TPM754A NXP Semiconductors, TPM754A Datasheet - Page 4

• Manufacturer Part Number: TPM754A
• Description: Microcontroller With Trackpoint Microcode From Ibm
• Manufacturer: NXP Semiconductors

Philips Semiconductors
OSCILLATOR CHARACTERISTICS
X1 and X2 are the input and output, respectively, of an inverting
amplifier which can be configured for use as an on-chip oscillator.
To drive the device from an external clock source, X1 should be
driven while X2 is left unconnected. There are no requirements on
the duty cycle of the external clock signal, because the input to the
internal clock circuitry is through a divide-by-two flip-flop. However,
minimum and maximum high and low times specified in the data
sheet must be observed.
IDLE MODE
The TPM754 includes the 80C51 power-down and idle mode
features. In idle mode, the CPU puts itself to sleep while all of the
on-chip peripherals except the D/A stays active. The functions that
continue to run while in the idle mode are the timers and the
interrupts. The instruction to invoke the idle mode is the last
instruction executed in the normal operating mode before the idle
mode is activated. The CPU contents, the on-chip RAM, and all of
the special function registers remain intact during this mode. The
idle mode can be terminated either by any enabled interrupt (at
which time the process is picked up at the interrupt service routine
and continued), or by a hardware reset which starts the processor in
the same manner as a power-on reset. Upon powering-up the
circuit, or exiting from idle mode, sufficient time must be allowed for
stabilization of the internal analog reference voltages before a D/A
conversion is started.
I/O Ports
The I/O pins provided by the TPM754 consist of port 1 and port 3.
Port 1
Port 1 is a 3-bit bidirectional I/O port and includes alternate functions
on some pins of this port. Pins P1.0 and P1.1 are provided with
internal pullups while the remaining pin (P1.2) has an open drain
output structure. The alternate functions for port 1 are:
INT0 – External interrupt 0.
CEX – PCA clock output.
Port 3
Port 3 is an 8-bit bidirectional I/O port structure.
The alternate functions for port 3 are:
RxD – Serial port receiver data input.
T1 – Timer 1 external clock input.
INT1 – External interrupt 1.
TxD – Serial port transmitter data.
T0 – Timer 0 external clock input.
ECI – PCA external clock input.
Analog Section
The analog section of the TPM754, shown in Figure 1, consists of
four major elements: a bandgap referenced voltage regulator, an
8-bit DAC, an input multiplexer and comparator, and a low
impedance pulldown device.
The bandgap voltage regulator uses the AV
produces a regulated output on the VREG pin. The regulator also
supplies the analog supply voltage for the DAC. The regulator may
be switched on/off by means of the AC1 bit in the analog control
register (ACON0). The regulator output may also be supplied to the
XYDACBIAS and XYSOURCE pins by means of bits AC3 and AC4,
respectively. The DECOUPLE pin is provided for decoupling the
regulator output.
1999 Nov 11
Microcontroller with TrackPoint
CC
pin as its supply and
microcode from IBM
4
The DAC is an 8-bit device and its output appears on the XYDAC
pin. In addition, the DAC output may also be routed to the ZDAC pin
by means of bit AC6 in the ACON0 register. The DAC output is not
buffered, so external load impedances should be taken into
consideration when using either of these outputs.
A 3-input multiplexer is provided, whose output is connected to the
positive reference of a comparator. The multiplexer output is
controlled by bits MUX2:0 of ACON1. A bandgap reference supplies
the negative reference of the comparator. The output of the
comparator may be used the trigger the capture input of module 4 of
the PCA.
A low impedance pulldown is supplied at the XYZRAMP pin and is
controlled by bit AC5 of ACON0.
The functions of the analog section are controlled by the IBM
TrackPoint code embedded within the Philips TPM754.
PC BOARD LAYOUT CONSIDERATIONS
The TrackPoint is a low-level analog circuit. While not difficult to
implement, careful consideration should be given to circuit board
layout to obtain proper operation of the TrackPoint. The
considerations are similar to that used for radio frequency
application. The circuit should be located far from the CPU and
video lines, and should also be shielded from any digital signals. A
100 V pulse picked up every 3 or 4 seconds is sufficient to cause
cursor drift. A good circuit board layout will result in a circuit that is
very stable and will hold the cursor on a pixel for days at a time.
However, the autorouters of most software board layout packages
will not do an adequate job, and manual routing of this portion of the
motherboard is recommended.
The TPM754 has excellent supply regulation for the analog portions
of the TrackPoint circuit. However, care should be taken with respect
to the circuit ground to avoid voltage shifts due to non-TrackPoint
loads. The analog part of the circuit must have its own ground
plane, isolated from everything else and connected to the main
ground at just one point (no ground loops). All of the analog portion
of the TrackPoint circuit, and nothing else, must be over this
ground island.
No digital traces can pass though the analog area on any level from
the ground plane out. The circuit should be confined to one side of
the ground plane, preferably on the first interior layer, with the
ground plane next. The circuit should be powered only at a single
point (pin 14), and that power should be filtered to ground before it
comes onto the analog area.
The signal lines from the TrackPoint sensor stick can be sensitive to
pickup, and should be run close together, and not too close to digital
lines. A grounded guard trace is a good idea. Most important, the
stick common line is not a ground line, but rather a signal line,
although it will eventually connect to the analog ground. A common
and serious error is to treat it as ground, connecting it to a general
ground at some convenient point.
The layout should be designed to keep things compact and minimize
trace lengths. The whole circuit, analog and digital, will fit
comfortably within 2cm
desirable to put the circuit on a separate card instead of on the
motherboard. In this case, a shielded cable should provide the best
means of connecting the stick signals to the circuit card.
3cm. In some situations, it may be
Preliminary specification
TPM754A