected to the microprocessor interrupt input. 
9.1.1 SPI Serial Port Mode 
The external processor bus can be configured to operate in SPI serial bus mode. See Section 
9.1.2
 for detailed 
timing diagrams.  
When SPI_SEL = 1, SPI bus mode is implemented using four signals: clock (SPI_SCLK), master out-slave in data 
(SPI_MOSI), master in-slave out data (SPI_MISO), and chip select (
CSB
). Clock polarity and phase can be set by 
the D[7]/SPI_CPOL and D[6]/SPI_CPHA pins.   
The order of the address and data bits in the serial stream is selectable using the D[5]/SPI_SWAP pin. The R/W bit 
is always first and B bit is always last in the initial control word and are not effected by the D[5]/SPI_SWAP pin 
setting. 
SPI mode is not recommended for HDLC operations because of the bandwidth constraints of SPI. 
9.1.2 SPI Functional Timing Diagrams 
Note: 
The transmit and receive order of the address and data bits are selected by the D[5]/SPI_SWAP pin. The 
R/W (read/write) MSB bit and B (burst) LSB bit position is not affected by the D[5]/SPI_SWAP pin setting. 
9.1.2.1 SPI Transmission Format and CPHA Polarity 
When SPI_CPHA = 0, 
CSB
 may be deasserted between accesses. An access is defined as one or two control 
bytes followed by a data byte. 
CSB
 cannot be deasserted between the control bytes, or between the last control 
byte and the data byte. When SPI_CPHA = 0, 
CSB
 may also remain asserted between accesses. If it remains 
asserted and the BURST bit is set, no additional control bytes are expected after the first control byte(s) and data 
are transferred. If the BURST bit is set, the address will be incremented for each additional byte of data transferred 
until 
CSB
 is deasserted. If 
CSB
 remains asserted and the BURST bit is not set, a control byte(s) is expected 
following the data byte, and the address for the next access will be received from that. Anytime 
CSB
 is deasserted, 
the BURST access is terminated. 
When SPI_CPHA = 1, 
CSB
 may remain asserted for more than one access without being toggled high and then 
low again between accesses. If the BURST bit is set, the address should increment and no additional control bytes 
are expected. If the BURST bit is not set, each data byte will be followed by the control byte(s) for the next access. 
Additionally, 
CSB
 may also be deasserted between accesses when SPI_CPHA = 1. In the case, any BURST 
access is terminated and the next byte received when 
CSB
 is reasserted will be a control byte. 
The following diagrams describe the functionality of the SPI port for the four combinations of SPI_CPOL and 
SPI_CPHA. They indicate the clock edge that samples the data and the level of the clock during no-transfer events 
(high or low). Since the SPI port of the DS26518 acts as a slave device, the master device provides the clock. The