Cirrus Logic CDB5378 - Manual (Page 24)

CDB5378
24 DS639DB4
2.3.1.2 Configuration - SPI Port
On CDB5378, configuration of the digital filter is through the SPI port by the on-board 8051 microcontrol-
ler, which receives commands from the PC evaluation software via the USB interface. Evaluation software
commands can write/read digital filter registers, specify digital filter coefficients and start/stop digital filter
operation. Alternately, the digital filter can automatically load configuration information from an on-board
serial EEPROM.
The configuration method for the digital filter is selected by the BOOT signal from a dip switch (S1, #4).
By default the BOOT signal is set low (S1, #4 - LO) to indicate configuration information is written by the
microcontroller. If BOOT is instead set high (S1, #4 - HI), the digital filter attempts to automatically read
configuration information from the serial EEPROM after reset. Configuration information is initially written
into the serial EEPROM by jumpering its chip select input (EECS) to the microcontroller chip select output
(SS) and sending EEPROM programming commands and data from the PC evaluation software.
2.3.1.3 Phase Locked Loop
To make synchronous analog measurements throughout a distributed system, a synchronous system
clock is required to be provided to each measurement node. For evaluation testing purposes, a BNC clock
input on CDB5378 can receive an external system clock and create a synchronous local clock using the
CS5378 PLL.
The system clock into the BNC clock input is applied to the CS5378 CLK input by selecting CLK_EXT on
the DIGITAL FILTER CLOCK jumper (J16). The CS5378 PLL input frequency is specified at reset by the
state of the GPIO[4..6]:PLL[0..2] pins, as detailed in the CS5378 data sheet.
Specification Value
Input Clock Frequency
1.024, 2.048, 4.096 MHz
Distributed Clock Synchronization ± 240 ns
Maximum Input Clock Jitter, RMS 1 ns
Specification Value
PLL Internal Clock Frequency
32.768 MHz
Maximum Jitter, RMS 300 ps
Loop Filter Architecture Internal