Compal Electronics Inc PICONODE Manual De Usuario
PicoNode Integration Specification
23
7 SPI Messaging Protocol
The SPI Messaging between the Host processor and the node is controlled/handled by the Host
Common Library, compiled onto the user’s apps processor. The following section (all of
Chapter 7) is informational only.
Common Library, compiled onto the user’s apps processor. The following section (all of
Chapter 7) is informational only.
The details of Host/Node messaging are typically not necessary for integrators to implement.
7.1 Arbitration
Arbitration is the process a Host uses to signal to the Node that it supports the bi-directional
messaging protocol. The arbitration sequence is designed to reduce the probability that an
arbitrary non-Host transfer sequence can mirror a valid arbitration sequence.
messaging protocol. The arbitration sequence is designed to reduce the probability that an
arbitrary non-Host transfer sequence can mirror a valid arbitration sequence.
Arbitration consists of both Host and Node transmitting an arbitration request/reply pair. After
a defined turn-around delay, both transmit a validation request/reply. The turn-around delay
avoids race conditions between Host and Node and provides enough time to allow ISR
execution to complete before the next SPI transfer.
a defined turn-around delay, both transmit a validation request/reply. The turn-around delay
avoids race conditions between Host and Node and provides enough time to allow ISR
execution to complete before the next SPI transfer.
If the Node does not reply to the Host request, the Host needs to wait for a turn-around delay
and retry the arbitration request.
and retry the arbitration request.
The Host must perform the arbitration sequence before any other SPI Bus communication can
take place between the Host and the Node.
take place between the Host and the Node.
The Host must initiate this arbitration sequence on boot up. Additionally, the Host must
perform the arbitration sequence when the Node sends to the Host an arbitration message.
This can occur due to the Node going into Deep Sleep and then waking up. Since the Node
requires the arbitration sequence after waking from Deep Sleep and since the Host is not aware
of when the Node goes to Deep Sleep, the Host must be able to detect that the Node is
requesting arbitration and the Host must then reset its Host interface state machine and
perform arbitration. For more information on the Host interface SPI bus state machine, refer to
section 7.3: Host Interface SPI Bus State Machine.
perform the arbitration sequence when the Node sends to the Host an arbitration message.
This can occur due to the Node going into Deep Sleep and then waking up. Since the Node
requires the arbitration sequence after waking from Deep Sleep and since the Host is not aware
of when the Node goes to Deep Sleep, the Host must be able to detect that the Node is
requesting arbitration and the Host must then reset its Host interface state machine and
perform arbitration. For more information on the Host interface SPI bus state machine, refer to
section 7.3: Host Interface SPI Bus State Machine.
7.2 Message Protocol
Host-to-Node transfers use master message command pairs and Node-to-Host transfers use
slave message command pairs. Both transfers use identical command sequences with only the
encoding of the commands differing. The command sequence for a message transfer consists
of a request/acknowledgement pair followed by a defined turn-around delay and then a
message composed of a header pair and a payload.
slave message command pairs. Both transfers use identical command sequences with only the
encoding of the commands differing. The command sequence for a message transfer consists
of a request/acknowledgement pair followed by a defined turn-around delay and then a
message composed of a header pair and a payload.
Variable length payloads are supported by encoding the payload size in the second half of the
message request. The second half of the message reply contains the available receive buffer
size. If the message payload size exceeds the receive buffer size, then a new request must be
made after a turn-around delay with a payload size that does not exceed the receive buffer size.
message request. The second half of the message reply contains the available receive buffer
size. If the message payload size exceeds the receive buffer size, then a new request must be
made after a turn-around delay with a payload size that does not exceed the receive buffer size.