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VSC056

Data Sheet

Revision 4.1

January 2008

3.2.77 8Dh: Pulse Train Control 91 (PTC91)

This register, along with the PTC90 register, provides a user-programmable LED flashing pulse train that

defines the 0.25 Hz flash rate (selectable in the bit control registers) at power on. The user can adjust the

pulse duration, the pulse train length, and the on/off time to derive a specific visual indication. The PTC90

contains two bits of pulse-width programmability, two bits of pulse train length programmability, and the

remaining four bits of the programmable pulse train.

As an example, the pulse train can be used to develop a visual heartbeat indication by programming the

PTC90 register with a 45h and the PTC91 register with a 01h. This combination develops a pulse train

with two blinks followed by a gap with the on times of the LED equal to 166.67 ms.

If the default values of this register or the PTC90 register are modified, and synchronization to other LED

flash rates is desired, the SYNC# pin must be enabled. To enable the SYNC# pin, tie the SYNCEN pin to

VDD, connect an external 10 kΩ pull-up resistor to the SYNC# pin, and place the proper synchronization

value in bits 3:0 of the Clock Select Control register. For information on the Clock Select Control register,

see “FDh: Clock Select Control (CSC),” page 111.

The following table shows the bit assignments for the Pulse Train Control 91 registers.

PTC91

Address:

8Bh

Reset Value:

1000_0000b

Bit

Bit Label

Access

Description

7:6

TPW1-0

R/W

Train Pulse Width

These two bits define the pulse width of each of the pulse train bits. The

available pulse widths are as follows:

5:4

PTL1-0

R/W

Pulse Train Length

These two bits define the pulse train length, which is the number of pulse train

bits that is shifted out before returning to bit 0. The available pulse train lengths

are given below.

3:0

PT11-8

R/W

Pulse Train

These four bits are the last bits shifted out from 0 to 3 and define the on/off time

for the flash rate.

1: defines an LED on time.

0: defines an LED off time.

TPW1

TPW0

Train Pulse Width

166.67 ms

250.0 ms

333.3 ms

500.0 ms

PTL1

PTL0

Pulse Train Length

12 pulse train bits from 0 to 11

10 pulse train bits from 0 to 9

9 pulse train bits from 0 to 8

8 pulse train bits from 0 to 7

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