The MAX705-MAX708/MAX813L from Maxim Integrated (now part of Analog Devices) are low-cost µP supervisory circuits that combine the housekeeping functions a processor board would otherwise build from discrete parts. The MAX705 integrates four: a reset generator, a watchdog timer, a power-fail comparator, and a manual-reset input. MAX705CSA+T is the RoHS ("+"), tape-and-reel ("T") 8-pin SO option; within the family the part code sets the reset threshold and RESET polarity (the MAX705 is the 4.65 V, active-low RESET̅ version).

The reset function holds the µP in a known state whenever the supply is invalid. The MAX705 monitors V_CC against a precision 4.65 V threshold and asserts RESET̅ on power-up, power-down or a brownout, holding it for a guaranteed 200 ms after V_CC recovers - and crucially, RESET̅ is guaranteed valid down to V_CC = 1 V, so the processor is held even as the rail collapses. A debounced manual-reset input (MR̅) lets a button force a clean reset.

The other two functions guard against software and supply failures. The watchdog timer expects the µP to toggle WDI within 1.6 s; if the software hangs, WDO̅ asserts. The independent power-fail comparator compares PFI to a 1.25 V reference and drives PFO̅ - typically used to sense the unregulated input through a divider for an early "power is failing" interrupt before the regulated rail drops. The MAX705 operates over the commercial 0 °C to +70 °C range in the 8-pin SO.

Note: Values are taken from the Maxim MAX705-MAX708/MAX813L datasheet. Confirm the exact threshold and RESET polarity for the MAX705CSA against the latest official documentation before committing a design.

MAX705CSA+T ships in the 8-pin SO (0.150-inch narrow SOIC, package outline 21-0041) - the small body that puts a four-function supervisor next to the processor. The layout is straightforward: route RESET̅ to the µP reset, WDI from a clean point in the main loop and WDO̅ back to reset/interrupt, divide the monitored rail into PFI with PFO̅ to an interrupt, add the MR̅ button, and bypass V_CC. The dimensional drawing below gives the full mechanical detail.

The 8-pin SO carries the manual reset, supply, the watchdog and power-fail pins, and the reset output (per the MAX705 datasheet):

• Pin 1 - MR̅: Manual-reset input (active-low, debounced).
• Pin 2 - V_CC: Supply input (monitored against 4.65 V).
• Pin 3 - GND: Ground.
• Pin 4 - PFI: Power-fail comparator input (compared to 1.25 V).
• Pin 5 - PFO̅: Power-fail comparator output.
• Pin 6 - WDI: Watchdog input; toggle within 1.6 s.
• Pin 7 - RESET̅: Active-low reset output; valid to V_CC = 1 V.
• Pin 8 - WDO̅: Watchdog output; asserts on watchdog timeout.

Within the MAX705-MAX708/MAX813L family the variants differ by reset threshold and polarity: the MAX706 drops the watchdog, the MAX707/MAX708 use a 4.40 V threshold and active-high RESET, and the MAX708 adds the power-fail comparator. Related supervisory and power parts are below.

Selection Tips: Choose the MAX705 for a 4.65 V, active-low-reset supervisor with watchdog, power-fail comparator and manual reset. Step to the MAX706 (no watchdog), the MAX707/MAX708 (4.40 V, active-high RESET) or the MAX813L per the threshold and polarity your processor needs. The MAX6368/MAX6369 are SOT23 supervisor/watchdog alternatives.

Maxim Integrated, now part of Analog Devices, Inc. (ADI), created the MAX705-MAX708/MAX813L to put a processor's reset, watchdog, power-fail and manual-reset housekeeping into one low-cost SO - replacing a handful of discrete comparators and timers.

ADI maintains the datasheet, the threshold/polarity option table and the watchdog and power-fail application guidance for the family - the basis for a reliable µP-supervision design.