4.4 F-Timer Function
An F-timer is integrated in the ERTEC 200 in addition to the system timers. This timer works independently of the system
clock and can be used for fail-safe applications, for example. The F-timer is triggered via the alternative “F_CLK” function
at the external “BYP_CLK” input. External triggering is not possible if the ARM946E-S is operated in a reserved test
mode (Config[4:3] = 11).
The following signal pins are available for the F-timer on the ERTEC 200.
•
External counter cable
1
F_CLK
Description of function:
The asynchronous input signal of the external independent time base is applied at a synchronization stage via the
BYP_CLK input pin (alternative F_CLK function). To rule out occurrences of metastable states at the counter input, the
synchronization stage is implemented with three flip-flop stages. The count pulses are generated in a series-connected
edge detection. All flip-flops run at the APB clock of 50 MHz.
The F_COUNTER_VAL register is reset using an asynchronous block reset or by writing the value 0x XXXX 55AA (X
means “don’t care) to the F-counter register “FCOUNT_RES”. The next count pulse sets the counter to 0xFFFF FFFF
and the counter is decremented at each additional count pulse. The FCOUNT_RES register is cleared again at the next
clock cycle.
The count value can be read out by a 32-bit read access. While an 8-bit or 16-bit read access is possible, it is not useful
because it can result in an inconsistency in the read count values.
Note on input frequency:
The maximum input frequency for the F-CLK is one-quarter of the APB clock. In the event of a quartz failure on the
ERTEC 200, a minimum output frequency between 40 and 90 MHz is set at the PLL. This yields a minimum APB-CLK
frequency of PLLOUT
min
40 MHz
/ 6 = 6.6666 MHz. To rule out a malfunction in the edge evaluation, the F-CLK can not
exceed APB-CLKmin 6.66 MHz/4 = 1.6666 MHz
The figure below shows the function blocks of the F-counter.
32-Bit Down-Counter
F-Counter-Val(31:0)
F-COUNTER-EN
BYP_CLK
Sync Stage:
3
edge
enable
DIRECT_IN
Read: F-Counter-Val
FCOUNT_RES
Reset
Write: F-Counter-Res
Data = 0xXXXX 55AAh
CLK APB
CLK_APB
EN
APB-Bus
F_CLK
CONFIG(4:3)
Figure 5: Block Diagram of F-Counter
Copyright © Siemens AG 2007. All rights reserved.
43
ERTEC 200 Manual
Technical data subject to change
Version 1.1.0