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AN-621
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
Programming the AD9832/AD9835
By Colm Slattery
INTRODUCTION
This application note details how to program 5 MHz on
the output of the AD9832/AD9835 parts. The frequency
register, defer register, and command sequence are
explained in detail.
THE FREQUENCY REGISTER
The AD9832 contains a 32-bit frequency register. The
register is shown below and is broken up into four bytes.
Since the AD9832 accepts only eight bits of data in any one
command, in order to fully program the 32-bit register the
user must input four commands, each delivering only eight
bits of data.
For addressing purposes, the 32-bit register is broken up
as follows: The 16 MSBs are dened as having eight higher
bits (H) and eight lower bits (L). Similarly, the 16 LSBs have
the same addressing scheme, eight higher (H) bits and
eight lower (L) bits.
32-BIT FREQUENCY REGISTER
MSB LSB
8 H MSBs 8 L MSBs 8 H LSBs 8 L LSBs
THE FREQUENCY WORD
The frequency word is the 32-bit word which the user
inputs into the frequency register. This determines the
output frequency and is based on the ratio of the required
output frequency to the reference clock frequency. In the
example below, the reference clock is 25 MHz. The re-
quired output needs to be 5 MHz, so the frequency word
is calculated as:
(5/25) (0xFFFFFFFF) = 0x33333333
THE DEFER REGISTER
The defer register (16 bits wide) acts as a buffer reg-
ister to the last register written to. The defer register
is written to in two writes. The rst eight bits will be
held (deferred) in the register until the next eight bits
are written in. When all 16 bits have been written into
the defer register, the frequency register will be updated.
The defer register always points to the last address
that was written to.
FREQ REG 0
FREQ REG 1
16 BITS 16 BITS
DEFER REGISTER
DEFER REGISTER POINTS
TO LAST FREQUENCY REGISTER
WRITTEN TO.
16 BITS 16 BITS
8 BITS 8 BITS
Since the defer register has stored the last 16 bits writ-
ten to it and is still pointing to the frequency register
previously addressed, it is possible to update either the
lower eight bits or the upper eight bits of that frequency
register with one write. This is called a direct write and is
actually the last part of a normal write sequence.
Note that if users want to write to any other register, they
will have to do a full write sequence as shown above.
