Cirrus Logic AN225 User Manual

Application Note

USING THE CS5460A AUTO-BOOT MODE

AN225

1. Which EEPROMs Can Be Used?

Several industry-standard serial EEPROMs that will successfully run auto-boot with the CS5460A are listed below:

•Atmel

AT25010 AT25020 AT25040

• National Semiconductor

NM25C040M8 NM25020M8

•Xicor

X25040SI

These types of serial EEPROMs expect a specific 8-bit command word (00000011) in order to perform a memory download. The CS5460A has been hardware programmed to transmit this 8-bit command word to the EEPROM at the beginning of the auto-boot sequence.

1.1 Further Explanation of the Auto-Boot Sequence

The auto-boot sequence is terminated by writing a ‘1’ to the STOP bit in the CS5460A’s Control Register. This action is performed as the last command in the EEPROM command sequence. At the completion of the write to the Control Register (provided STOP bit = “1”), SCLK stops, and CS thereby reducing power consumed by the EEPROM. At completion of the Auto-Boot sequence, the serial port will revert to functioning as a slavemode interface. Therefore, if desired, the CS5460A registers can still be read by an external device, such as a central office controller, connected to the meter assembly by a bus interface.

1.2 Sample Auto-Boot Sequence

The serial data for such a sample sequence is shown below in single-byte hexidecimal notation:

rises,

40 00 00 61 ;In Configuration Register,

turn high-pass filters on, set K=1.

44 7F C4 A9 ;Write value of 0x7FC4A9 to

Current Channel Gain Register.

46 7F B2 53 ;Write value of 0x7FB253 to

Voltage Channel DC Offset Register.

4C 00 00 14 ;Set Pulse Rate Register to

0.625 Hz.

74 00 00 04 ;Unmask bit #2 (“LSD” bit in

the Mask Register).

E8 ;Start performing continuous

computation cycles.

78 00 01 40 ;Write STOP bit to Control

When the CS5460A is commanded by the EEPROM to perform a certain operation, the operation will not be pre-maturely terminated by the assertion of the Control Register’s STOP bit. In the above example, the ‘Start Conversions’ command (0xE8) is issued from the EEPROM, and therefore the CS5460A will continue to perform continuous A/D conversions even after the STOP bit is asserted.

1.3 How do I reset the CS5460A in Auto-Boot Mode during Brown-Out/Black-Out conditions?

The power line that is to be metered may enter a black-out or brown-out condition at certain times, due to problems at the power plant or other environmental conditions (ground fault, electrical storms, etc.) In such conditions, it is important for the meter assembly to accomplish a proper reset,

pulse

www.cirrus.com

FEB ‘03

AN225REV1

AN225

so that it can continue normal metering operations once the line power is restored. When the CS5460A is controlled by a microcontroller, the microcontroller is typically programmed (by the user) to handle these power-fail-reset situations. In the case of auto-boot, the CS5460A may be expected to reset itself (by re-executing the Auto-Boot sequence) whenever the line-power is restored. Figure 1 shows a reasonably reliable way to configure the CS5460A’s RESET

and INT pins of the CS5460A to restart the Auto-Boot sequence after a brown-out or black-out condition. This configuration employs a diode, a resistor, and a capacitor on the RESET

pininanattempttoallowtheCS5460A to reboot after a sudden loss of power, followed by a reinstatement of power.

Note that in the above auto-boot example code set (see Section 1.2) the LSD bit is un-masked, in order to cause a high-to-low transition on the INT

pin whenever the PFMON low-supply threshold is reached on the PFMON pin. If a power supply loss condition is sensed on PFMON, then the INT

pin is asserted to low (because LSD is un-masked), which allows the BAT85 diode to quickly drain the charge on C

. But whenever the +5V power is

BOOT

restored, the resistor-capacitor network will force RESET the RESET

to recharge slowly. The slow rise-time on

pin can help to allow the oscillator circuitry and the CS5460A’s internal reference circuitry enough time to stabilize before the device attempts to re-execute with the Auto-Boot sequence. This will allow the CS5460A to resume its normal metering operations after power is restored. (User must provide suitable resistor divider configuration on the PFMON pin, see Figure 1.) Use of this configuration does not guarantee that the CS5460A will reset properly, when exposed to any sudden disturbance in power.

In addition to the configuration described above, the designer should include sizeable commonmode capacitors to the VA+/VD+ pins (see Figure 1). Such capacitance on the analog/digital power supply pins will increase the amount of time over which the CS5460A will remain operational after power is lost, which therefore increases the chances that the CS5460A will successfully re-execute a proper reboot upon restoration of power. Suggested values are >47 µF (per pin) or >100 µF (total).

LOAD

Figure 1. CS5460A Auto-Boot Configuration: Automatic Restart After Power Failure

47 uF

AGND

4.096 MHz CRYSTAL

AGND

+5V

VA+ VD+

PFMON

VIN+

VIN-

CS5460A

IIN+

IIN-

XIN

XOUT

REFIN REFOUT

VA- DGND

+5V

Mode

EOUT

EDIR

INT

RESET

DGND

10k

30uF

SPI

47 uF

BOOT

DGND

E2PROM

BAT85

00000

TOTALIZER

DGND

This resistor-capacitor- diode configuration helps to ensure a smooth power-down, as well as a proper power-up/reset during and after a power black-out or brown-out event.

+ 2 hidden pages