Texas Instruments 99/4A - Page 65

INTERVAL TIMER
Figure 26, p . 63 is a block diagram of the TMS 9901 interval timer
section. The clock consists of a 14 bit counter that decrements
at a rate o-F (03) /64 (at 3MHz this results in a maximum interval
349 milliseconds with a resolution o-F 21.3 microseconds), The
clock can be used as either an interval timer or an event timer.
To access the clock, select bit zero (control bit) must be set to
a one. The clock is enabled to cause interrupts by writing a
nonzero value to it and is then disabled -From interrupting by
writing zero to it or by an RST 1. The clock starts operating at
no more than two 03 times after it is loaded. When the clock
decrementer is running, it will decrement down to zero and will
then be reloaded -From the clock register and decrementing will
start again. (The zero state is counted as any other decrementer
state). The decrementer always runs, but it will not issue
interrupts until enabled; o-F course, the contents o-F the not
enabled clock read registers' are meaningless.
ROWER UR
During hardware reset, RST1 must be active (low) -For a minimum o-F
two clock cycles to -Force the TMS 9901 into a known state. RST 1
will disable all interrupts, disable the clock, program all I/O
ports to the input mode, and -Force IC0-IC3 to all zero's with
INTREO held high. The system software must enable the appropriate
interrupts, program the clock, and configure the I/O ports as
required. After initial power-up the TMS 9901 is accessed only as
needed to service the clock, enable (or disable) interrupts, or
read (write) data to the I/O ports. The I/O ports can be
configured by use of the RST2 software reset command bit.
HARDWARE DESCRIPTION
Connections to the THS 9901 as used on the 99/4A are given in
figure 22,p.57. The THS 9901 is reset during power up by using
the buffered reset signal of the THS 9900. The THS 9901 enabled
by the RAMELK signal (see memory selection part, figure 11 p.35).
When enabled, the- 9901 communicates with the TMS 9900 through the
CRUIN, CRUOUT and CRUCLK lines. The CRU bits are selected by
decoding A10 through A14. Connections with the devices which
interface to the TMS 9901 are discussed in other sections.
56