System Overview
3-12
S6/CLKDIV2
Bus Cycle Status Bit 6 (output, synchronous)
Clock Divide by 2 (input, internal pullup)
S6—During the second and remaining periods of a cycle (t
2
, t
3
, and t
4
),
this pin is asserted High to indicate a DMA-initiated bus cycle. During
a bus hold or reset condition, S6 floats.
CLKDIV2—If S6/CLKDIV2 is held Low during power-on reset, the chip
enters clock divide-by-2 mode where the processor clock is derived by
dividing the external clock input by 2. If this mode is selected, the PLL
is disabled. The pin is sampled on the rising edge of RES.
If S6 is to be used as PIO29 in input mode, the device driving PIO29
must not drive the pin Low during power-on reset. S6/PIO29 defaults
to a PIO input with pullup, so the pin does not need to be driven High
externally.
SCLK
Serial Clock (output, synchronous, three-state)
This pin supplies the synchronous serial interface (SSI) clock to a slave
device, allowing transmit and receive operations to be synchronized
between the microcontroller and the slave. SCLK is derived from the
microcontroller internal clock and then divided by 2, 4, 8, or 16,
depending on register settings. An access to any of the SSR or SSD
registers activates SCLK for eight SCLK cycles (see Figure 11-5 and
Figure 11-6 on page 11-8). When SCLK is inactive, it is held High by
the microcontroller.
SDATA
Serial Data (input/output, synchronous)
This pin transmits and receives synchronous serial interface (SSI) data
to and from a slave device. When SDATA is inactive, a weak keeper
holds the last value of SDATA on the pin.
SDEN1–SDEN0
Serial Data Enables (output, synchronous)
These pins enable data transfers on ports 1 and 0 of the synchronous
serial interface (SSI). The microcontroller asserts either SDEN1 or
SDEN0 at the beginning of a transfer and deasserts it after the transfer
is complete. When SDEN1–SDEN0 are inactive, they are held Low by
the microcontroller.
S2
S1
S0
Bus Cycle
0
0
0
Interrupt acknowledge
0
0
1
Read data from I/O
0
1
0
Write data to I/O
0
1
1
Halt
1
0
0
Instruction fetch
1
0
1
Read data from memory
1
1
0
Write data to memory
1
1
1
None (passive)
Summary of Contents for AM186EM
Page 1: ...Am186 EM and Am188 EM Microcontrollers User s Manual...
Page 4: ...iv...
Page 12: ...Table of Contents xii...
Page 62: ...Peripheral Control Block 4 10...
Page 76: ...Chip Select Unit 5 14...
Page 122: ...Timer Control Unit 8 8...
Page 136: ...DMA Controller 9 14...
Page 144: ...Asynchronous Serial Port 10 8...
Page 158: ...Programmable I O Pins 12 6...
Page 186: ...Index I 12...