Fifo control – Rainbow Electronics DS21458 User Manual
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DS21455/DS21458 Quad T1/E1/J1 Transceivers
145 of 270
24.2.1 FIFO Control
Control of the transmit and receive FIFOs is accomplished via the FIFO control (HxFC). The FIFO
control register sets the watermarks for both the transmit and receive FIFO. Bits 3–5 set the transmit low
watermark and the lower 3 bits set the receive high watermark.
When the transmit FIFO empties below the low watermark, the TLWM bit in the appropriate HDLC
status register SR6 or SR7 will be set. TLWM is a real-time bit and will remain set as long as the transmit
FIFO’s read pointer is below the watermark. If enabled, this condition can also cause an interrupt via the
INT pin.
When the receive FIFO fills above the high watermark, the RHWM bit in the appropriate HDLC status
register will be set. RHWM is a real-time bit and will remain set as long as the receive FIFO’s write
pointer is above the watermark. If enabled, this condition can also cause an interrupt via the
INT pin.
Register Name:
H1FC, H2FC
Register Description:
HDLC # 1 FIFO Control, HDLC # 2 FIFO Control
Register Address:
91h, A1h
Bit
# 7
6 5 4 3 2 1 0
Name — —
TFLWM2
TFLWM1
TFLWM0 RFHWM2 RFHWM1 RFHWM0
Default
0
0 0 0 0 0 0 0
Bits 0 to 2/Receive FIFO High Watermark Select (RFHWM0 to RFHWM2).
RFHWM2 RFHWM1 RFHWM0
RECEIVE FIFO WATERMARK (BYTES)
0 0 0
4
0 0 1
16
0 1 0
32
0 1 1
48
1 0 0
64
1 0 1
80
1 1 0
96
1 1 1
112
Bits 3 to 5/Transmit FIFO Low Watermark Select (TFLWM0 to TFLWM2).
TFLWM2 TFLWM1 TFLWM0
TRANSMIT FIFO WATERMARK (BYTES)
0 0 0
4
0 0 1
16
0 1 0
32
0 1 1
48
1 0 0
64
1 0 1
80
1 1 0
96
1 1 1
112
Bit 6/Unused, must be set to zero for proper operation.
Bit 7/Unused, must be set to zero for proper operation.