Complex Arbritration Example

Consider a complex mote based on the telosb mote that includes the following:

• serial communication for direct connection.
• serial communication for gps.
• serial communication disabled.
• radio communication (SPI based)
• mass storage using a Secure Digital card (SPI based)

• Serial communication utilizes UART1/USART1 with an external multiplexer for switching between the three choices.
  
• Radio communication uses a CC2420 ChipCom radio that uses the SPI bus. This uses SPI1/USART1.
  
• SD Mass Storage uses the SPI and uses SPI1/USART1.

Communications can be in one of three modes, NONE, SERIAL, or RADIO. This will determine which DefaultOwner will own the resource. NONE being selected allows USART1 and associated h/w to be powered down. SERIAL being selected configures the serial hardware (including connecting clocks and timers which prevent the CPU from going into a lower power mode). SERIAL will own the h/w unless another module requests it. This ownership allows the serial port to receive packets. The SERIAL driver manages incoming bytes on a packet level and will only release the h/w at a packet boundary.

Impact of GPS

The addition of GPS allows multiple serial stacks to be active at the same time. Assume that SERIAL is selected when the GPS is turned on. The GPS driver will request the UART1 h/w and will need to keep it while receiving packets from the GPS. But while the GPS is functioning it is desirable to be able to receive control packets from the SERIAL port. The SERIAL module is a default owner so the GPS module obtains the port for some amount of time, releases (at a message boundary), and then requests again after an interval (allowing the SERIAL port to potentially start receiving a packet), the SERIAL port won't release if a packet has started to be received. The SERIAL driver will release when it gets to a packet boundary.

In addition to interacting with the SERIAL driver, the GPS module should be prepared to release (at a message boundary) if another module (such as the SD driver) requests the hardware. If too much time passes other modules can be starved. Serious problems can develop (current code will panic). This argues for the GPS releasing either on request or via timing.

Other Considerations

• Forcing the GPS to release/request at message boundaries costs significant overhead from the unconfigure/configure at each release/request/grant. It is advantagous to minimize how often the release/request/grant occurs.
• Basing module release purely on time seems rigid. Policy based on requests seems to be more flexible. However it does increase complexity of both implementation as well as symantics.
• During normal operation, the GPS module and SERIAL (direct connect) are mutually exclusive. When the mote is plugged in we know where it is and there is no need for the GPS to tell us. When the radio is active, it can receive while being disconnected from the CPU. A h/w event is generated which signals the RADIO driver. In this sense, the RADIO and GPS do not compete with each other. The RADIO driver when presented with the receive event will request, when granted it obtains the SPI bus and handles the receive packet.
  
  This argues for the normal access mechanism being based on request rather than special casing using timers. The GPS releasing/requesting at every message boundary would be expensive and is counter-indicated. An efficient mechanism for switching between different modules could use the ResourceRequested.requested interface. Another possibility is to make GPS a DefaultOwner. Basically any module that needs to sit on the hardware would be a DefaultOwner.