Maxim Integrated MAXQ7666 - Manual (Page 85)

2.5.1 Power-Up Counter
An independent power-up counter functions as the startup counter to count 65,536 cycles of the internal 7.6MHz RC oscillator from ini-
tial power-on. This time period is verified by the counter after the DVDD level reaches the reset threshold (V
RST ). The counter is active
only during initial power-up and is completely shut off during normal operation.
2.5.2 DVDD Brownout Reset (BOR)
The DVDD brownout reset monitoring is enabled when the VDPE bit in the APE register is set to logic 1. The BOR circuitry monitors the
DVDD voltage and invokes a brownout reset state to halt program execution if the DVDD voltage falls in the threshold range deter-
mined by the VDBR[1:0] bits in the VMC register. The MAXQ7665/MAXQ7666 are held in the brownout reset state (Figure 2-5) while
the DVDD voltage is below the reset threshold level and the
RESET pin is asserted to be logic-low. Table 2-2 shows the supported
brownout reset threshold range. When the DVDD power sources return above the threshold level, a brownout reset cycle is performed.
For the MAXQ7665/MAXQ7666 to exit brownout reset, the following condition must apply:
• DVDD is above the brownout reset threshold level determined by the VDBR[1:0] bits.
Once the above condition is satisfied, the brownout reset condition is removed automatically ( RESET pin goes high) and software exe-
cution will begin at the reset vector location 8000h (in the utility ROM). A brownout reset cycle is similar to power-on reset cycle shown
in Figure 2-4, except that there is no power-up counter delay, V
RST is determined by the VDBR[1:0] bits, and some selected register
bits are maintained and not reset to default state. For example, the VDBR1 and VDBR0 bits are only cleared by POR, not by BOR.
A brownout reset caused by a DVDD drop below the selected threshold level appears to be the same as a power-on reset, only if DVDD
voltage falls below ~1.2V and rises back above the default POR rising voltage threshold. In such a case, the MAXQ7665/MAXQ7666
go through a complete POR reset (see Figure 2-6) as described in Section 2.5 and the POR flag in the WDCN register will be set.
Note: The DVDD brownout reset monitoring is enabled (VDPE = 1) by default after all forms of reset. The VDBR1 and VDBR0 bits are
only cleared by POR (only if DVDD goes below ~1.2V in the case of BOR) and retain the selected level after all other forms of reset.
Table 2-2. DVDD Brownout Reset Threshold Range
VDBR1:VDBR0
DVDD BROWNOUT
RESET THRESHOLD RANGE (V)*
00 2.70–2.99 (default)
01 2.77–3.06
10 2.84–3.13
11 2.91–3.20
MAXQ7665/MAXQ7666 User’s Guide
2-13
* Reconfirm the values provided in this table with those in the latest MAXQ7665 and MAXQ7666 data sheets.
Maxim Integrated