Si5345-44-42-D-RM
Rev. 1.0
51
8.2. DCO with Direct Register Writes
When an N divider numerator (Nx_NUM) and its corresponding update bit (Nx_UPDATE) are written, the new
numerator value will take effect, and the output frequency will change without any glitches. The N divider
numerator and denominator terms (Nx_NUM and Nx_DEN) can be left- and right-shifted so that the least
significant bit of the numerator word represents the exact step resolution that is needed for your application. Each
N divider has an update bit (Nx_UPDATE) that must be written to cause the written values to take effect. All N
dividers can be updated at the same time by writing the N_UPDATE_ALL bit. Writing this bit will NOT cause any
output glitching on an N divider that did not have its numerator or denominator changed.
When changing the N divider denominator (Nx_DEN), it is remotely possible that a small phase hit of ~550 fs may
occur at the exact time of the frequency change. However, with the proper setup, it is possible to change Nx_DEN
and never have a phase hit. If your application requires changing an N divider denominator, contact Silicon Labs at
https://www.silabs.com/support/pages/contacttechnicalsupport.aspx
for support.
Table 34. Frequency Increment/Decrement Control Registers
Register Name
Hex Address [Bit Field]
Function
Si5345
Si5344
Si5342
FINC
0x001D[0]
0x001D[0]
0x001D[0]
Asserting this bit will increase the DSPLL
output frequency by the frequency step
word.
FDEC
0x001D[1]
0x001D[1]
0x001D[1]
Asserting this bit will decrease the DSPLL
output frequency by the frequency step
word.
N0_FSTEPW
0x033B[7:0]-
0x0340[7:0]
0x033B[7:0]-
0x0340[7:0]
0x033B[7:0]-
0x0340[7:0]
This is a 44-bit frequency step word for
each of the Multi-
Synths. The Nx_FSTEPW will be added or
subtracted to the output frequency during
assertion of the FINC/FDEC bits or pins.
The Nx_FSTEPW is calculated based on
the frequency configuration and is easily
determined using the ClockBuilder Pro
N1_FSTEPW
0x0341[7:0]-
0x0346[7:0]
0x0341[7:0]-
0x0346[7:0]
0x0341[7:0]-
0x0346[7:0]
N2_FSTEPW
0x0347[7:0]-
0x034C[7:0]
0x0347[7:0]-
0x034C[7:0]
—
N3_FSTEPW
0x034D[7:0]-
0x0352[7:0]
0x034D[7:0]-
0x0352[7:0]
—
N4_FSTEPW
0x0353[7:0]-
0x0358[7:0]
—
—
N_FSTEP_MSK
0x0339[4:0]
0x0339[3:0]
0x0339[1:0]
This mask bit determines if a FINC or FDEC
affects N0, N1, N2, N3, N4. 0 = FINC/FDEC
will Increment/decrement
the Nx_FSTEPW to the selected Multi-
Synth(s), 1 = Ignores FINC/FDEC.