Maxim-integrated MAXQ7666 Bedienungsanleitung Seite 84

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MAXQ7665/MAXQ7666 Users Guide
2-12
2.4 Linear Regulator
The MAXQ7665/MAXQ7666 contain a +3.3V, low dropout (LDO) linear regulator. The regulator powers the MAXQ7665/MAXQ7666 dig-
ital core functions including the CPU, flash, SRAM, oscillator, and all the digital peripherals. The linear regulator is powered by the +5V
DVDDIO supply. The REGEN signal must be connected to GNDIO to enable the internal regulator. When the internal linear regulator is
disabled (REGEN connected to DVDDIO), an external +3.3V supply must be used.
2.5 Power-On Reset
The MAXQ7665/MAXQ7666 support an on-chip power-on reset (POR) circuit to ensure proper initialization of internal device states. The
POR circuit provides a power-on rising voltage threshold and a minimum power-on delay sufficient to accomplish this initialization. When
power is first applied to the MAXQ7665/MAXQ7666, the MAXQ7665/MAXQ7666 are held in a power-on reset state (Figure 2-4). The
MAXQ7665/MAXQ7666 power-on circuitry (POR) monitors the DVDD supply voltage in relation to the on-chip band gap voltage reference.
On power-up, once DVDD exceeds ~1.2V, the RESET pin is asserted to be logic-low. All the internal system and peripheral registers are
reset if DVDD from cold start exceeds ~1.2. Also, above this voltage, the power-on-reset delay counter is started.
For the MAXQ7665/MAXQ7666 to exit power-on reset, the following two conditions must apply:
DVDD is above the power-on-reset rising voltage threshold level V
RST
(2.7V–2.99V power-on default)
The internal RC oscillator has completed 65,536 cycles (power-on-reset delay for power supply to stabilize; about 8.6ms at 7.6MHz)
Once the power-up period has elapsed, the reset condition is removed automatically (RESET pin goes high) and software execution will
begin at the reset vector location 8000h (in the utility ROM). Software can determine whether a reset was caused by a power-on reset by
checking the POR flag in the WDCN register. This flag is set to 1 following a power-on reset, and should be cleared by software after it
has been checked.
Figure 2-4. MAXQ7665/MAXQ7666 Power-On Reset
POWER-UP DELAY (65,536 RC CYCLES
OR 8.6ms AT 7.6MHz)
INTERNAL RESET
NOMINAL DVDD
(+3.3V)
V
RST
(+2.7V)
+1.2V
DGND
INTERNAL
RC
STARTUP TIME
RESET PIN
Note: In a brownout reset (BOR) situation (see
Section 2.5.2
), where the voltage drops below the DVDD BOR threshold (e.g., 2.7V) and
rises back above the default power-on-reset rising voltage threshold level (2.7V), the POR flag in WDCN register will not be set unless
DVDD drops below ~1.2V. The POR flag will be set if DVDD voltage drops below ~1.2V and rises back above the default POR rising
voltage threshold (2.7V). In such a case, the MAXQ7665/MAXQ7666 go through a complete POR reset as described above.
Maxim Integrated
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