timer mode 1 and edsim giving different result

Question

IN 8051, Timer 0 in mode 1, if the maximum count reaches, the timer should displayed with 0000H, and should stay 0000h until we stop it and start again . But with this program

 MOV TMOD,#01H
 MOV TH0,#0FFH
 MOV TH1,#0DCH
 SETB TR0

The timer counts up to FFFFH then again starts from 0000, is not it unexpected?

1. in some internet reference it says, when it finishes counting to ffffh it "stops", does it really stop or is it just running in the background and pretending as if it has stopped by staying at 0000?

2. after the counting is finished and when it is showing 0000, at this moment if we really stop the counter by CLR TR0, will it imply now the counter is really reset? and if just after that SETB TR0 is used, then will it count from 0000?

Answer 1

The 8051 timers definitely keep counting unless you explicitly stop them.¹

So:

• No, it is not unexpected.
• No, it does not actually stop.
• No, it does not guarantee that it is reset, because the counter may have advanced by one or more counts before the firmware actually got around to stopping it.

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¹ In fact, there's a technique called "precision reload" that takes advantage of this to eliminate accumulated error when using the timer repeatedly to create interrupts at regular intervals, despite some jitter in the interrupt latency.