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manual/getting_started: fix minor errors found during tutorial

This commit is contained in:
Sebastien Bourdeauducq 2014-11-17 14:34:00 -07:00
parent 07e4749421
commit f11fb0f553
1 changed files with 2 additions and 1 deletions
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doc/manual/getting_started.rst
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@ -99,7 +99,7 @@ Create a new file ``rtio.py`` containing the following: ::
Connect an oscilloscope or logic analyzer to the RTIO channel 1 (pin C11 on the Papilio Pro) and run ``python3 rtio.py``. Notice that the generated signal's period is precisely 4 microseconds, and that it has a duty cycle of precisely 50%. This is not what you would expect if the delay and the pulse were implemented with CPU-controlled GPIO: overhead from the loop management, function calls, etc. would increase the signal's period, and asymmetry in the overhead would cause duty cycle distortion. Connect an oscilloscope or logic analyzer to the RTIO channel 1 (pin C11 on the Papilio Pro) and run ``python3 rtio.py``. Notice that the generated signal's period is precisely 4 microseconds, and that it has a duty cycle of precisely 50%. This is not what you would expect if the delay and the pulse were implemented with CPU-controlled GPIO: overhead from the loop management, function calls, etc. would increase the signal's period, and asymmetry in the overhead would cause duty cycle distortion.
Instead, inside the core device, output timing is generated by the gateware and the CPU only programs switching commands with certain timestamps that the CPU computes. This guarantees precise timing as long as the CPU can keep generating timestamps that are increasing fast enough. In case it fails to do that (and attempts to program an event with a timestamp in the past), the :class:`artiq.devices.runtime_exceptions.RTIOUnderflow` exception is raised. The kernel causing it may catch it (using a regular ``try... except...`` construct), or it will be propagated to the host. Instead, inside the core device, output timing is generated by the gateware and the CPU only programs switching commands with certain timestamps that the CPU computes. This guarantees precise timing as long as the CPU can keep generating timestamps that are increasing fast enough. In case it fails to do that (and attempts to program an event with a timestamp in the past), the :class:`artiq.coredevice.runtime_exceptions.RTIOUnderflow` exception is raised. The kernel causing it may catch it (using a regular ``try... except...`` construct), or it will be propagated to the host.
Try reducing the period of the generated waveform until the CPU cannot keep up with the generation of switching events and the underflow exception is raised. Then try catching it: :: Try reducing the period of the generated waveform until the CPU cannot keep up with the generation of switching events and the underflow exception is raised. Then try catching it: ::
@ -111,6 +111,7 @@ Try reducing the period of the generated waveform until the CPU cannot keep up w
class Tutorial(AutoContext): class Tutorial(AutoContext):
parameters = "led o" parameters = "led o"
@kernel
def run(self): def run(self):
self.led.set(0) self.led.set(0)
try: try: