
DS26518 8-Port T1/E1/J1 Transceiver 
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9.10.1.1 
HDLC FIFO Control 
Control of the transmit and receive FIFOs is accomplished via the Receive HDLC FIFO Control (
RHFC
) and 
Transmit HDLC FIFO Control (
THFC
) registers. The FIFO control registers set the watermarks for the FIFO.  
When the receive FIFO fills above the high watermark, the RHWM bit (
RRTS5
.1) will be set. RHWM and THRM are 
real-time bits and will remain set as long as the FIFO’s write pointer is above the watermark. When the transmit 
FIFO empties below the low watermark, the TLWM bit in the 
TRTS2
 register will be set. TLWM is a real-time bit 
and will remain set as long as the transmit FIFO’s write pointer is below the watermark. If enabled, this condition 
can also cause an interrupt via the 
INTB
 pin. 
If the receive HDLC FIFO does overrun the current packet being processed is dropped. The receive FIFO is 
emptied. The packet status bit in 
RRTS5
 and 
RLS5
.5 (ROVR) indicate an overrun. 
9.10.1.2 
Receive Packet Bytes Available 
The lower 7 bits of the Receive HDLC Packet Bytes Available Register (
RHPBA
) indicates the number of bytes (0 
to 64) that can be read from the receive FIFO. The value indicated by this register informs the host as to how many 
bytes can be read from the receive FIFO without going past the end of a message. This value will refer to one of 
four possibilities, the first part of a packet, the continuation of a packet, the last part of a packet, or a complete 
packet. After reading the number of bytes indicated by this register the host then checks the HDLC status registers 
for detailed message status. 
If the value in the 
RHPBA
 register refers to the beginning portion of a message or continuation of a message, then 
the MSB of the RHPBA register will return a value of 1. This indicates that the host can safely read the number of 
bytes returned by the lower 7 bits of the RHPBA register, but there is no need to check the information register 
since the packet has not yet terminated (successfully or otherwise). 
9.10.1.3 
HDLC Status and Information 
RRTS5
, 
RLS5
, and 
TLS2
 provide status information for the HDLC controller. When a particular event has occurred 
(or is occurring), the appropriate bit in one of these registers will be set to a one. Some of the bits in these registers 
are latched and some are real-time bits that are not latched. This section contains register descriptions that list 
which bits are latched and which are real-time. With the latched bits, when an event occurs and a bit is set to a 
one, it will remain set until the user reads and clears that bit. The bit will be cleared when a 1 is written to the bit 
and it will not be set again until the event has occurred again. The real-time bits report the current instantaneous 
conditions that are occurring and the history of these bits is not latched. 
Like the other latched status registers, the user will follow a read of the status bit with a write. The byte written to 
the register will inform the device which of the latched bits the user wishes to clear (the real-time bits are not 
affected by writing to the status register). The user will write a byte to one of these registers, with a one in the bit 
positions he or she wishes to clear and a zero in the bit positions he or she does not wish to clear. 
The HDLC status registers 
RLS5
 and 
TLS2
 have the ability to initiate a hardware interrupt via the 
INTB
 output 
signal. Each of the events in this register can be either masked or unmasked from the interrupt pin via the HDLC 
interrupt mask registers 
RIM5
 and 
TIM2
. Interrupts will force the 
INTB
 signal low when the event occurs. The INTB 
pin will be allowed to return high (if no other interrupts are present) when the user reads the event bit that caused 
the interrupt to occur. 
9.10.1.4 
HDLC Receive Example 
The HDLC status registers in the DS26518 allow for flexible software interface to meet the user’s preferences. 
When receiving HDLC messages, the host can choose to be interrupt driven, to poll to desired status registers, or a 
combination of polling and interrupt processes can be used. An example routine for using the DS26518 HDLC 
receiver is given in 
Figure 9-17
.