PNX1300/01/02/11 Data Book
Philips Semiconductors
8-2
PRELIMINARY SPECIFICATION
8.3
CLOCK SYSTEM
Figure 8-1
illustrates the different clock capabilities of the
AI unit. At the heart of the clock system is a square wave
DDS (Direct Digital Synthesizer). The DDS can be pro-
grammed to emit frequencies from approx. 1 Hz to 40
MHz with a resolution of better than 0.3 Hz.
The output of the DDS is always sent on the AI_OSCLK
output pin. This output is intended to be used as the
256f
or 384f
system clock source instead of a fixed fre-
quency crystal for oversampling A/D converters, such as
the Philips SAA7366T, or Analog Devices AD1847.
The PNX1300 AI DDS frequency is set by writing to the
FREQUENCY MMIO register. The programmer can
change the FREQUENCY setting dynamically, so as to
adjust the input sampling rate to track an application de-
pendent master reference.
Depending on bit 31 (MSB), the DDS runs in one of two
modes:
bit 31 = 1 (PNX1300 improved mode)
bit 31 = 0 (TM-1000 compatibility mode)
8.3.1
PNX1300 Improved Mode
In improved mode, a high quality, low-jitter AI_OSCLK is
generated. The setting of the FREQUENCY register to
accomplish a given AI_OSCLK frequency is given by:
This mode, and the above formula, should be used for all
new software development on PNX1300. It is not avail-
able on TM-1000.
In the improved mode the DDS synthesizer maximum jit-
ter can be computed as follows:
9
f
DSPCPU
Example of jitter values can be found in
Table 8-2
.
8.3.2
TM-1000 Compatibility Mode
TM-1000 compatibility mode is provided so that TM-1000
software runs without changes. It should NOT be used
for new PNX1300 software development. TM-1000
mode is automatically entered whenever FREQUEN-
CY[31] = 0. In TM-1000 mode, AI_OSCLK frequency is
set as follows:
f
2
32
3
f
DSPCPU
8.4
CLOCK SYSTEM OPERATION
AI_SCK and AI_WS can be configured as input or out-
put, as determined by the SER_MASTER control field.
As output, AI_SCK is a divider of the DDS output fre-
quency. Whether input or output, the AI_SCK pin signal
is used as the bit clock for serial-parallel conversion.
If set as output, AI_WS can similarly be programmed us-
ing WSDIV to control the serial frame length from 1 to
512 bits.
The preferred application of the clock system options is
to use AI_OSCLK as A/D master clock, and let the A/D
converter be timing master over the serial interface
(SER_MASTER=0).
In case an external codec (e.g. the AD1847 or CS4218)
is used for common audio I/O, it may not be possible to
independently control the A/D and D/A system clocks. In
that case it is recommended that the Audio Out AO) unit
FREQUENCY
AI_OSCLK
AI_SCK
AI_WS
div N+1
SCKDIV
div N+1
Square Wave DDS
9
×
DSPCPUCLK
AI_SD
SER_MASTER
Serial To Parallel Converter
16
16
LEFT[15:0]
RIGHT[15:0]
sample_clock
(e.g. 64
×
f
s
)
WSDIV
31
0
7
0
0
8
(e.g. 256
×
f
s
)
Figure 8-1. AI clock system and I/O interface.
FREQUENCY
2
31
f
2
32
9
f
DSPCPU
-----------------------------
+
=
jitter
-------------1
=
Table 8-2. Jitter values for common DSPCPU MHz
f
DSPCPU
(MHz)
jitter
(nSec)
f
DSPCPU
(MHz)
jitter
(nSec)
143
166
0.777
0.669
180
200
0.617
0.555
FREQUENCY
-----------------------------
=
SCKDIV
0 255
[ ,
]
∈
f
AISCK
f
1
+
SCKDIV
=