DSP56000 INTERFACING
similar to the QSPI interface seen in Figure 14. As mentioned
in the QSPI section, the DSP56000 must be programmed to
enable the interface when a LOW to HIGH transition on SC1
is observed (BUSY going HIGH at the end of conversion).
The DSP56000 serial interface has SPI compatibility mode
with some enhancements. Figure 16 shows an interface
between the ADS7807 and the DSP56000 which is very
The DSP56000 can also provide the convert pulse by includ-
ing a monostable multi-vibrator, as seen in Figure 17. The
receive and transmit sections of the interface are decoupled
(asynchronous mode) and the transmit section is set to
generate a word length frame sync every other transmit
frame (frame rate divider set to 2). The prescale modulus
should be set to 3.
Convert Pulse
DSP56000
ADS7807
R/C
The monostable multi-vibrator in this circuit will provide
varying pulse widths for the convert pulse. The pulse width
will be determined by the external R and C values used with
the multi-vibrator. The 74HCT123N data sheet shows that
the pulse width is (0.7) RC. Choosing a pulse width as close
to the minimum value specified in this data sheet will offer the
best performance. See the Starting A Conversion section of
this data sheet for details on the conversion pulse width.
SC1
BUSY
SRD
SCO
SDATA
DATACLK
CS
EXT/INT
BYTE
The maximum conversion rate for a 20.48MHz DSP56000 is
exactly 40kHz. Note that this will not be the case for the
ADS7806. See the ADS7806 data sheet (SBAS021B) for
more information.
SYN = 0 (Asychronous)
GCK = 1 (Gated clock)
SCD1 = 0 (SC1 is an input)
SHFD = 0 (Shift MSB first)
WL1 = 1 WL0 = 0 (Word length = 16 bits)
FIGURE 16. DSP56000 Interface to the ADS7807.
74HCT123N
+5V
+5V
DSP56000
R
B1
REXT1
CEXT1
Q1
C
ADS7807
SC2
CLR1
A1
R/C
SC0
SRD
DATACLK
SDATA
CS
EXT/INT
BYTE
SYN = 0 (Asychronous)
GCK = 1 (Gated clock)
SCD2 = 1 (SC2 is an output)
SHFD = 0 (Shift MSB first)
WL1 = 1 WL0 = 0 (Word length = 16 bits)
FIGURE 17. DSP56000 Interface to the ADS7807. Processor Initiates Conversions.
ADS7807
SBAS022C
19
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