Datasheet

13.4 Enabling a Peripheral
In order to enable a peripheral that is clocked by a Generic Clock, the following parts of the system needs
to be configured:
• A running Clock Source
• A clock from the Generic Clock Generator must be configured to use one of the running Clock
Sources, and the Generator must be enabled.
• The Peripheral Channel that provides the Generic Clock signal to the peripheral must be configured
to use a running Generic Clock Generator, and the Generic Clock must be enabled.
• The user interface of the peripheral needs to be unmasked in the PM. If this is not done the
peripheral registers will read all 0’s and any writing attempts to the peripheral will be discarded.
13.5 On Demand Clock Requests
Figure 13-4. Clock Request Routing
DFLL48M
Generic Clock
Generator
Clock request
Generic Clock
Periph. Channel
Clock request
Peripheral
Clock request
ENABLE
RUNSTDBY
ONDEMAND
CLKEN
RUNSTDBY
ENABLE
RUNSTDBY
GENEN
All clock sources in the system can be run in an on-demand mode: the clock source is in a stopped state
unless a peripheral is requesting the clock source. Clock requests propagate from the peripheral, via the
GCLK, to the clock source. If one or more peripheral is using a clock source, the clock source will be
started/kept running. As soon as the clock source is no longer needed and no peripheral has an active
request, the clock source will be stopped until requested again.
The clock request can reach the clock source only if the peripheral, the generic clock and the clock from
the Generic Clock Generator in-between are enabled. The time taken from a clock request being
asserted to the clock source being ready is dependent on the clock source startup time, clock source
frequency as well as the divider used in the Generic Clock Generator. The total startup time T
start
from a
clock request until the clock is available for the peripheral is between:
T
start_max
= Clock source startup time + 2 × clock source periods + 2 × divided clock source periods
T
start_min
= Clock source startup time + 1 × clock source period + 1 × divided clock source period
The time between the last active clock request stopped and the clock is shut down, T
stop
, is between:
T
stop_min
= 1 × divided clock source period + 1 × clock source period
T
stop_max
= 2 × divided clock source periods + 2 × clock source periods
The On-Demand function can be disabled individually for each clock source by clearing the ONDEMAND
bit located in each clock source controller. Consequently, the clock will always run whatever the clock
request status is. This has the effect of removing the clock source startup time at the cost of power
consumption.
The clock request mechanism can be configured to work in standby mode by setting the RUNSDTBY bits
of the modules, see Figure 13-4.
SAM D5x/E5x Family Data Sheet
Clock System
© 2019 Microchip Technology Inc.
Datasheet
DS60001507E-page 150