Description of Operation
18-4
ADSP-BF537 Blackfin Processor Hardware Reference
RTC Clock Requirements
The RTC timer is clocked by a 32.768 kHz crystal external to the proces-
sor. The RTC system memory-mapped registers (MMRs) are clocked by
this crystal. When the prescaler is disabled, the RTC MMRs are clocked at
the 32.768 kHz crystal frequency. When the prescaler is enabled, the
RTC MMRs are clocked at the 1 Hz rate.
There is no way to disable the RTC counters from software. If a given sys-
tem does not require the RTC functionality, then it may be disabled with
hardware tie-offs. Tie the
RTXI
and
RTCGND
pins to
EGND
, tie the
RTCVDD
pin to
EVDD
, and leave the
RTXO
pin unconnected. Additionally, writing
RTC_PREN
to 0 saves a small amount of power.
Prescaler Enable
The single active bit of the RTC prescaler enable register (
RTC_PREN
) is
written using a synchronization path. Clearing of the bit is synchronized
to the 32.768 kHz clock. This faster synchronization allows the module to
be put into high-speed mode (bypassing the prescaler) without waiting the
full 1 second for the write to complete that would be necessary if the mod-
ule were already running with the prescaler enabled. When this bit is
cleared, the prescaler is disabled, and the RTC runs at the 32.768 kHz
crystal frequency.
When setting the
RTC_PREN
bit, the first positive edge of the 1 Hz clock
occurs 1 to 2 cycles of the 32.768 kHz clock after the prescaler is enabled.
The write complete status/interrupt works as usual when enabling or dis-
abling the prescale counter. The new RTC clock rate is in effect before the
write complete status is set. In order for the RTC to operate at the proper
rate, software must set the prescaler enable bit after initial powerup. When
this bit is set, the prescaler is enabled, and the RTC runs at a frequency of
1 Hz.
Summary of Contents for Blackfin ADSP-BF537
Page 42: ...Contents xlii ADSP BF537 Blackfin Processor Hardware Reference ...
Page 90: ...Development Tools 1 32 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 138: ...Programming Examples 4 26 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 340: ...SDC Programming Examples 6 84 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 606: ...Programming Examples 9 94 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 660: ...Programming Examples 10 54 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 720: ...Electrical Specifications 11 60 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 840: ...Programming Examples 13 42 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 876: ...Programming Examples 14 36 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 938: ...Programming Examples 15 62 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 958: ...Programming Examples 17 12 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 986: ...Programming Examples 18 28 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 1162: ...G 26 ADSP BF537 Blackfin Processor Hardware Reference ...
Page 1218: ...Index I 56 ADSP BF537 Blackfin Processor Hardware Reference ...