Rainbow Electronics DS1086L - Manual (Page 8)

DS1086
DS1086 Spread-Spectrum EconOscillator
8 _____________________________________________________________________
Detailed Description
A block diagram of the DS1086 is shown in Figure 3.
The internal master oscillator generates a square wave
with a 66MHz to 133MHz frequency range. The fre-
quency of the master oscillator can be programmed
with the DAC register over a two-to-one range in 10kHz
steps. The master oscillator range is larger than the
range possible with the DAC step size, so the OFFSET
register is used to select a smaller range of frequencies
over which the DAC spans. The prescaler can then be
set to divide the master oscillator frequency by 2
x
(where x equals 0 to 8) before routing the signal to the
output (OUT) pin.
A programmable triangle-wave generator injects an off-
set element into the master oscillator to dither its output
2% or 4%. The dither is controlled by the J0 bit in the
PRESCALER register and enabled with the SPRD pin.
The maximum spectral attenuation occurs when the
prescaler is set to 1. The spectral attenuation is
reduced by 2.7dB for every factor of 2 that is used in
the prescaler. This happens because the prescaler’s
divider function tends to average the dither in creating
the lower frequency. However, the most stringent spec-
tral emission limits are imposed on the higher frequen-
cies where the prescaler is set to a low divider ratio.
The external control input, OE, gates the clock output
buffer. The
PDN pin disables the master oscillator and
turns off the clock output for power-sensitive applica-
tions*. On power-up, the clock output is disabled until
power is stable and the master oscillator has generated
512 clock cycles. Both controls feature a synchronous
enable that ensures there are no output glitches when
the output is enabled, and a constant time interval (for a
given frequency setting) from an enable signal to the
first output transition.
The control registers are programmed through a 2-wire
interface and are used to determine the output frequen-
cy and settings. Once programmed into EEPROM,
since the register settings are NV, the settings only
need to be reprogrammed if it is desired to reconfigure
the device.
OFFSET FREQUENCY RANGE (MHz)
OS - 6 61.44 to 71.67
OS - 5 66.56 to 76.79
OS - 4 71.68 to 81.91
OS - 3 76.80 to 87.03
OS - 2 81.92 to 92.15
OS - 1 87.04 to 97.27
OS* 92.16 to 102.39
OS + 1 97.28 to 107.51
OS + 2 102.40 to 112.63
OS + 3 107.52 to 117.75
OS + 4 112.64 to 122.87
OS + 5 117.76 to 127.99
OS + 6 122.88 to 133.11
*Factory default setting. OS is the integer value of the 5 LSBs
of the RANGE register.
REGISTER ADDR MSB BINARY LSB
FACTORY
DEFAULT
ACCESS
PRESCALER 02h X 1 X 1 X X J0 P3 P2 P1 P0 11100000b R/W
DAC HIGH 08h b9 b8 b7 b6 b5 b4 b3 b2 01111101b R/W
DAC LOW 09h b1 b0 X 0 X 0 X 0 X 0 X 0 X 0 00000000b R/W
OFFSET 0Eh X 1 X 1 X 1 b4 b3 b2 b1 b0 1 1 1 - - - - - b R/W
ADDR 0Dh X
1
X
1
X
1
X
1
WC A2 A1 A0 11110000b R/W
RANGE 37h X
X
X
X
X
X
b4 b3 b2 b1 b0 x x x - - - - - b R
WRITE EE 3Fh NO DATA — —
Table 1. Register Summary
X 0 = Don’t care, reads as zero.
X 1 = Don’t care, reads as one.
X X = Don’t care, reads indeterminate.
X = Don’t care.
Table 2. Offset Settings
*The power-down command must persist for at least two out-
put frequency cycles plus 10µs for deglitching purposes.