at89c55-16qa ATMEL Corporation, at89c55-16qa Datasheet - Page 8

• Manufacturer Part Number: at89c55-16qa
• Description: 8-bit Microcontroller With Bytes Flash
• Manufacturer: ATMEL Corporation

1, a 16 bit reload can be triggered either by an overflow or
by a 1-to-0 transition at external input T2EX. This transition
also sets the EXF2 bit. Both the TF2 and EXF2 bits can
generate an interrupt if enabled.
Setting the DCEN bit enables Timer 2 to count up or down,
as shown in Figure 3. In this mode, the T2EX pin controls
the direction of the count. A logic 1 at T2EX makes Timer 2
count up. The timer will overflow at 0FFFFH and set the
TF2 bit. This overflow also causes the 16 bit value in
Figure 2. Timer 2 Auto Reload Mode (DCEN = 0)
Table 4. T2MOD—Timer 2 Mode Control Register
4-176
Symbol
—
T20E
DCEN
T2MOD Address = 0C9H
Not Bit Addressable
Bit
Function
Not implemented, reserved for future use.
Timer 2 Output Enable bit.
When set, this bit allows Timer 2 to be configured as an up/down counter.
—
7
AT89C55
—
6
—
5
—
4
RCAP2H and RCAP2L to be reloaded into the timer regis-
ters, TH2 and TL2, respectively.
A logic 0 at T2EX makes Timer 2 count down. The timer
underflows when TH2 and TL2 equal the values stored in
RCAP2H and RCAP2L. The underflow sets the TF2 bit and
causes 0FFFFH to be reloaded into the timer registers.
The EXF2 bit toggles whenever Timer 2 overflows or
underflows and can be used as a 17th bit of resolution. In
this operating mode, EXF2 does not flag an interrupt.
—
3
—
2
Reset Value = XXXX XX00B
T20E
1
DCEN
0