2 tima counter registers, Tima counter registers – Freescale Semiconductor MC68HC908MR32 User Manual

Page 227

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I/O Registers

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

Freescale Semiconductor

227

PS[2:0] — Prescaler Select Bits

These read/write bits select either the

PTE3/TCLKA

pin or one of the seven prescaler outputs as the

input to the TIMA counter as

Table 16-1

shows. Reset clears the PS[2:0] bits.

16.7.2 TIMA Counter Registers

The two read-only TIMA counter registers contain the high and low bytes of the value in the TIMA counter.
Reading the high byte (TACNTH) latches the contents of the low byte (TACNTL) into a buffer. Subsequent
reads of TACNTH do not affect the latched TACNTL value until TACNTL is read. Reset clears the TIMA
counter registers. Setting the TIMA reset bit (TRST) also clears the TIMA counter registers.

NOTE

If TACNTH is read during a break interrupt, be sure to unlatch TACNTL by
reading TACNTL before exiting the break interrupt. Otherwise, TACNTL
retains the value latched during the break.

Table 16-1. Prescaler Selection

PS[2:0]

TIMA Clock Source

000

Internal bus clock

÷1

001

Internal bus clock

÷ 2

010

Internal bus clock

÷ 4

011

Internal bus clock

÷ 8

100

Internal bus clock

÷ 16

101

Internal bus clock

÷ 32

110

Internal bus clock

÷ 64

111

PTE3/TCLKA

Register Name and Address:

TACNTH — $000F

Bit 7

6

5

4

3

2

1

Bit 0

Read:

Bit 15

Bit 14

Bit 13

Bit 12

Bit 11

Bit 10

Bit 9

Bit 8

Write:

R

R

R

R

R

R

R

R

Reset:

0

0

0

0

0

0

0

0

Register Name and Address:

TACNTL — $0010

Bit 7

6

5

4

3

2

1

Bit 0

Read:

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Write:

R

R

R

R

R

R

R

R

Reset:

0

0

0

0

0

0

0

0

R

= Reserved

Figure 16-6. TIMA Counter Registers (TACNTH and TACNTL)

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