Cirrus Logic AN130 - Manual (Page 2)

AN130
2 AN130REV2
3. SOFTWARE DESCRIPTION
This note presents algorithms to initialize the
PIC16C84 and the CS5521/22/23/24/28, perform
calibrations, modify the CS5521/22/23/24/28’s in-
ternal registers, and acquire a conversion. Figure 2
depicts a block diagram of the main program struc-
ture. While reading this application note, please re-
fer to Section 8. “APPENDIX: PIC16C84
Microcode to Interface to the
CS5521/22/23/24/28” on page 6 for the code list-
ing.
3.1 Initialize
Initialize is a subroutine that configures Port A
(RA) on the PIC16C84 and places the serial port of
the CS5521/22/23/24/28 into the command state.
RA’s data direction is configured as depicted in
Figure 1 by writing to the TRISA register (for more
information on configuring ports, see the
PIC16C84 Data Sheet). The controller then enters
a number of delay states to allow the appropriate
time for the ADC’s oscillator to start up and stabi-
lize (oscillator start-up time for a 32.768 KHz crys-
tal is typically about 500ms). Finally, the ADCs
serial port is reset by sending fifteen bytes of logic
1’s followed by a single byte with its LSB at logic
0 to SDI (the serial port is initialized after any pow-
er-on reset, and this software re-initialization is for
demonstration purposes) Once the proper sequence
of bits has been received, the serial port on the
ADC is in the command state, where it waits for a
valid command.
3.2 Write Channel Setup Registers
The subroutine write_csrs is an example of how to
write to the CS5521/22/23/24/28’s Channel Setup
Registers (CSRs). For this example, two CSRs
(four Setups) are written. The number of CSRs to
be accessed is determined by the Depth Pointer bits
(DP3-DP0) in the configuration register. The
Depth Pointer bits are set to “0011” to access the
two CSRs. The value “0011” is calculated by tak-
ing the number of Setups to be accessed and sub-
tracting 1. Because each CSR holds two Setups,
this number must always be an odd value, that is,
START
INITIALIZE
SELF-OFFSET CAL.
MODIFY GAIN
ACQUIRE CONVERSION
MICROCONTROLLER & ADC
Figure 2. CS5521/22/23/24/28 Software Flowchart
WRITE CSRs
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