Texas Instruments 99/4A - Page 25

TIMING SIGNAL
The TMS 9900 has an additional timing signal IAQ which becomes high
during any memory cycle when the TMS 9900 is acquiring an
instruction. This signal can be used tor timing purposes.
SUPPLY VOLTAGES
The
TMS 9900 requires 3
voltages with respect to ground.
VBB
- 5 volt supply
voltage
VCC
4- 5 volt supply
voltage
VDD
4-12 volt supply
voltage
VSS
system ground
CLOCK INPUTS
Four
clock signals, QI,
Q2, Q3 and Q4, have to be provided on the
four clock input pins for internal timing of the TMS 9900. Note
that these inputs require a 12 volt swing.
SYSTEM START UP
•The following signals can be used for system start up.
RESET
When active (low), RESET resets the processor. When RESET
is released, the TMS 9900 starts execution of its programs
with the first memory address fetched from memory location
0002.
LOAD When active (low), the processor starts execution of its
programs on the address in memory location FFFE.
During a reset or load, the microprocessor also fetches the
workspace pointer. This is the starting address of a memory field
of 16 words in RAM memory which is used as a register field.
During reset the workspace pointer is fetched from address 0000,
during load from address FFFC.
HARDWARE DESCRIPTION
A drawing of the connections to the TMS 9900 as used in the 99/4 is
given in Figure 4,p.21.
The following list describes the connections in more detail.
POWER CONNECTIONS
+ 5 volt. The +5 volt power supply is connected to FIN 2 and 59
of the TMS 9900. L600, C600 and C611 are acting as a
filter to reduce system noise on PIN2 and C603 is used
for the same purpose on FIN 59.
+12 volt.
The -+-12 volt power supply is connected to PIN 27 L601
and C612 are acting as a filter to reduce noise.
- 5 volt. The -5 volt power supply is connected to PIN 1 with
C602 for decoupling.
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