
DALLAS SEMICONDUCTOR
DS21FF42/DS21FT42
101899
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11. STATUS AND INFORMATION REGISTERS
There is a set of nine registers per channel that contain information on the current real time
status of a framer in the DS21Q42, Status Register 1 (SR1), Status Register 2 (SR2),
Receive Information Registers 1 to 3 (RIR1/RIR2/RIR3) and a set of four registers for the
onboard HDLC and BOC controller. The specific details on the four registers pertaining to
the HDLC and BOC controller are covered in Section 19 but they operate the same as the
other status registers in the DS21Q42 and this operation is described below.
When a particular event has occurred (or is occurring), the appropriate bit in one of these
nine registers will be set to a one. All of the bits in SR1, SR2, RIR1, RIR2, and RIR3
registers operate in a latched fashion. This means that if an event or an alarm occurs and a
bit is set to a one in any of the registers, it will remain set until the user reads that bit. The
bit will be cleared when it is read and it will not be set again until the event has occurred
again (or in the case of the RBL, RYEL, LRCL, and RLOS alarms, the bit will remain set
if the alarm is still present). There are bits in the four HDLC and BOC status registers that
are not latched and these bits are listed in Section 19.
The user will always precede a read of any of the nine registers with a write. The byte
written to the register will inform the DS21Q42 which bits the user wishes to read and have
cleared. The user will write a byte to one of these registers, with a one in the bit positions
he or she wishes to read and a zero in the bit positions he or she does not wish to obtain the
latest information on. When a one is written to a bit location, the read register will be
updated with the latest information. When a zero is written to a bit position, the read
register will not be updated and the previous value will be held. A write to the status and
information registers will be immediately followed by a read of the same register. The read
result should be logically AND’ed with the mask byte that was just written and this value
should be written back into the same register to insure that bit does indeed clear. This
second write step is necessary because the alarms and events in the status registers occur
asynchronously in respect to their access via the parallel port. This write–read– write scheme
allows an external microcontroller or microprocessor to individually poll certain bits
without disturbing the other bits in the register. This operation is key in controlling the
DS21Q42 with higher–order software languages.
The SR1, SR2, and FDLS registers have the unique ability to initiate a hardware interrupt
via the INT* output pin. Each of the alarms and events in the SR1, SR2, and HSR can be
either masked or unmasked from the interrupt pin via the Interrupt Mask Register 1 (IMR1),
Interrupt Mask Register 2 (IMR2), and HDLC Interrupt Mask Register (HIMR) respectively.
The FIMR register is covered in Section 19. The INTERRUPT STATUS REGISTER can
be used to determine which framer is requesting interrupt servicing and the type of the
request: status or the HDLC controller.