Cirrus Logic CS5460A, Manual

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CS5460A
32 DS284PP4
These types of serial EEPROMs expect a specific
8-bit command word (00000011) in order to per-
form a memory download. The CS5460A has been
hardware programmed to transmit this 8-bit com-
mand word to the EEPROM at the beginning of the
auto-boot sequence.
The auto-boot sequence is terminated by writing a
‘1’ to the STOP bit in the CS5460A’s Control Reg-
ister. This action is performed as the last command
in the EEPROM command sequence. At the com-
pletion of the write to the Control Register (provid-
ed STOP bit = “1”), SCLK stops, and CS rises,
thereby reducing power consumed by the EE-
PROM. At completion of the Auto-Boot sequence,
the serial port will revert to functioning as a
slave-mode interface.
Therefore, if desired, the
CS5460A registers can still be read by an external
device, such as a central office controller, connect-
ed to the meter assembly by a bus interface.
When the CS5460A is commanded by the EE-
PROM to perform a certain operation, the opera-
tion 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 assert-
ed.Auto-Boot Reset 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 envi-
ronmental conditions (ground fault, electrical
storms, etc.) In such conditions, it is important for
the meter assembly to accomplish a proper reset, 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 expect-
ed to reset itself (by re-executing the Auto-Boot se-
quence) whenever the line-power is restored.
Figure 15 shows a reasonably reliable way to con-
figure the CS5460A’s RESET and INT pins so that
CS5460A to restart the Auto-Boot sequence after a
brown-out or black-out condition. This configura-
tion employs a diode, a resistor, and a capacitor on
the RESET pin in attempt to allow the CS5460A to
reboot after a sudden loss of power, followed by a
reinstatement of power.
Note in the above auto-boot example code set (see
Section 4.3.2) that the LSD bit is un-masked, in or-
der 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
BOOT
. But whenever the +5V power is restored,
the resistor-capacitor network will force RESET to
recharge slowly. The slow rise-time on the RESET
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 pro-
vide suitable resistor divider configuration on the
PFMON pin, see Figure 15.) Use of this configu-
ration does not guarantee that the CS5460A will re-
set properly, when exposed to any sudden
disturbance in power.
In addition to the configuration described above,
the
designer should include sizeable com-
mon-mode capacitors to the VA+/VD+ pins (see
Figure 15). 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-ex-
ecute a proper reboot upon restoration of power.
Suggested values are >47 µF (per pin) or >100 µF
(total).