EMAC PRIMER Instruction Manual page 19

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termination pulse is reached if so desired. The user can also determine the timer interval by programming the counter

Mode 0 No operation mode (NOP) does not affect the timer.

Mode 1 Stop mode stops the timer/counter if it is running otherwise NOP.

Mode 2 Stops the timer if running immediately after the terminal count has been reached otherwise NOP.

Mode 3 The start mode loads the output mode and count length and starts the timer/counter immediately if timer is not

running, otherwise it waits for the terminal count then starts the timer/counter.

The timer/counter has four output modes which are as follows:

Mode 0 Outputs a low during the second half of the count, which is equivalent to a single square wave.

Mode 1 Outputs a continuous square wave when the terminal count is reached.

Mode 2 Outputs a single pulse when the terminal count is reached.

Mode 3 Outputs a single pulse and reloads automatically.

The timer/counter operating modes are programmed through the 8155 control register (I/O address 10H). The lower 8 bits

of the count length is written to I/O address 14H. The upper 6 bits of the count length along with the 2 bit output mode is

written to I/O address 15H. The timer/counter can be used as an external program interval timer. If you wish to perform a

software operation at a specific time interval, then the timer/counter can be programmed to that interval. Upon the resulting

timer interrupt your program can execute the desired software.

The timer/counter is also used to drive the PRIMER's speaker. Different frequencies can be output from the speaker using

a MOS service.

See the 8155 PDF for more details.

EXPANSION CONNECTOR

The PRIMER has a 40 pin expansion connector (CN1) on board which provides additional expansion capabilities.

Primarily, this port gives access to the Data and Low Address Busses and Control Lines. EMAC offers modular expansion

boards that plug into this connector.

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