2001 May 07
13
Philips Semiconductors
Preliminary specification
Digital Signal Processor
SAA7715H
8.6
I
2
C-bus interface (pins SCL and SDA)
The I
2
C-bus format is described in “The I
2
C-bus and how
to use it” order no. 9398 393 40011.
For the external control of the SAA7715 a fast I
2
C-bus is
implemented. This is a 400 kHz bus which is downward
compatible with the standard 100 kHz bus.
There are two different types of control instructions:
Loading of the Program RAM (PRAM) with the required
DSP program
– Programming the coefficient RAM (YRAM)
– Instructions to control the DSP program.
Selection of the digital serial input/output format to be
used, the DSP clock speed.
The detailed description of the I
2
C-bus and the description
of the different bits in the memory map is given in
Chapter 9.
8.7
Reset
The reset (pin DSP_RESET) is active LOW and needs an
external 22 k
pull-up resistor. Between this pin and the
V
SSI
ground a capacitor of 1
μ
F should be connected to
allow a proper switch-on of the supply voltage. The
capacitor value is such that the chip is in reset as long as
the power supply is not stabilized. A more or less fixed
relationship between the DSP reset and the POM time
constantisobligatory.Thevoltageonpin POMdetermines
the current flowing in the DACs.
The reset sets all I
2
C-bus bits to their default value and it
restarts the DSP program.
8.8
Power-down mode
The Power-down mode switches off all activity on the chip.
The Power-down mode can be switched on and off using
pin POWERDOWN. This pin needs to be connected to
ground if not used. The following applies for the
Power-down mode:
Power-down mode may only be switched on when there
is no I
2
C-bus activity to or from the SAA7715
Power-down mode may not be switched on before the
complete chip has been reset (DSP_RESET
active LOW)
The clock signal on pin CLK_IN should be running
during Power-down mode
It is advised to set pin POM to logic 0 before switching
on the Power-down mode and set it back to logic 1 after
the chip actually returns from Power-down mode as
shown in Fig.4
All on-chip registers and memories will keep their values
during Power-down mode
Digital serial outputs are not muted, the last value is kept
on the output
The SAA7715 will not ‘lock-up’ the I
2
C-bus during
Power-down mode (SDA line).
Figure 4 shows the time the chip actually is in Power-down
mode after switching on/off pin POWERDOWN.
handbook, full pagewidth
MGT828
tA
tB
POWERDOWN
device actually in
Power-down mode
POM
CLK_IN
Fig.4 Power-down mode.
t
A
= 4
×
(256/CLK_IN); 8.192 MHz < CLK_IN < 12.288 MHz.
t
A
= 4
×
(512/CLK_IN); 16.384 MHz < CLK_IN < 24.576 MHz.
t
B
= 128
×
(256/CLK_IN); 8.192 MHz < CLK_IN < 12.288 MHz.
t
B
= 128
×
(512/CLK_IN); 16.384 MHz < CLK_IN < 24.576 MHz.