Rainbow Electronics AT89C5132 - Manual (Page 27)

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AT8xC5132
4173A–8051–08/02
External Bus Cycles This section describes the bus cycles that AT8xC5132 execute to read (see Figure 18),
and write data (see Figure 19) in the external data memory.
External memory cycle takes 6 CPU clock periods. This is equivalent to 12 oscillator
clock periods in standard mode or 6 oscillator clock periods in X2 mode. For further
information on X2 mode, refer to the section “X2 Feature”, page 12.
Slow peripherals can be accessed by stretching the read and write cycles. This is done
using the M0 bit in AUXR register. Setting this bit changes the width of the RD and WR
signals from 3 to 15 CPU clock periods.
For simplicity, the accompanying figures depict the bus cycle waveforms in idealized
form and do not provide precise timing information. For bus cycle timing parameters
refer to the section “AC Characteristics”.
Figure 18. External Data Read Waveforms
Notes: 1. RD signal may be stretched using M0 bit in AUXR register.
2. When executing MOVX @Ri instruction, P2 outputs SFR content.
3. When executing MOVX @DPTR instruction, if DPHDIS is set (Page Access Mode),
P2 outputs SFR content instead of DPH.
Figure 19. External Data Write Waveforms
Notes: 1. WR signal may be stretched using M0 bit in AUXR register.
2. When executing MOVX @Ri instruction, P2 outputs SFR content.
3. When executing MOVX @DPTR instruction, if DPHDIS is set (Page Access Mode),
P2 outputs SFR content instead of DPH.
ALE
P0
P2
RD# (1)
DPL or Ri D7:0
DPH or P2 (2),(3) P2
CPU Clock
ALE
P0
P2
WR (1)
DPL or Ri D7:0
P2
CPU Clock
DPH or P2 (2),(3)