Rainbow Electronics AT90LS8535 - Manual (Page 46)

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AT90S/LS8535
1041H–11/01
In PWM mode, the Timer Overflow Flag (TOV2) is set when the counter advances from
$00. Timer Overflow Interrupt2 operates exactly as in normal Timer/Counter mode, i.e.,
it is executed when TOV2 is set, provided that Timer Overflow Interrupt and global inter-
rupts are enabled. This also applies to the Timer Output Compare flag and interrupt.
The frequency of the PWM will be Timer Clock Frequency divided by 510.
Asynchronous Status
Register – ASSR
• Bit 7..4 – Res: Reserved Bits
These bits are reserved bits in the AT90S8535 and always read as zero.
• Bit 3 – AS2: Asynchronous Timer/Counter2
When AS2 is set (one), Timer/Counter2 is clocked from the TOSC1 pin. Pins PC6 and
PC7 become connected to a crystal oscillator and cannot be used as general I/O pins.
When cleared (zero), Timer/Counter2 is clocked from the internal system clock, CK.
When the value of this bit is changed, the contents of TCNT2, OCR2 and TCCR2 might
get corrupted.
• Bit 2 – TCN2UB: Timer/Counter2 Update Busy
When Timer/Counter2 operates asynchronously and TCNT2 is written, this bit becomes
set (one). When TCNT2 has been updated from the temporary storage register, this bit
is cleared (zero) by hardware. A logical “0” in this bit indicates that TCNT2 is ready to be
updated with a new value.
• Bit 1 – OCR2UB: Output Compare Register2 Update Busy
When Timer/Counter2 operates asynchronously and OCR2 is written, this bit becomes
set (one). When OCR2 has been updated from the temporary storage register, this bit is
cleared (zero) by hardware. A logical “0” in this bit indicates that OCR2 is ready to be
updated with a new value.
• Bit 0 – TCR2UB: Timer/Counter Control Register2 Update Busy
When Timer/Counter2 operates asynchronously and TCCR2 is written, this bit becomes
set (one). When TCCR2 has been updated from the temporary storage register, this bit
is cleared (zero) by hardware. A logical “0” in this bit indicates that TCCR2 is ready to be
updated with a new value.
If a write is performed to any of the three Timer/Counter2 registers while its Update Busy
flag is set (one), the updated value might get corrupted and cause an unintentional inter-
rupt to occur.
The mechanisms for reading TCNT2, OCR2 and TCCR2 are different. When reading
TCNT2, the actual timer value is read. When reading OCR2 or TCCR2, the value in the
temporary storage register is read.
1 0 $FF H
1 1 $00 H
1 1 $FF L
Table 20. PWM Outputs OCR2 = $00 or $FF
COM21 COM20 OCR2 Output PWM2
Bit 7 6 5 4 3 2 1 0
$22 ($22) – – – – AS2 TCN2UB OCR2UB TCR2UB ASSR
Read/Write R R R R R/W R R R
Initial Value 0 0 0 0 0 0 0 0