How to know whether Power on reset or Software reset has occured in 8051 microcontrller

Question

I am developing an application on ATMEL AT89C51 of 8051 family.

Could anyone suggest how to determine in coding whether the reset has been done due to power cycle or through software?

Answer 1

I don't think there is a method to determine how reset has occurred because once reset everything starts from the beginning in 8051.

One method i guess would work is,

Say take a variable X, before every software code of reset, just set X=1 (indicating software reset) and store this variable in any ROM if you interfaced externally.

On every reset, at the beginning include an instance which checks this variable X to see which reset had occurred and change X to 0, for next time detection.

If you do not have an external ROM, interface an D-latch atleast.

I hope this works. Do tell me if this works.

Answer 2

According to the Atmel 8051 Microcontrollers Hardware Manual (PDF link), the power-off flag (POF / bit 4) in the power control register (PCON / 87h) is set by hardware when VCC rises from 0 to its nominal voltage. The power-off flag reset value will be 1 only after a power on (cold reset). A warm reset (e.g. software reset) does not affect the value of this bit.

I've often found that different vendors implement their own registers in the SFR space that can be taken advantage of for cases such as this. For example, Silicon Labs uses a power-on reset flag (PORSF) in their reset source register (RSTSRC).

Answer 3

It really depends if you wanted to depend on some specific 8051 variant vendor. It is best to use vendor provided registers, but if you changed vendor your code will brake, or even worse, it will misbehave.

If you had external RAM in your system (and it was not battery powered), than you could write a sequence of bytes (like 0xAA, 0x55...) somewhere in the reserved part of the memory, and check if it was still there after start up. If not, you have had a cold start. Of course, you should modify assembler start up code to make sure it does not initialize this part of memory (or it would be zero at each start), and you should instruct your linker to exclude this memory from linkage so that it does not get used by anything else.

Finally, include conditional compilation in your code so that if you had some 8051 variant with special registers, it would be used, if not, try the plan B.

I have done that with few bytes of internal 8051 memory /all my external RAM was battery powered/ and then I have learned than not every 8051 variant has had consistent policy at start up - some have all their internal memory initialized, some have initialized only SFR and some other specific areas leaving me few bytes to play with the procedure described.