Renesas Hitachi H8S/2191, Hardware Manual

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Rev. 2.0, 11/00, page 773 of 1037
(2) VASS Detect Mode
VASS detection is carried out by the duty discriminator. Software can detect index
sequences by reading the duty I/O flag at each CTL pulse.
At each CTL pulse, the duty discriminator sends the result of duty discrimination to the duty
I/O flag, and simultaneously generates an interrupt request. The duty I/O flag is cleared to 0
if the CTL pulse is a 1 (duty cycle below 43%), and is set to 1 if the CTL pulse is a 0 (duty
cycle equal to or above 44%).
The duty I/O flag is modified at each CTL pulse. It should be read by the interrupt-handling
routine within the period of the PB-CTL signal. The VASS detection format is illustrated in
figure 28.58.
1
Tape direction
Written three times
1 1 1 1 1 1 1 1 1 1
M
S
B
L
S
B
L
S
B
M
S
B
M
S
B
L
S
B
L
S
B
M
S
B
Thousands Header (11 bits) Hundreds
Data (16 bits: 4 digits of 4-bit BCD)
Tens Ones
Figure 28.58 VASS (Index) Format
(3) Assemble (ASM) Mark Detect Mode
ASM mark detection is carried out by the duty discriminator. If the duty discriminator
detects that the duty cycle of the PB-CTL signal is 66% or higher, it generates an interrupt
request, and simultaneously clears the duty I/O flag to 0.
The duty I/O flag is updated at every CTL pulse. It should be read by the interrupt-handling
routine within the period of the PB-CTL signal.