Datasheet

06 February 2012 Broadcom Europe Ltd. 406 Science Park Milton Road Cambridge CB4 0WW Page 105
© 2012 Broadcom Corporation. All rights reserved
6.3 General Purpose GPIO Clocks
The General Purpose clocks can be output to GPIO pins. They run from the peripherals clock
sources and use clock generators with noise-shaping MASH dividers. These allow the GPIO
clocks to be used to drive audio devices.
The fractional divider operates by periodically dropping source clock pulses, therefore the
output frequency will periodically switch between:
DIVI
frequencysource _
&
1
_
+
DIVI
frequencysource
Jitter is therefore reduced by increasing the source clock frequency. In applications where
jitter is a concern, the fastest available clock source should be used.
The General Purpose clocks have MASH noise-shaping dividers which push this fractional
divider jitter out of the audio band.
MASH noise-shaping is incorporated to push the fractional divider jitter out of the audio band
if required. The MASH can be programmed for 1, 2 or 3-stage filtering. MASH filter, the
frequency is spread around the requested frequency and the user must ensure that the module
is not exposed to frequencies higher than 25MHz. Also, the MASH filter imposes a low limit
on the range of DIVI.
MASH min DIVI min output freq average output freq max output freq
0 (int divide) 1 source / ( DIVI ) source / ( DIVI ) source / ( DIVI )
1 2 source / ( DIVI ) source / ( DIVI + DIVF / 1024 ) source / ( DIVI + 1 )
2 3 source / ( DIVI - 1 ) source / ( DIVI + DIVF / 1024 ) source / ( DIVI + 2 )
3 5 source / ( DIVI - 3 ) source / ( DIVI + DIVF / 1024 ) source / ( DIVI + 4 )
Table 6-32 Effect of MASH Filter on Frequency
The following example illustrates the spreading of output clock frequency resulting from the
use of the MASH filter. Note that the spread is greater for lower divisors.
PLL
freq
(MHz)
target
freq
(MHz)
MASH divisor DIVI DIVF
min
freq
(MHz)
ave
freq
(MHz)
max
freq
(MHz)
error
650 18.32 0 35.480 35 492 18.57 18.57 18.57 ok
650 18.32 1 35.480 35 492 18.06 18.32 18.57 ok
650 18.32 2 35.480 35 492 17.57 18.32 19.12 ok
650 18.32 3 35.480 35 492 16.67 18.32 20.31 ok
400 18.32 0 21.834 21 854 19.05 19.05 19.05 ok
400 18.32 1 21.834 21 854 18.18 18.32 19.05 ok
400 18.32 2 21.834 21 854 17.39 18.32 20.00 ok
400 18.32 3 21.834 21 854 16.00 18.32 22.22 ok
200 18.32 0 10.917 10 939 20.00 20.00 20.00 ok
200 18.32 1 10.917 10 939 18.18 18.32 20.00 ok
200 18.32 2 10.917 10 939 16.67 18.32 22.22 ok
200 18.32 3 10.917 10 939 14.29 18.32 28.57 error
Table 6-33 Example of Frequency Spread when using MASH Filtering
It is beyond the scope of this specification to describe the operation of a MASH filter or to
determine under what conditions the available levels of filtering are beneficial.