1.5. Dual-Port RAM Interface
Command 0x24: Load DAC
The TFP’s 10MHz oscillator frequency is voltage controlled using the output from a 16-bit DAC. This
command allows the user to directly load a 16-bit value to the DAC. This feature allows the user to
fine tune the TFP time base when operating in the Free Running Mode. This voltage is also routed out
of the TFP via the J1 connector (pin 9) allowing external oscillators to be disciplined. The DAC output
voltage ranges nominally from 0 V (value = 0x0000) to 5 V (value = 0xFFFF).
JP3 is used to select the DAC voltage range. When the 2mm jumper is OFF, the DAC voltage is 0-5
VDC; when ON the voltage is 0-10 VDC. Both on-board oscillators that are offered for this card use
the 0-5 VDC control voltage range. If an external oscillator requires a 0-10 VDC control voltage range,
a 2mm jumper must be placed on JP3.
Byte
Type
Item
Value or Range
0
UINT8
ID
0x24
1-2
UINT16
DAC value
0x0000 - 0xFFFF
Command 0x25: Set Disciplining Gain
This command allows the gain and sense of the disciplining process to be set by the user. A positive
gain indicates that the voltage-controlled oscillator source frequency increases with increasing con-
trol voltage. This feature is valuable to anyone using the TFP to discipline an external oscillator.
Byte
Type
Item
Value or Range
0
UINT8
ID
0x25
1-2
INT16
gain
-100 to +100
Note: Use this command with caution, as it will affect the TFP disciplining routine.
Gain Default:
n
0x02 = TCXO
n
0x20 = OCXO
Command 0x26: Request Battery Connection Status
This request (used with command 0x19) reads the current RTC battery connection status which is
either connected (0x1) or disconnected (0x0).
Byte
Type
Item
Value or Range
0
UINT8
ID
0x26
1-2
INT16
battstat
0x0, 0x1
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