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3.5
Clock Setup
Figure 3-1 shows the relationship of the various clock sources on the SIO4BX board. These clock sources can be
most simply viewed in three sections: On-Board Programmable Clocks, IO connector Clocks, and USC Clocks.
The Programmable Clocks consist of a single on-board programmable oscillator and four post divide clocks (one for
each channel). The single programmable oscillator clock is used as the input for each of the programmable clock
post dividers, which will allow each channel to have a unique programmable clock input. These programmable
clocks are further described in sections 2.1.12 and 3.6.
The IO Connector Clocks consist of the cable RxC and cable TxC for each channel, and an Auxiliary Clock signal
(AuxC) which may be configured as either input or output. The RxC is always an input and may be used as a clock
source for either the cable TxC or the USC Clocks. The cable TxC is always an output configured by the Pin Source
register. The Auxiliary clock may be used as an output or input clock signal, or as a general purpose IO, configured
by the Pin Source register. See Section 2.1.11 for further information on the Pin Source register.
The USC Clocks (USC_RxC and USC_TxC) are bidirectional signals. Even though the names of these clocks seem
to imply a receive clock and a transmit clock, both clocks are bidirectional, fully programmable, and identical in
function – either clock may be used for transmit or receive. The USC clocks may be sourced from either the USC or
the FPGA (via the Pin Source register). The user must be careful to ensure that both the USC and Pin Source
Register are setup to agree. If a USC clock is set as an output in the USC, it should be programmed as an input in
the Pin Source register. Likewise, if a USC clock source is driven from the Pin Source register, the user should
program the pin as an input to the USC. Section 2.1.11 describes the Pin Source Registers.
On-Board
Programmable
Oscillator
tristate when
Output
from USC
tristate when
Output
from USC
8302A Whitesburg Drive Huntsville, AL 35802, Phone: (256) 880-8787
FPGA
Prog Clk
RxAuxC
Prog Clk
Prog Clk
RxAuxC
RxC
USC TxC
Source
Pin Source Reg
D2:D0
USC RxC
Source
Pin Source Reg
D5:D3
Figure 4-1 – Clock Configuration
PC104P-SIO4BX User Manual, Revision: 0
General Standards Corporation
TxAuxC
Pin Source Reg
D18:D17
RxAuxC
RxC
RxC
TxC Source
Pin Source Reg
D8:D6
USC TxC
USC RxC
RxAuxC/TxAuxC
IO
RxC
Connector
TxC
USC TxC
USC
USC RxC
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