2.2.1 WRITE A BYTE
TO SP2
The sequence for a byte transfer from the Host to the SP2 (i.e. TX download) is asynchronous and
proceeds as follows:
1. HOST asserts TX Request line low to initiate transfer
2. Wait for SP2 to pull TX Accept low (i.e. request is accepted)
3. Set data lines to output and place LS nibble on the data lines
4. Negate TX Request (set to 1) to tell SP2 that data is present.
5. Wait for SP2 to negate TX Accept (i.e. data has been accepted)
Repeat steps 1-5 with MS nibble.
data present
transfer request
TX-REQUEST
(TXR)
request accept
data has been read
TX-ACCEPT
(TXA)
data may change
DATA
LS nibble
least significant nibble
MS nibble
most significant nibble
(4 LINES D0-D3)
1 byte
figure 6: TX download timing diagram
Notes:
• The data bus must not be set to output until step 3. i.e. after the SP2 has accepted the request. The
bus may be left as an output until the entire packet has been transferred to the SP2, it should then
be set back to input (default state).
• The SP2's normal response time to the initial TX Request may be up to 1ms, thereafter, for the
duration of the packet, the response will be fast.
• The SP2 will ignore a TX Request from the Host while it is receiving a packet from the radio. If the
incoming packet fails it's error check the SP2 will respond to the TX Request as normal, i.e. the TX
Accept from the SP2 will be delayed until the incoming packet has finished. If a valid packet is
received this must be uploaded to the Host before the SP2 can respond to the Host’s TX Request.
Thus an RX Request will be signalled to the Host and not the expected TX Accept and the Host must
upload the incoming packet before the TX packet can be downloaded. The TX Request should be left
asserted (low) during the upload. The SP2 will respond as normal after the upload is completed.
• For the above reason it is often easier to use RX Request to trigger a HOST interrupt and upload the
SP2 to the HOST under interrupt control.
• See Appendix B and C. for example SP2 driver subroutines.
Radiometrix Ltd, SP2 data sheet
page 7
RADIOMETRIX [ RADIOMETRIX LTD ]
