Cirrus Logic AN131 - Manual (Page 2)

AN131
2 AN131REV2
3. SOFTWARE DESCRIPTION
This note presents algorithms to initialize the
68HC05 and the CS5521/22/23/24/28, modify the
CS5521/22/23/24/28’s internal registers, perform
calibrations, and acquire conversions. Figure 2 de-
picts a block diagram of the program structure.
While reading this application note, please refer to
Section 6. “APPENDIX: 68HC05 Microcode to In-
terface to the CS5521/22/23/24/28” on page 6 for
the code listing.
3.1 Initialize
Initialize is a subroutine that configures the SPI on
the 68HC05 and places the CS5521/22/23/24/28
into the command state. The SPI is configured as
depicted in Figure 1 by selecting the 68HC05 as the
master. To function properly with the
CS5521/22/23/24/28, the SPI must be set up to use
a clock which idles low, and begins clocking data
with a rising edge in the center of the first bit (for
more information on configuring the SPI refer to
Motorola’s 68HC05 Application Guide). After
configuring the SPI, the controller enters a delay
state to allow time for the CS5521/22/23/24/28’s
power-on-reset and oscillator to start-up (oscillator
start-up time for a 32.768 KHz crystal is typically
500 ms). The last step is to reinitialize the serial
port on the ADC (reinitializing the serial port is un-
necessary here, and the code is for demonstration
purposes only). This is implemented by sending the
converter fifteen bytes of logic 1’s followed by one
final byte, with its LSB at logic 0. This sequence
places the serial port of the ADC into 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-
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START
INITIALIZE MICROCONTROLLER
SELF-OFFSET CAL.
MODIFY GAIN
ACQUIRE CONVERSION
AND CS5521/22/23/24/28
Figure 2. CS5521/22/23/24/28 Software Flowchart