Datasheet

AD7888
–12–
CS and the second rising edge of SCLK as shown in Figure 14a.
In microcontroller applications, this is readily achievable by
driving the CS input from one of the port lines and ensuring
that the serial data read (from the microcontrollers serial port) is
not initiated for 5 µs. In DSP applications, where the CS is
generally derived from the serial frame synchronization line, it is
not possible to separate the first falling edge and second rising
edge of SCLK after the CS falling edge by up to 5 µs. There-
fore, the user will need to write to the Control Register to exit
this mode and (by writing PM1 = 0 and PM0 = 0) put the part
into normal mode. A second conversion will then need to be
initiated when the part is powered up to obtain a conversion
result as shown in Figure 14b.
SCLK
4 LEADING ZEROES
+ CONVERSION RESULT
CS
DOUT
DATA INDIN
1
168
4 LEADING ZEROES
+ CONVERSION RESULT
DATA IN
1
168
4 LEADING ZEROES
+ CONVERSION RESULT
DATA IN
116
8
PM1 AND PM0 = 0 TO PLACE
THE PART IN NORMAL MODE
PM1 = 1 AND PM0 = 0 TO
PLACE THE PART BACK IN
AUTOSHUTDOWN MODE
CONTROL REGISTER DATA IS LOADED ON
THE FIRST 8 CLOCKS. PM1 = 1 AND PM0 = 0
THE PART ENTERS
SHUTDOWN AT THE END
OF CONVERSION AS
PM1 = 1 AND PM0 = 0
THE PART REMAINS POWERED
UP AS PM1 AND PM0 = 0
THE PART BEGINS TO POWER-
UP FROM SHUTDOWN
THE PART ENTERS
SHUTDOWN AT THE END OF
CONVERSION AS PM1 = 1
AND PM0 = 0
Figure 14b. Autoshutdown Operation
SCLK
4 LEADING ZEROES + CONVERSION RESULT
CS
DOUT
DATA IN
DIN
CONTROL REGISTER DATA IS LOADED ON
THE FIRST 8 CLOCKS. PM1 = 1 AND PM0 = 1
1
16
4 LEADING ZEROES + CONVERSION RESULT
DATA IN
PM1 = 1 AND PM0 = 1 TO KEEP
THE PART IN THIS MODE
1
16
THE PART POWERS UP
FROM STANDBY ON SCLK
FALLING EDGE AS PM1 = 1
AND PM0 = 1
THE PART ENTERS
STANDBY AT THE END OF
CONVERSION AS
PM1 = 1 AND PM0 = 1
Figure 15. Autostandby Operation
Autostandby (PM1 = 1, PM0 = 1)
In this mode, the AD7888 automatically enters a standby (or
sleep) mode at the end of every conversion. In this standby
mode, all on-chip circuitry, apart from the on-chip reference, is
powered down. This mode is similar to the autoshutdown but
in this case, the power-up time is much shorter as the on-chip
reference remains powered up at all times.
Figure 15 shows the general diagram of the operation of the
AD7888 in this mode. On the first falling SCLK edge after CS
goes low, the AD7888 comes out of standby. The AD7888
wake-up time is very short in this mode so it is possible to wake
up the part and carry out a valid conversion in the same read/
write operation. The input signal is sampled on the second
rising edge of SCLK following the CS falling edge. At the end
of conversion (last rising edge of SCLK) the part automatically
enters its standby mode.
REV. C