MSP430 reset circuit

Hi, Tony,i too have little knowledge on reset operation,as per my knowledge if u want to rst ur Controller only at the time of power ON then it is enough to use a RC combination rst circuit.

As per ur design, ( input supply from DC/DC converter) i don't think, for u to use a IC for resetting and it is more expensive over a RC circuit generally we use an IC when we have to monitor the input supply conditions(in ur case i think it is always constant).

In rst IC there is extra facility for detecting ur input voltage out of tolerance generally VCC>=2.7 volts if detects then the reset signal is kept in the active state for approximately 150 ms to allow the power supply and processor to stabilize and i think this facility is not in ur RC combination circuit

You are welcome.

Note that you do not even need a "RC reset circuit". You only need a pull-up resister (say 47 k-ohm). You may add a 10pF or 100pF C for noise immunity. If you are going to use SBW, you should not use a C higher than 10pF.

So why in most reliable MCU circuits designers usually use an external Reset / Power-on Reset IC ?

for example (MAX16058)

here is datasheet:

http://www.maxim-ic.com/datasheet/index.mvp/id/6046)

The MSP has an internal RC delay that holds the MSP in reset state for a certain time after the brownout circuit has detected the presence of VCC. However, some older MSPs do not have a brownout circuit and require an exernal RC.
On thsoe with brownout, the external RC might still be required if VCC is rising slowly (depends on teh power supply). The Brownout will start the RC delay ona voltage that is actually still lower than the required minimum operating voltage. This is because there is a high tolerance on the BOR voltage level (no prcision reference is available as a reference requires a minimum voltage too and also consumes too much power).
Assumption is that when VCC already has exceeded the brownout level, it will in the valid range after the additional delay. But depending on the supply this might not be true. Then the external RC is still needed. When powering with a lab supply or through the FET, no RC is needed. But if you have a battewry supply that requires some time to charge the circuitry capacitors, or a switchign regulator (especially the boost types) that needs some time to reach its output level, then the external RC is required.

Since the SBW protocol (if used instead of the 4-wire JTAG) puts a clock signal on RST pin, the maximum capacitance is 2.2nF, or the RC parts need to be separated from RST pin by another series resistor, so the RC won't act as low pass on the SBW signals.