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A Prototype Optical Tracking System Investigation and Development - page 91 / 170

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6.1 Physical layer

77

U

CTS

A

Rx

R

T

Tx

Soft

FTDI

Hub

USB

Bridge

UART

Module 1

UART

Module 2

UART

Module 3

Bus Control

Figure 6.3 The revised communications system uses a dedicated timing line. This line is asserted when the bus is in use.

TX from modules

Bus Control

1

2

3

1

Figure 6.4 A timing diagram showing three modules sending data to the Soft Hub. The bus con- trol signal is being driven by each module in turn. Each numbers represents the module that is controlling the bus, i.e., module number 1 first controls the bus.

low latency associated with detecting the falling edge of the bus control signal. However, one potential problem is that noise on the bus control signal could cause a falling edge to be registered spuriously. In the first method the likelihood of this happening is smaller as only a valid packet can trigger a module to send its data. During testing this has not been a problem. nother problem that affects both approaches is that of the firmware in a camera module crashing causing the module to become unresponsive and the system to stall when the module’s time slot becomes active. This can be fixed by allowing the Soft Hub to reset the order of time slots when it does not receive data from the modules after a certain period. However, an attempt to recover may not be the best option as the failure of one or more modules may render the system ineffective. In this case recovery would only

be useful if the offending module were reset.

Figure 6.4 shows a timing diagram for three camera modules sharing the bus. The TX signal represents the transmission path from the modules to the Soft Hub. The bus control signal shows the status of the shared bus control signal. The numbers below the signal represent which module is driving the signal. Four time slots are shown. Initially, module 1 asserts the bus control signal and transmits its data. It then enters a high impedance mode allowing the signal to fall. This triggers the second module to raise the bus control signal again and transmit its data. The same process occurs for Module 3 and following the falling edge of the bus control signal Module 1 begins sending data again.

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